Aircraft Routine/Required Inspections

For the purpose of determining their overall condition, 14 CFR provides for the inspection of all civil aircraft at specific intervals, depending generally upon the type of operations that they are engaged in. The pilot in command (PIC) of a civil aircraft is responsible for determining whether that aircraft is in a condition for safe flight. Therefore, the aircraft must be inspected before each flight. More detailed inspections must be conducted by aviation maintenance technicians (AMTs at least once each 12 calendar months, while inspection is required for others after each 100 hours of flight. In other instances, an aircraft may be inspected in accordance with a system set up to provide for total inspection of the aircraft over a calendar or flight time period. These include phase-type inspections.

To determine the specific inspection requirements and rules for the performance of inspections, refer to the CFR that prescribes the requirements for the inspection and maintenance of aircraft in various types of operations.

Preflight/Postflight Inspections

Pilots are required to follow a checklist contained within the Pilot’s Operating Handbook (POH) when operating aircraft. The first section of the checklist is entitled “Preflight Inspection.” The preflight inspection checklist includes a “walk-around” section listing items that the pilot is to visually check for general condition as he or she walks around the airplane. Also, the pilot must ensure that fuel, oil, and other items required for flight are at the proper levels and not contaminated. Additionally, it is the pilot’s responsibility to review the aircraft maintenance records, and other required paperwork to verify that the aircraft is indeed airworthy. After each flight, it is recommended that the pilot or mechanic conduct a postflight inspection to detect any problems that might require repair or servicing before the next flight.

Annual/100-Hour Inspections

The basic requirements for annual and 100-hour inspections are discussed in 14 CFR part 91. With some exceptions, all aircraft must have a complete inspection annually. Aircraft that are used for commercial purposes (carrying any person, other than a crewmember, for hire or flight instruction for hire) and are likely to be used more frequently than noncommercial aircraft must have this complete inspection every 100 hours. The scope and detail of items to be included in annual and 100-hour inspections is included as Appendix D to part 43. [Figure 1]

Figure 1. Title 14 CFR Appendix D to Part 43—Scope and detail of items (as applicable to the particular aircraft) to be included in annual and 100-hour inspections

A properly written checklist, such as the one shown in this site, includes all the items of Appendix D. Although the scope and detail of annual and 100-hour inspections are identical, there are two significant differences. One difference involves persons authorized to conduct them. A certified airframe and powerplant (A&P) maintenance technician can conduct a 100-hour inspection, whereas an annual inspection must be conducted by a certified A&P maintenance technician with inspection authorization (IA). The other difference involves authorized overflight of the maximum 100 hours before inspection. An aircraft may be flown up to 10 hours beyond the 100-hour limit if necessary to fly to a destination where the inspection is to be conducted.

Progressive Inspections

Because the scope and detail of an annual inspection is very extensive and could keep an aircraft out of service for a considerable length of time, alternative inspection programs designed to minimize down time may be utilized. A progressive inspection program allows an aircraft to be inspected progressively. The scope and detail of an annual inspection is essentially divided into segments or phases (typically four to six). Completion of all the phases completes a cycle that satisfies the requirements of an annual inspection. The advantage of such a program is that any required segment may be completed overnight and thus enable the aircraft to fly daily without missing any revenue earning potential. Progressive inspection programs include routine items, such as engine oil changes, and detailed items, such as flight control cable inspection. Routine items are accomplished each time the aircraft comes in for a phase inspection, and detailed items focus on detailed inspection of specific areas. Detailed inspections are typically done once each cycle. A cycle must be completed within 12 months. If all required phases are not completed within 12 months, the remaining phase inspections must be conducted before the end of the 12th month from when the first phase was completed.

Each registered owner or operator of an aircraft desiring to use a progressive inspection program must submit a written request to the FAA Flight Standards District Office (FSDO) having jurisdiction over the area that the applicant is located. Section 91.409(d) of 14 CFR part 91 establishes procedures to be followed for progressive inspections. [Figure 2]

Figure 2. Title 14 CFR Section 91.409(d), Progressive Inspection

Continuous Inspections

Continuous inspection programs are similar to progressive inspection programs, except that they apply to large or turbine-powered aircraft and are therefore more complicated. Like progressive inspection programs, they require approval by the FAA Administrator. The approval may be sought based upon the type of operation and the CFR parts that the aircraft is operated under. The maintenance program for commercially operated aircraft must be detailed in the approved operations specifications (OpSpecs) of the commercial certificate holder.

Airlines utilize a continuous maintenance program that includes both routine and detailed inspections. However, the detailed inspections may include different levels of detail. Often referred to as “checks,” the A-checks, B-checks, C-checks, and D-checks involve increasing levels of detail. A-checks are the least comprehensive and occur frequently. D-checks, on the other hand, are extremely comprehensive, involving major disassembly, removal, overhaul, and inspection of systems and components. They might occur only three to six times during the service life of an aircraft.

Altimeter and Transponder Inspections

Aircraft that are operated in controlled airspace under instrument flight rules (IFR) must have each altimeter and static system tested in accordance with procedures described in 14 CFR part 43, Appendix E, within the preceding 24 calendar months. Aircraft having an air traffic control (ATC) transponder must also have each transponder checked within the preceding 24 months. All these checks must be conducted by appropriately certified individuals.

Aircraft Special Inspections

During the service life of an aircraft, occasions may arise when something out of the ordinary care and use of an aircraft could possibly affect its airworthiness. When these situations are encountered, special inspection procedures, also called conditional inspections, are followed to determine if damage to the aircraft structure has occurred. The procedures outlined are general in nature and are intended to acquaint the aviation mechanic with the areas to be inspected. As such, they are not all inclusive. When performing any of these special inspections, always follow the detailed procedures in the aircraft maintenance manual. In situations where the manual does not adequately address the situation, seek advice from other maintenance technicians who are highly experienced with them. The following paragraphs describe some typical types of special inspections.

Hard or Overweight Landing Inspection

The structural stress induced by a landing depends not only upon the gross weight at the time, but also upon the severity of impact. The hard landing inspection is for hard landings at or below the maximum design landing limits. An overweight landing inspection must be performed when an airplane lands at a weight above the maximum design landing weight. However, because of the difficulty in estimating vertical velocity at the time of contact, it is hard to judge whether or not a landing has been sufficiently severe to cause structural damage. For this reason, a special inspection is performed after a landing is made at a weight known to exceed the design landing weight or after a rough landing, even though the latter may have occurred when the aircraft did not exceed the design landing weight.

Wrinkled wing skin is the most easily detected sign of an excessive load having been imposed during a landing. Another indication easily detected is fuel leakage along riveted seams. Other possible locations of damage are spar webs, bulkheads, nacelle skin and attachments, firewall skin, and wing and fuselage stringers. If none of these areas show adverse effects, it is reasonable to assume that no serious damage has occurred. If damage is detected, a more extensive inspection and alignment check may be necessary.

Severe Turbulence Inspection/Over “G”

When an aircraft encounters a gust condition, the airload on the wings exceeds the normal wingload supporting the aircraft weight. The gust tends to accelerate the aircraft while its inertia acts to resist this change. If the combination of gust velocity and airspeed is too severe, the induced stress can cause structural damage.

A special inspection is performed after a flight through severe turbulence. Emphasis is placed upon inspecting the upper and lower wing surfaces for excessive buckles or wrinkles with permanent set. Where wrinkles have occurred, remove a few rivets and examine the rivet shanks to determine if the rivets have sheared or were highly loaded in shear.

Through the inspection doors and other accessible openings, inspect all spar webs from the fuselage to the tip. Check for buckling, wrinkles, and sheared attachments. Inspect for buckling in the area around the nacelles and in the nacelle skin, particularly at the wing leading edge. Check for fuel leaks. Any sizeable fuel leak is an indication that an area may have received overloads that have broken the sealant and opened the seams.

If the landing gear was lowered during a period of severe turbulence, inspect the surrounding surfaces carefully for loose rivets, cracks, or buckling. The interior of the wheel well may give further indications of excessive gust conditions. Inspect the top and bottom fuselage skin. An excessive bending moment may have left wrinkles of a diagonal nature in these areas.

Inspect the surface of the empennage for wrinkles, buckling, or sheared attachments. Also, inspect the area of attachment of the empennage to the fuselage. These inspections cover the critical areas. If excessive damage is noted in any of the areas mentioned, the inspection must be continued until all damage is detected.

Lightning Strike

Although lightning strikes to aircraft are extremely rare, if a strike has occurred, the aircraft is carefully inspected to determine the extent of any damage that might have occurred. When lightning strikes an aircraft, the electrical current must be conducted through the structure and be allowed to discharge or dissipate at controlled locations. These controlled locations are primarily the aircraft’s static discharge wicks, or on more sophisticated aircraft, null field dischargers. When surges of high-voltage electricity pass through good electrical conductors, such as aluminum or steel, damage is likely to be minimal or nonexistent. When surges of high-voltage electricity pass through non-metallic structures, such as a fiberglass radome, engine cowl or fairing, glass or plastic window, or a composite structure that does not have built-in electrical bonding, burning and more serious damage to the structure could occur. Visual inspection of the structure is required. Look for evidence of degradation, burning, or erosion of the composite resin at all affected structures, electrical bonding straps, static discharge wicks, and null field dischargers.

Bird Strike

When the aircraft is hit by birds during flight, the external areas of the airplane are inspected in the general area of the bird strike. If the initial inspection shows structural damage, then the internal structure of the airplane must be inspected as well. Also, inspect the hydraulic, pneumatic, and any other systems in the area of the bird strike.

Fire Damage

Inspection of aircraft structures that have been subjected to fire or intense heat can be relatively simple if visible damage is present. Visible damage requires repair or replacement. If there is no visible damage, the structural integrity of an aircraft may still have been compromised. Since most structural metallic components of an aircraft have undergone some sort of heat treatment process during manufacture, an exposure to high heat not encountered during normal operations could severely degrade the design strength of the structure. The strength and airworthiness of an aluminum structure that passes a visual inspection, but is still suspect, can be further determined by use of a conductivity tester. This is a device that uses eddy current. Since strength of metals is related to hardness, possible damage to steel structures might be determined by use of a hardness tester, such as a Rockwell C hardness tester. [Figure]

Figure. Rockwell C Hardness Tester

Flood Damage

Like aircraft damaged by fire, aircraft damaged by water can range from minor to severe. This depends on the level of the flood water, whether it was fresh or salt water, and the elapsed time between the flood occurrence and when repairs were initiated. Any parts that were totally submerged are completely disassembled, thoroughly cleaned, dried, and treated with a corrosion inhibitor. Many parts might have to be replaced, particularly interior carpeting, seats, side panels, and instruments. Since water serves as an electrolyte that promotes corrosion, all traces of water and salt must be removed before the aircraft can again be considered airworthy.

Seaplanes

Because they operate in an environment that accelerates corrosion, seaplanes must be carefully inspected for corrosion and conditions that promote corrosion. Inspect bilge areas for waste hydraulic fluids, water, dirt, drill chips, and other debris. Additionally, since seaplanes often encounter excessive stress from the pounding of rough water at high speeds, inspect for loose rivets and other fasteners; stretched, bent or cracked skins; damage to the float attach fitting; and general wear and tear on the entire structure.

Aerial Application Aircraft

Two primary factors that make inspecting these aircraft different from other aircraft are the corrosive nature of some of the chemicals used and the typical flight profile. Damaging effects of corrosion may be detected in a much shorter period of time than normal use aircraft. Chemicals may soften the fabric or loosen the fabric tapes of fabric-covered aircraft. Metal aircraft may need to have the paint stripped, cleaned, and repainted and corrosion treated annually. Leading edges of wings and other areas may require protective coatings or tapes. Hardware may require more frequent replacement.

During peak use, these aircraft may fly up to 50 cycles (takeoffs and landings) or more in a day, most likely from an unimproved or grass runway. This can greatly accelerate the failure of normal fatigue items. Landing gear and related items require frequent inspections. Because these aircraft operate almost continuously at very low altitudes, air filters tend to become obstructed more rapidly.

Nondestructive Inspection/Testing (Part 1)

The preceding information in this site provided general details regarding aircraft inspection. The remainder of this site deals with several methods often used on specific components or areas on an aircraft when carrying out the more specific inspections. They are referred to as nondestructive inspection (NDI) or nondestructive testing (NDT). The objective of NDI and NDT is to determine the airworthiness of a component, without damaging it, that would render it unairworthy. Some of these methods are simple, requiring little additional expertise, while others are highly sophisticated and require that the technician be highly trained and specially certified.

Training, Qualification, and Certification

The product manufacturer or the FAA generally specifies the particular NDI method and procedure to be used in inspection. These NDI requirements are specified in the manufacturer’s inspection, maintenance, or overhaul manual, FAA ADs, supplemental structural inspection documents (SSID), or SBs.

The success of any NDI method and procedure depends upon the knowledge, skill, and experience of the NDI personnel involved. The person(s) responsible for detecting and interpreting indications, such as eddy current, x-ray, or ultrasonic NDI, must be qualified and certified to specific FAA or other acceptable government or industry standards, such as MIL-STD-410, Nondestructive Testing Personnel Qualification and Certification or A4A iSPec 2200, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods. The person must be familiar with the test method, know the potential types of discontinuities peculiar to the material, and be familiar with their effect on the structural integrity of the part. Additional information on NDI may be found by referring to Chapter 5 of FAA AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair.

Advantages and Disadvantages of NDI Methods

Figure 1 provides a table of the advantages and disadvantages of common NDI methods. This table could be used as a guide for evaluating the most appropriate NDI method when the manufacturer or the FAA has not specified a particular NDI method to be used.

Method	Advantages	Disadvantages
Visual	• Inexpensive• Highly portable• Immediate results• Minimum training• Minimum part preparation	• Surface discontinuities only• Generally only large discontinuities• Misinterpretation of scratches
Penetrant Dye	• Portable• Inexpensive• Sensitive to very small discontinuities• 30 minutes or less to accomplish• Minimum skill required	• Locate surface defects only• Rough or porous surfaces interfere with test• Part preparation required (removal of finishes and sealant, etc.)• High degree of cleanliness required• Direct visual detection on results required
Magnetic Particle	• Can be portable• Inexpensive• Sensitive to small discontinuities• Immediate results• Moderate skill required• Detects surface and subsurface discontinuities• Relatively fast	• Surface must be accessible• Rough surfaces interfere with test• Part preparation required (removal of finishes and sealant, etc.)• Semi-directional requiring general orientation of field to discontinuity• Ferro-magnetic materials only• Part must be demagnetized after test
Eddy Current	• Portable• Detects surface and subsurface discontinuities• Moderate speed• Immediate results• Sensitive to small discontinuities• Thickness sensitive• Can detect many variables	• Surface must be accessible to probe• Rough surfaces interfere with test• Electrically conductive materials• Skill and training required• Time consuming for large areas
Ultrasonic	• Portable• Inexpensive• Sensitive to very small discontinuities• Immediate results• Little part preparation• Wide range of materials and thickness can be inspected	• Surface must be accessible to probe• Rough surfaces interfere with test• Highly sensitive to sound beam discontinuity orientation• High degree of skill and experience required for exposure and interpretation• Depth of discontinuity not indicated
X-Ray Radiography	• Detects surface and internal flaws• Can inspect hidden areas• Permanent test record obtained• Minimum part preparation	• Safety hazard• Very expensive (slow process)• Highly directional, sensitive to flaw orientation• High degree of skill and experience required for exposure and interpretation• Depth of discontinuity not indicated
Isotope Radiography	• Portable• Less inexpensive than x-ray• Detects surface and internal flaws• Can inspect hidden areas• Permanent test record obtained• Minimum part preparation	• Safety hazard• Must conform to federal and state regulations for handling and use• Highly directional, sensitive to flaw orientation• High degree of skill and experience required for exposure and interpretation• Depth of discontinuity not indicated

Figure 1. Advantages and disadvantages of NDI methods

General Techniques

Before conducting NDI, it is necessary to follow preparatory steps in accordance with procedures specific to that type of inspection. Generally, the parts or areas must be thoroughly cleaned. Some parts must be removed from the aircraft or engine. Others might need to have any paint or protective coating stripped. A complete knowledge of the equipment and procedures is essential and, if required, calibration and inspection of the equipment must be current.

Visual Inspection

Visual inspection can be enhanced by looking at the suspect area with a bright light, a magnifying glass, and a mirror. Some defects might be so obvious that further inspection methods are not required. The lack of visible defects does not necessarily mean further inspection is unnecessary. Some defects may lie beneath the surface or may be so small that the human eye, even with the assistance of a magnifying glass, cannot detect them.

Surface Cracks

When searching for surface cracks with a flashlight, direct the light beam at a 5 to 45 degree angle to the inspection surface towards the face. [Figure 2] Do not direct the light beam at such an angle that the reflected light beam shines directly into the eyes. Keep the eyes above the reflected light beam during the inspection. Determine the extent of any cracks found by directing the light beam at right angles to the crack and tracing its length. Use a 10-power magnifying glass to confirm the existence of a suspected crack. If this is not adequate, use other NDI techniques, such as penetrant, magnetic particle, or eddy current to verify cracks.

Figure 2. Using a flashlight to inspect for cracks

Borescope

Inspection by use of a borescope is essentially a visual inspection. A borescope is a device that enables the inspector to see inside areas that could not otherwise be inspected without disassembly. Borescopes are used in aircraft and engine maintenance programs to reduce or eliminate the need for costly teardowns. Aircraft turbine engines have access ports that are specifically designed for borescopes. Borescopes are also used extensively in a variety of aviation maintenance programs to determine the airworthiness of difficult to reach components. Borescopes typically are used to inspect interiors of hydraulic cylinders and valves for pitting, scoring, porosity, and tool marks; search for cracked cylinders in aircraft reciprocating engines; inspect turbojet engine turbine blades and combustion cans; verify the proper placement and fit of seals, bonds, gaskets, and subassemblies in difficult to reach areas; and assess foreign object damage (FOD) in aircraft, airframe, and powerplants. Borescopes may also be used to locate and retrieve foreign objects in engines and airframes.

Borescopes are available in two basic configurations. The simpler of the two is a rigid type, small diameter telescope with a tiny mirror at the end that enables the user to see around corners. The other type uses fiber optics that enable greater flexibility. [Figure 3] Many borescopes provide images that can be displayed on a computer or video monitor for better interpretation of what is being viewed and to record images for future reference. Most borescopes also include a light to illuminate the area being viewed.

Figure 3. Rigid and flexible borescopes

Liquid Penetrant Inspection

Penetrant inspection is a nondestructive test for defects open to the surface in parts made of any nonporous material. It is used with equal success on such metals as aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium. It may also be used on ceramics, plastics, molded rubber, and glass.

Penetrant inspection detects defects, such as surface cracks or porosity. These defects may be caused by fatigue cracks, shrinkage cracks, shrinkage porosity, cold shuts, grinding and heat treat cracks, seams, forging laps, and bursts. Penetrant inspection also indicates a lack of bond between joined metals. The main disadvantage of penetrant inspection is that the defect must be open to the surface in order to let the penetrant get into the defect. For this reason, if the part in question is made of material that is magnetic, the use of magnetic particle inspection is generally recommended.

Penetrant inspection uses a penetrating liquid that enters a surface opening and remains there, making it clearly visible to the inspector. It calls for visual examination of the part after it has been processed, increasing the visibility of the defect so that it can be detected. Visibility of the penetrating material is increased by the addition of one or two types of dye: visible or fluorescent.

The visible penetrant kit consists of dye penetrant, dye remover emulsifier, and developer. The fluorescent penetrant inspection kit contains a black light assembly, as well as spray cans of penetrant, cleaner, and developer. The light assembly consists of a power transformer, a flexible power cable, and a hand-held lamp. Due to its size, the lamp may be used in almost any position or location.

The steps for performing a penetrant inspection are:

1. Clean the metal surface thoroughly.
2. Apply penetrant.
3. Remove penetrant with remover emulsifier or cleaner.
4. Dry the part.
5. Apply the developer.
6. Inspect and interpret results.

Interpretation of Results

The success and reliability of a penetrant inspection depends upon the thoroughness that the part was prepared with. Several basic principles applying to penetrant inspection are:

1. The penetrant must enter the defect in order to form an indication. It is important to allow sufficient time so the penetrant can fill the defect. The defect must be clean and free of contaminating materials so that the penetrant is free to enter.
2. If all penetrant is washed out of a defect, an indication cannot be formed. During the washing or rinsing operation, prior to development, it is possible that the penetrant is removed from within the defect, as well as from the surface.
3. Clean cracks are usually easy to detect. Surface openings that are uncontaminated, regardless of how fine, are seldom difficult to detect with the penetrant inspection.
4. The smaller the defect, the longer the penetrating time. Fine crack-like apertures require a longer penetrating time than defects such as pores.
5. When the part to be inspected is made of a material susceptible to magnetism, it should be inspected by a magnetic particle inspection method if the equipment is available.
6. Visible penetrant-type developer, when applied to the surface of a part, dries to a smooth, white coating. As the developer dries, bright red indications appear where there are surface defects. If no red indications appear, there are no surface defects.
7. When conducting the fluorescent penetrant-type inspection, the defects show up (under black light) as a brilliant yellow-green color and the sound areas appear deep blue-violet.
8. It is possible to examine an indication of a defect and to determine its cause as well as its extent. Such an appraisal can be made if something is known about the manufacturing processes that the part has been subjected to.

The size of the indication, or accumulation of penetrant, shows the extent of the defect and the brilliance is a measure of its depth. Deep cracks hold more penetrant and are broader and more brilliant. Very fine openings can hold only small amounts of penetrants and appear as fine lines. [Figure 4]

Figure 4. Dye penetrant inspection

False Indications

With the penetrant inspection, there are no false indications in the sense that they occur in the magnetic particle inspection. There are, however, two conditions that may create accumulations of penetrant that are sometimes confused with true surface cracks and discontinuities.

The first condition involves indications caused by poor washing. If all the surface penetrant is not removed in the washing or rinsing operation following the penetrant dwell time, the unremoved penetrant is visible. Evidences of incomplete washing are usually easy to identify since the penetrant is in broad areas rather than in the sharp patterns found with true indications. When accumulations of unwashed penetrant are found on a part, the part must be completely reprocessed. Degreasing is recommended for removal of all traces of the penetrant. False indications may also be created where parts press fit to each other. If a wheel is press fit onto a shaft, penetrant shows an indication at the fit line. This is perfectly normal since the two parts are not meant to be welded together. Indications of this type are easy to identify since they are regular in form and shape.

Nondestructive Inspection/Testing (Part 2)

Eddy Current Inspection

Electromagnetic analysis is a term describing the broad spectrum of electronic test methods involving the intersection of magnetic fields and circulatory currents. The most widely used technique is the eddy current. Eddy currents are composed of free electrons under the influence of an induced electromagnetic field that are made to “drift” through metal. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine alloy and heat-treat condition.

Eddy current is used in aircraft maintenance to inspect jet engine turbine shafts and vanes, wing skins, wheels, bolt holes, and spark plug bores for cracks, heat, or frame damage. Eddy current may also be used in repair of aluminum aircraft damaged by fire or excessive heat. Different meter readings are seen when the same metal is in different hardness states. Readings in the affected area are compared with identical materials in known unaffected areas for comparison. A difference in readings indicates a difference in the hardness state of the affected area. In aircraft manufacturing plants, eddy current is used to inspect castings, stampings, machine parts, forgings, and extrusions. Figure 1 shows a technician performing an eddy current inspection on a fan blade.

Figure 1. Eddy current inspection

Basic Principles

When an alternating current (AC) is passed through a coil, it develops a magnetic field around the coil, which in turn induces a voltage of opposite polarity in the coil and opposes the flow of original current. If this coil is placed in such a way that the magnetic field passes through an electrically conducting specimen, eddy currents are induced into the specimen. The eddy currents create their own field that varies the original field’s opposition to the flow of original current. The specimen’s susceptibility to eddy currents determines the current flow through the coil.

The magnitude and phase of this counter field is dependent primarily upon the resistance and permeability of the specimen under consideration and enables us to make a qualitative determination of various physical properties of the test material. The interaction of the eddy current field with the original field results is a power change that can be measured by utilizing electronic circuitry similar to a Wheatstone bridge.

Principles of Operations

Eddy currents are induced in a test article when an AC is applied to a test coil (probe). The AC in the coil induces an alternating magnetic field in the article, causing eddy currents to flow in the article. [Figure 2]

Figure 2. Generating an eddy current

Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accordingly. [Figure 3] Instruments display the impedance changes either by impedance plane plots or by needle deflection. [Figure 4]

Figure 3. Detecting an eddy current

Figure 4. Impedance plane test

The specimen is either placed in or passed through the field of an electromagnetic induction coil, and its effect on the impedance of the coil or on the voltage output of one or more test coils is observed. The process that involves electric fields made to explore a test piece for various conditions involves the transmission of energy through the specimen much like the transmission of x-rays, heat, or ultrasound.

Eddy current inspection can frequently be performed without removing the surface coatings, such as primer, paint, and anodized films. It can be effective in detecting surface and subsurface corrosion, pots, and heat treat condition.

Eddy Current Instruments

A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, receiving, and displaying. The generating portion of the unit provides an alternating current to the test coil. The receiving section processes the signal from the test coil to the required form and amplitude for display. Instrument outputs or displays consist of a variety of visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays.

A reference standard is required for the calibration of eddy current test equipment. A reference standard is made from the same material as the item is to be tested. A reference standard contains known flaws or cracks and could include items, such as a flat surface notch, a fastener head, a fastener hole, or a countersink hole. Figures 5, 6, and 7 show typical surface cracks, subsurface cracks, and structural corrosion that can be detected with eddy current techniques.

Figure 5. Typical surface cracks

Figure 6. Typical subsurface cracks

Figure 7. Typical structural corrosion

Ultrasonic Inspection

Ultrasonic inspection is an NDI technique that uses sound energy moving through the test specimen to detect flaws. The sound energy passing through the specimen is displayed on a cathode ray tube (CRT), a liquid crystal display (LCD) computer data program, or video/camera medium. Indications of the front and back surface and internal/external conditions appear as vertical signals on the CRT screen or nodes of data in the computer test program. [Figure 8] There are three types of display patterns: “A” scan, “B” scan, and “C” scan. Each scan provides a different picture or view of the specimen being tested. [Figure 9]

Figure 8. Ultrasonic inspection

Figure 9. Typical structural corrosion

Ultrasonic detection equipment makes it possible to locate defects in all types of materials. Minute cracks, checks, and voids too small to be seen by x-ray can be located by ultrasonic inspection. An ultrasonic test instrument requires access to only one surface of the material to be inspected and can be used with either straight line or angle beam testing techniques.

Two basic methods are used for ultrasonic inspection. The first of these methods is immersion testing. In this method of inspection, the part under examination and the search unit are completely immersed in a liquid couplant, such as water or other suitable fluids.

The second method is called contact testing. It is readily adapted to field use and is the method discussed. In this method, the part under examination and the search unit are coupled with a viscous material, liquid, or a paste that wets both the face of the search unit and the material under examination.

There are three basic ultrasonic inspection methods: pulse echo, through transmission, and resonance. Through transmission and pulse echo are shown in Figure 10.

Figure 10. Through-transmission and pulse echo indications

Pulse Echo

Flaws are detected by measuring the amplitude of signals reflected and the time required for these signals to travel between specific surfaces and the discontinuity. [Figure 11]

Figure 11. Block diagram of basic pulse-echo system

The time base, triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen of the CRT or LCD. The spot sweeps from left to right across the face of the scope 50 to 5,000 times per second or higher if required for high-speed automated scanning. Due to the speed of the cycle of transmitting and receiving, the picture on the oscilloscope appears to be stationary.

A few microseconds after the sweep is initiated, the rate generator electrically excites the pulser, and the pulser in turn emits an electrical pulse. The transducer converts this pulse into a short train of ultrasonic sound waves. If the interfaces of the transducer and the specimen are properly oriented, the ultrasound is reflected back to the transducer when it reaches the internal flaw and the opposite surface of the specimen. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen are measured by the timing circuits. The reflected pulse received by the transducer is amplified, transmitted to, and displayed on the instrument screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is in relation to the front and back surfaces of the specimen. [Figure 12]

Figure 12. Pulse-echo display in relationship to flaw detection

Pulse-echo instruments may also be used to detect flaws not directly underneath the probe by use of the angle beam testing method. Angle beam testing differs from straight beam testing only in the manner that the ultrasonic waves pass through the material being tested. As shown in Figure 13, the beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam, or a portion thereof, reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight beam testing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.

Figure 13. Pulse-echo angle beam testing

Through Transmission

Through transmission inspection uses two transducers, one to generate the pulse and another placed on the opposite surface to receive it. A disruption in the sound path indicates a flaw and is displayed on the instrument screen. Through transmission is less sensitive to small defects than the pulse-echo method.

Resonance

This system differs from the pulse method in that the frequency of transmission may be continuously varied. The resonance method is used principally for thickness measurements when the two sides of the material being tested are smooth and parallel and the backside is inaccessible. The point where the frequency matches the resonance point of the material being tested is the thickness determining factor. It is necessary that the frequency of the ultrasonic waves corresponding to a particular dial setting be accurately known. Checks are made with standard test blocks to guard against possible drift of frequency.

If the frequency of an ultrasonic wave is such that its wavelength is twice the thickness of a specimen (fundamental frequency), then the reflected wave arrives back at the transducer in the same phase as the original transmission so that strengthening of the signal occurs. This results from constructive interference or a resonance and is shown as a high amplitude value on the indicating screen. If the frequency is increased such that three times the wavelength equals four times the thickness, the reflected signal returns completely out of phase with the transmitted signal and cancellation occurs. Further increase of the frequency causes the wavelength to be equal to the thickness again and gives a reflected signal in phase with the transmitted signal and a resonance once more. By starting at the fundamental frequency and gradually increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. [Figure 14]

Figure 14. Conditions of ultrasonic resonance in a metal plate

In some instruments, the oscillator circuit contains a motor-driven capacitor that changes the frequency of the oscillator. [Figure 15] In other instruments, the frequency is changed by electronic means. The change in frequency is synchronized with the horizontal sweep of a CRT. The horizontal axis represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to present these vertically. Calibrated transparent scales are then placed in front of the tube and the thickness can be read directly. The instruments normally operate between 0.25 millicycle (mc) and 10 mc in four or five bands.

Figure 15. Block diagram of resonance thickness measuring system

The resonance thickness instrument can be used to test the thickness of such metals as steel, cast iron, brass, nickel, copper, silver, lead, aluminum, and magnesium. In addition, areas of corrosion or wear on tanks, tubing, airplane wing skins, and other structures or products can be located and evaluated. Direct reading dial-operated units are available that measure thickness between 0.025 inch and 3 inches with an accuracy of better than ±1 percent. Ultrasonic inspection requires a skilled operator who is familiar with the equipment being used, as well as the inspection method to be used for the many different parts being tested. [Figure 16]

Figure 16. Ultrasonic inspection of a composite structure

Ultrasonic Instruments

A portable, battery-powered ultrasonic instrument is used for field inspection of airplane structure. The instrument generates an ultrasonic pulse, detects and amplifies the returning echo, and displays the detected signal on a CRT or similar display. Piezoelectric transducers produce longitudinal or shear waves, the most commonly used wave forms for aircraft structural inspection.

Reference Standards

Reference standards are used to calibrate the ultrasonic instrument. Reference standards serve two purposes: to provide an ultrasonic response pattern that is related to the part being inspected and to establish the required inspection sensitivity. To obtain a representative response pattern, the reference standard configuration is the same as that of the test structure or is a configuration that provides an ultrasonic response pattern representative of the test structure. The reference standard contains a simulated defect (notch) that is positioned to provide a calibration signal representative of the expected defect. The notch size is chosen to establish inspection sensitivity (response to the expected defect size). The inspection procedure gives a detailed description of the required reference standard.

Couplants

Inspection with ultrasonics is limited to the part in contact with the transducer. A layer of couplant is required to couple the transducer to the test piece, because ultrasonic energy does not travel through air. Some typical couplants used are water, glycerin, motor oils, and grease.

nspection of Aircraft Bonded Structures (Part 1) – Inspection Concepts and Techniques

Ultrasonic inspection is finding increasing application in aircraft bonded construction and repair. Many configurations and types of bonded structures are in use in aircraft. All of these variations complicate the application of ultrasonic inspections. An inspection method that works well on one part or one area of the part may not be applicable for different parts or areas of the same part. Some of the variables in the types of bonded structures are as follows:

• Top skin material is made from different materials and thickness
• Different types and thickness of adhesives are used in bonded structures
• Underlying structures contain differences in core material, cell size, thickness, height, back skin material and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates (number of layers, layer thickness, and layer material)
• The top only or top and bottom skin of a bonded structure may be accessible

Types of Defects

Defects can be separated into five general types to represent the various areas of bonded and laminate structures as follows:

1. Type I—disbonds or voids in an outer skin-toadhesive interface.
2. Type II—disbonds or voids at the adhesive-to-core interface.
3. Type III—voids between layers of a laminate.
4. Type IV—voids in foam adhesive or disbonds between the adhesive and a closure member at core-to-closure member joints.
5. Type V—water in the core.

Acoustic Emission Inspection

Acoustic emission is an NDI technique that involves the placing of acoustic emission sensors at various locations on an aircraft structure and then applying a load or stress. The materials emit sound and stress waves that take the form of ultrasonic pulses. Cracks and areas of corrosion in the stressed airframe structure emit sound waves that are registered by the sensors. These acoustic emission bursts can be used to locate flaws and to evaluate their rate of growth as a function of applied stress. Acoustic emission testing has an advantage over other NDI methods in that it can detect and locate all of the activated flaws in a structure in one test. Because of the complexity of aircraft structures, application of acoustic emission testing to aircraft has required a new level of sophistication in testing technique and data interpretation.

Magnetic Particle Inspection

Magnetic particle inspection is a method of detecting invisible cracks and other defects in ferromagnetic materials, such as iron and steel. It is not applicable to nonmagnetic materials. In rapidly rotating, reciprocating, vibrating, and other highly-stressed aircraft parts, small defects often develop to the point that they cause complete failure of the part. Magnetic particle inspection has proven extremely reliable for the rapid detection of such defects located on or near the surface. With this method of inspection, the location of the defect is indicated and the approximate size and shape are outlined.

The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected. The ferromagnetic particles (indicating medium) may be held in suspension in a liquid that is flushed over the part; the part may be immersed in the suspension liquid; or the particles, in dry powder form, may be dusted over the surface of the part. The wet process is more commonly used in the inspection of aircraft parts.

If a discontinuity is present, the magnetic lines of force are disturbed and opposite poles exist on either side of the discontinuity. The magnetized particles thus form a pattern in the magnetic field between the opposite poles. This pattern, known as an “indication,” assumes the approximate shape of the surface projection of the discontinuity. A discontinuity may be defined as an interruption in the normal physical structure or configuration of a part, such as a crack, forging lap, seam, inclusion, porosity, and the like. A discontinuity may or may not affect the usefulness of a part.

Development of Indications

When a discontinuity in a magnetized material is open to the surface and a magnetic substance (indicating medium) is available on the surface, the flux leakage at the discontinuity tends to form the indicating medium into a path of higher permeability. (Permeability is a term used to refer to the ease that a magnetic flux can be established in a given magnetic circuit.) Because of the magnetism in the part and the adherence of the magnetic particles to each other, the indication remains on the surface of the part in the form of an approximate outline of the discontinuity that is immediately below it. The same action takes place when the discontinuity is not open to the surface, but since the amount of flux leakage is less, fewer particles are held in place and a fainter and less sharply defined indication is obtained.

If the discontinuity is very far below the surface, there may be no flux leakage and no indication on the surface. The flux leakage at a transverse discontinuity is shown in Figure 1. The flux leakage at a longitudinal discontinuity is shown in Figure 2.

Figure 1. Flux leakage at transverse discontinuity

Figure 2. Flux leakage at longitudinal discontinuity

Types of Discontinuities Disclosed

The following types of discontinuities are normally detected by the magnetic particle test: cracks, laps, seams, cold shuts, inclusions, splits, tears, pipes, and voids. All of these may affect the reliability of parts in service.

Cracks, splits, bursts, tears, seams, voids, and pipes are formed by an actual parting or rupture of the solid metal. Cold shuts and laps are folds that have been formed in the metal, interrupting its continuity.

Inclusions are foreign material formed by impurities in the metal during the metal processing stages. They may consist, for example, of bits of furnace lining picked up during the melting of the basic metal or of other foreign constituents.

Inclusions interrupt the continuity of the metal, because they prevent the joining or welding of adjacent faces of the metal.

Preparation of Parts for Testing

Grease, oil, and dirt must be cleaned from all parts before they are tested. Cleaning is very important since any grease or other foreign material present can produce nonrelevant indications due to magnetic particles adhering to the foreign material as the suspension drains from the part.

Grease or foreign material in sufficient amount over a discontinuity may also prevent the formation of a pattern at the discontinuity. It is not advisable to depend upon the magnetic particle suspension to clean the part. Cleaning by suspension is not thorough and any foreign materials so removed from the part contaminates the suspension, thereby reducing its effectiveness.

In the dry procedure, thorough cleaning is absolutely necessary. Grease or other foreign material holds the magnetic powder, resulting in nonrelevant indications and making it impossible to distribute the indicating medium evenly over the part’s surface. All small openings and oil holes leading to internal passages or cavities must be plugged with paraffin or other suitable nonabrasive material.

Coatings of cadmium, copper, tin, and zinc do not interfere with the satisfactory performance of magnetic particle inspection, unless the coatings are unusually heavy or the discontinuities to be detected are unusually small.

Chromium and nickel plating generally do not interfere with indications of cracks open to the surface of the base metal, but prevent indications of fine discontinuities, such as inclusions. Because it is more strongly magnetic, nickel plating is more effective than chromium plating in preventing the formation of indications.

Effect of Flux Direction

To locate a defect in a part, it is essential that the magnetic lines of force pass approximately perpendicular to the defect. It is, therefore, necessary to induce magnetic flux in more than one direction, since defects are likely to exist at any angle to the major axis of the part. This requires two separate magnetizing operations, referred to as circular magnetization and longitudinal magnetization. The effect of flux direction is illustrated in Figure 3.

Figure 3. Effect of flux direction on strength of indication

Circular magnetization is the induction of a magnetic field consisting of concentric circles of force about and within the part. This is achieved by passing electric current through the part, locating defects running approximately parallel to the axis of the part. Figure 4 illustrates circular magnetization of a crankshaft. In longitudinal magnetization, the magnetic field is produced in a direction parallel to the long axis of the part. This is accomplished by placing the part in a solenoid excited by electric current. The metal part then becomes the core of an electromagnet and is magnetized by induction from the magnetic field created in the solenoid.

Figure 4. Circular magnetization of a crankshaft

In longitudinal magnetization of long parts, the solenoid must be moved along the part in order to magnetize it. [Figure 5] This is necessary to ensure adequate field strength throughout the entire length of the part.

Figure 5. Longitudinal magnetization of camshaft (solenoid method)

Solenoids produce effective magnetization for approximately 12 inches from each end of the coil, thus accommodating parts or sections approximately 30 inches in length. Longitudinal magnetization equivalent to that obtained by a solenoid may be accomplished by wrapping a flexible electrical conductor around the part. Although this method is not as convenient, it has an advantage in that the coils conform more closely to the shape of the part, producing a somewhat more uniform magnetization. The flexible coil method is also useful for large or irregularly-shaped parts when standard solenoids are not available.

Effect of Flux Density

The effectiveness of the magnetic particle inspection also depends on the flux density or field strength at the surface of the part when the indicating medium is applied. As the flux density in the part is increased, the sensitivity of the test increases, because of the greater flux leakages at discontinuities and the resulting improved formation of magnetic particle patterns.

Excessively high flux densities may form nonrelevant indications, such as patterns of the grain flow in the material. These indications interfere with the detection of patterns resulting from significant discontinuities. It is therefore necessary to use a field strength high enough to reveal all possible harmful discontinuities, but not strong enough to produce confusing nonrelevant indications.

Magnetizing Methods

When a part is magnetized, the field strength in the part increases to a maximum for the particular magnetizing force and remains at this maximum as long as the magnetizing force is maintained.

When the magnetizing force is removed, the field strength decreases to a lower residual value depending on the magnetic properties of the material and the shape of the part. These magnetic characteristics determine whether the continuous or residual method is used in magnetizing the part.

In the continuous inspection method, the part is magnetized and the indicating medium applied while the magnetizing force is maintained. The available flux density in the part is thus at a maximum. The maximum value of flux depends directly upon the magnetizing force and the permeability of the material that the part is made of.

The continuous method may be used in practically all circular and longitudinal magnetization procedures. The continuous procedure provides greater sensitivity than the residual procedure, particularly in locating subsurface discontinuities. The highly critical nature of aircraft parts and assemblies and the necessity for subsurface inspection in many applications have resulted in the continuous method being more widely used. Since the continuous procedure reveals more nonsignificant discontinuities than the residual procedure, careful and intelligent interpretation and evaluation of discontinuities revealed by this procedure are necessary.

The residual inspection procedure involves magnetization of the part and application of the indicating medium after the magnetizing force has been removed. This procedure relies on the residual or permanent magnetism in the part and is more practical than the continuous procedure when magnetization is accomplished by flexible coils wrapped around the part. In general, the residual procedure is used only with steels that have been heat treated for stressed applications.

Identification of Indications

The correct evaluation of the character of indications is extremely important but is sometimes difficult to make from observation of the indications alone. The principal distinguishing features of indications are shape, buildup, width, and sharpness of outline. These characteristics are more valuable in distinguishing between types of discontinuities than in determining their severity. Careful observation of the character of the magnetic particle pattern must always be included in the complete evaluation of the significance of an indicated discontinuity.

The most readily distinguished indications are those produced by cracks open to the surface. These discontinuities include fatigue cracks, heat treat cracks, shrink cracks in welds and castings, and grinding cracks. An example of a fatigue crack is shown in Figure 6.

Figure 6. Fatigue crack on the bottom end fitting of a Hydrosorb shock absorber

Inspection of Aircraft Bonded Structures (Part 2) – Inspection Concepts and Techniques

Magnaglo Inspection

Magnaglo inspection is similar to the preceding method, but differs in that a fluorescent particle solution is used and the inspection is made under black light. [Figure 1] Efficiency of inspection is increased by the neon-like glow of defects allowing smaller flaw indications to be seen. This is an excellent method for use on gears, threaded parts, and aircraft engine components. The reddish-brown liquid spray or bath that is used consists of Magnaglo paste mixed with a light oil at the ratio of 0.10 to 0.25 ounce of paste per gallon of oil. After inspection, the part must be demagnetized and rinsed with a cleaning solvent.

Figure 1. Magnaglo inspection

Magnetizing Equipment

Fixed (Nonportable) General Purpose Unit A fixed, general purpose unit provides direct current (DC) for wet, continuous, or residual magnetization procedures. [Figure 2]

Figure 2. Fixed general-purpose magnetizing unit

Circular or longitudinal magnetization may be used, and it may be powered with rectified AC, as well as DC. The contact heads provide the electrical terminals for circular magnetization. One head is fixed in position with its contact plate mounted on a shaft surrounded by a pressure spring so that the plate may be moved longitudinally. The plate is maintained in the extended position by the spring until pressure transmitted through the work from the movable head forces it back.

The motor-driven movable head slides horizontally in longitudinal guides and is controlled by a switch. The spring allows sufficient overrun of the motor-driven head to avoid jamming it and also provides pressure on the ends of the work to ensure good electrical contact.

A plunger-operated switch in the fixed head cuts out the forward motion circuit of the movable head motor when the spring has been properly compressed. In some units, the movable head is hand operated, and the contact plate is sometimes arranged for operation by an air ram. Both contact plates are fitted with various fixtures for supporting the work.

The magnetizing circuit is closed by depressing a pushbutton on the front of the unit. It is set to open automatically, usually after about one-half second. The strength of the magnetizing current may be set manually to the desired value by means of the rheostat or increased to the capacity of the unit by the rheostat short circuiting switch. The current utilized is indicated on the ammeter. Longitudinal magnetization is produced by the solenoid that moves in the same guide rail as the movable head and is connected in the electrical circuit by means of a switch.

The suspension liquid is contained in a sump tank and is agitated and circulated by a pump. The suspension is applied to the work through a nozzle. The suspension drains from the work through a nonmetallic grill into a collecting pan that leads back to the sump. The circulating pump is operated by a pushbutton switch.

Portable General Purpose Unit

It is often necessary to perform the magnetic particle inspection at locations where fixed general purpose equipment is not available or to perform an inspection on members of aircraft structures without removing them from the aircraft. It is particularly useful for inspecting landing gear and engine mounts suspected of having developed cracks in service. Portable units supply both AC and DC magnetization.

This unit is a source of magnetizing and demagnetizing current but does not provide a means for supporting the work or applying the suspension. It operates on 200 volt, 60 cycle AC and contains a rectifier for producing DC when required.[Figure 3]

Figure 3. Portable magnetic particle inspection equipment

The magnetizing current is supplied through the flexible cables with prods or contact clamps, as shown in Figure 4. The cable terminals may be fitted with prods or with contact clamps. Circular magnetization may be developed by using either the prods or clamps.

Figure 4. Magnetic particle inspection accessories

Longitudinal magnetization is developed by wrapping the cable around the part. The strength of the magnetizing current is controlled by an eight-point tap switch, and the duration that it is applied is regulated by an automatic cutoff similar to that used in the fixed general purpose unit.

This portable unit also serves as a demagnetizer and supplies high amperage, low-voltage AC for this purpose. For demagnetization, the AC is passed through the part and gradually reduced by means of a current reducer.

In testing large structures with flat surfaces where current must be passed through the part, it is sometimes impossible to use contact clamps. In such cases, contact prods are used.

Prods can be used with the fixed general purpose unit, as well as the portable unit. The part or assembly being tested may be held or secured above the standard unit and the suspension hosed onto the area, while excess suspension drains into the tank. The dry procedure may also be used.

Prods are held firmly against the surface being tested. There is a tendency for a high-amperage current to cause burning at contact areas, but with proper care, such burning is usually slight. For applications where prod magnetization is acceptable, slight burning is normally acceptable.

Indicating Mediums

The various types of indicating mediums available for magnetic particle inspection may be divided into two general material types: wet and dry. The basic requirement for any indicating medium is that it produce acceptable indications of discontinuities in parts.

The contrast provided by a particular indicating medium on the background or part surface is particularly important. The colors most extensively used are black and red for the wet procedure and black, red, and gray for the dry procedure.

For acceptable operation, the indicating medium must be of high permeability and low retentivity. High permeability ensures that a minimum of magnetic energy is required to attract the material to flux leakage caused by discontinuities. Low retentivity ensures that the mobility of the magnetic particles is not hindered by the particles themselves becoming magnetized and attracting one another.

Demagnetizing

The permanent magnetism remaining after inspection must be removed by a demagnetization operation if the part is to be returned to service. Parts of operating mechanisms must be demagnetized to prevent magnetized parts from attracting filings, grindings, or chips inadvertently left in the system or steel particles resulting from operational wear. An accumulation of such particles on a magnetized part may cause scoring of bearings or other working parts. Parts of the airframe must be demagnetized so they do not affect instruments.

Demagnetization between successive magnetizing operations is not normally required unless experience indicates that omission of this operation results in decreased effectiveness for a particular application. Demagnetization may be accomplished in a number of different ways. A convenient procedure for aircraft parts involves subjecting the part to a magnetizing force that is continually reversing in direction and, at the same time, gradually decreasing in strength. As the decreasing magnetizing force is applied first in one direction and then the other, the magnetization of the part also decreases.

Standard Demagnetizing Practice

The basic procedure for developing a reversing and gradually decreasing magnetizing force in a part involves the use of a solenoid coil energized by AC. As the part is moved away from the alternating field of the solenoid, the magnetism in the part gradually decreases.

A demagnetizer whose size approximates that of the work is used. For maximum effectiveness, small parts are held as close to the inner wall of the coil as possible. Parts that do not readily lose their magnetism are passed slowly in and out of the demagnetizer several times and, at the same time, tumbled or rotated in various directions. Allowing a part to remain in the demagnetizer with the current on accomplishes very little practical demagnetization.

The effective operation in the demagnetizing procedure is that of slowly moving the part out of the coil and away from the magnetizing field strength. As the part is withdrawn, it is kept directly opposite the opening until it is 1 or 2 feet from the demagnetizer. The demagnetizing current is not cut off until the part is 1 or 2 feet from the opening as the part may be remagnetized if current is removed too soon. Another procedure used with portable units is to pass AC through the part being demagnetized, while gradually reducing the current to zero.

Radiographic

Because of their unique ability to penetrate material and disclose discontinuities, X and gamma radiations have been applied to the radiographic (x-ray) inspection of metal fabrications and nonmetallic products.

The penetrating radiation is projected through the part to be inspected and produces an invisible or latent image in the film. When processed, the film becomes a radiograph or shadow picture of the object. This inspection medium and portable unit provides a fast and reliable means for checking the integrity of airframe structures and engines. [Figure 5]

Figure 5. Radiograph

Radiographic Inspection

Radiographic inspection techniques are used to locate defects or flaws in airframe structures or engines with little or no disassembly. This is in marked contrast to other types of nondestructive testing that usually require removal, disassembly, and stripping of paint from the suspected part before it can be inspected. Due to the radiation risks associated with x-ray, extensive training is required to become a qualified radiographer. Only qualified radiographers are allowed to operate the x-ray units.

Three major steps in the x-ray process discussed in subsequent paragraphs are: exposure to radiation, including preparation; processing of film; and interpretation of the radiograph.

Preparation and Exposure

The factors of radiographic exposure are so interdependent that it is necessary to consider all factors for any particular radiographic exposure. These factors include, but are not limited to, the following:

• Material thickness and density

• Shape and size of the object

• Type of defect to be detected

• Characteristics of x-ray machine used

• The exposure distance

• The exposure angle

• Film characteristics

• Types of intensifying screen, if used

Knowledge of the x-ray unit’s capabilities form a background for the other exposure factors. In addition to the unit rating in kilovoltage, the size, portability, ease of manipulation, and exposure particulars of the available equipment must be thoroughly understood. Previous experience on similar objects is also very helpful in the determination of the overall exposure techniques. A log or record of previous exposures provides specific data as a guide for future radiographs. After exposure to x-rays, the latent image on the film is made permanently visible by processing it successively through a developer chemical solution, an acid bath, and a fixing bath, followed by a clear water wash.

Radiographic Interpretation

From the standpoint of quality assurance, radiographic interpretation is the most important phase of radiography. It is during this phase that an error in judgment can produce disastrous consequences. The efforts of the whole radiographic process are centered in this phase, where the part or structure is either accepted or rejected. Conditions of unsoundness or other defects that are overlooked, not understood, or improperly interpreted can destroy the purpose and efforts of radiography and can jeopardize the structural integrity of an entire aircraft. A particular danger is the false sense of security imparted by the acceptance of a part or structure based on improper interpretation.

As a first impression, radiographic interpretation may seem simple, but a closer analysis of the problem soon dispels this impression. The subject of interpretation is so varied and complex that it cannot be covered adequately in this type of document. Instead, this post gives only a brief review of basic requirements for radiographic interpretation, including some descriptions of common defects.

Experience has shown that, whenever possible, it is important to conduct radiographic interpretation close to the radiographic operation. When viewing radiographs, it is helpful to have access to the material being tested. The radiograph can thus be compared directly with the material being tested, and indications due to such things as surface condition or thickness variations can be immediately determined. The following paragraphs present several factors that must be considered when analyzing a radiograph.

There are three basic categories of flaws: voids, inclusions, and dimensional irregularities. The last category, dimensional irregularities, is not pertinent to these discussions, because its prime factor is one of degree and radiography is not exact. Voids and inclusions may appear on the radiograph in a variety of forms ranging from a two-dimensional plane to a three-dimensional sphere. A crack, tear, or cold shut most nearly resembles a two-dimensional plane, whereas a cavity looks like a three-dimensional sphere. Other types of flaws, such as shrink, oxide inclusions, porosity, and so forth, fall somewhere between these two extremes of form.

It is important to analyze the geometry of a flaw, especially for items such as the sharpness of terminal points. For example, in a crack-like flaw, the terminal points appear much sharper in a sphere-like flaw, such as a gas cavity. Also, material strength may be adversely affected by flaw shape. A flaw having sharp points could establish a source of localized stress concentration. Spherical flaws affect material strength to a far lesser degree than do sharp-pointed flaws. Specifications and reference standards usually stipulate that sharp-pointed flaws, such as cracks, cold shuts, and so forth, are cause for rejection.

Material strength is also affected by flaw size. A metallic component of a given area is designed to carry a certain load plus a safety factor. Reducing this area by including a large flaw weakens the part and reduces the safety factor. Some flaws are often permitted in components due to these safety factors. In this case, the interpreter must determine the degree of tolerance or imperfection specified by the design engineer. Both flaw size and flaw shape are considered carefully, since small flaws with sharp points can be just as bad as large flaws with no sharp points.

Another important consideration in flaw analysis is flaw location. Metallic components are subjected to numerous and varied forces during their effective service life. Generally, the distribution of these forces is not equal in the component or part, and certain critical areas may be rather highly stressed. The interpreter must pay special attention to these areas. Another aspect of flaw location is that certain types of discontinuities close to one another may potentially serve as a source of stress concentrations creating a situation that must be closely scrutinized.

An inclusion is a type of flaw that contains entrapped material. Such flaws may be of greater or lesser density than the item being radiographed. The foregoing discussions on flaw shape, size, and location apply equally to inclusions and to voids. In addition, a flaw containing foreign material could become a source of corrosion.

Radiation Hazards

Radiation from x-ray units and radioisotope sources is destructive to living tissue. It is universally recognized that in the use of such equipment, adequate protection must be provided. Personnel must keep outside the primary x-ray beam at all times.

Radiation produces change in all matter that it passes through. This is also true of living tissue. When radiation strikes the molecules of the body, the effect may be no more than to dislodge a few electrons, but an excess of these changes could cause irreparable harm. When a complex organism is exposed to radiation, the degree of damage, if any, depends on the body cells that have been changed.

Vital organs in the center of the body that are penetrated by radiation are likely to be harmed the most. The skin usually absorbs most of the radiation and reacts earliest to radiation.

If the whole body is exposed to a very large dose of radiation, death could result. In general, the type and severity of the pathological effects of radiation depend on the amount of radiation received at one time and the percentage of the total body exposed. Smaller doses of radiation could cause blood and intestinal disorders in a short period of time. The more delayed effects are leukemia and other cancers. Skin damage and loss of hair are also possible results of exposure to radiation.