Instruments mechanically measure physical quantities or properties with varying degrees of accuracy. Much of a navigator’s work consists of applying corrections to the indications of various instruments and interpreting the results. Therefore, navigators must be familiar with the capabilities and limitations of the instruments available to them.
A navigator obtains the following flight information from basic instruments: direction, altitude, temperature, airspeed, drift, and groundspeed (GS). Some of the basic instruments are discussed in this category. The more complex instruments that make accurate and long distance navigation possible are discussed in later categories.
Direction
Basic Instruments
The navigator must have a fundamental background in navigation to ensure accurate positioning of the aircraft. Dead reckoning (DR) procedures aided by basic instruments give the navigator the tools to solve the three basic problems of navigation: position of the aircraft, direction to destination, and time of arrival. Using only a basic instrument, such as the compass and drift information, you can navigate directly to any place in the world. Various fixing aids, such as celestial and radar, can greatly improve the accuracy of basic DR procedures. This chapter discusses the basic instruments used for DR and then reviews the mechanics of DR, plotting, wind effect, and computer solutions.
Directional information needed to navigate is obtained by use of the earth’s magnetic lines of force. A compass system uses a device that detects and converts the energy from these lines of force to an indicator reading. The magnetic compass operates independently of the aircraft electrical systems. Later developed compass systems require electrical power to convert these lines of force to an aircraft heading.
Earth’s Magnetic Field
The earth has some of the properties of a bar magnet; however, its magnetic poles are not located at the geographic poles, nor are the two magnetic poles located exactly opposite each other as on a straight bar. The north magnetic pole is located approximately at 73° N latitude and 100° W longitude on Prince of Wales Island. The south magnetic pole is located at 68° S latitude and 144° E longitude on Antarctica.
The earth’s magnetic poles, like those of any magnet, can be considered to be connected by a number of lines of force. These lines result from the magnetic field that envelops the earth. They are considered to be emanating from the south magnetic pole and terminating at the north magnetic pole. [Figure 3-1]
Figure 3-1. Earth’s magnetic field.
The force of the magnetic field of the earth can be divided into two components: the vertical and the horizontal. The relative intensity of these two components varies over the earth so that, at the magnetic poles, the vertical component is at maximum strength and the horizontal component is minimum strength. At approximately the midpoint between the poles, the horizontal component is at maximum strength and the vertical component is at minimum strength. Only the horizontal component is used as a directive force for a magnetic compass. Therefore, a magnetic compass loses its usefulness in an area of weak horizontal force, such as the area around the magnetic poles. The vertical component causes the end of the needle nearer to the magnetic pole to tip as the pole is approached. [Figure 3-1] This departure from the horizontal is called magnetic dip.
Compasses (Part One)
A compass may be defined as an instrument that indicates direction over the earth’s surface with reference to a known datum. Various types of compasses have been developed, each of which is distinguished by the particular datum used as the reference from which direction is measured. Two basic types of compasses are in current use: the magnetic and gyrocompass.
The magnetic compass uses the lines of force of the earth’s magnetic field as a primary reference. Even though the earth’s field is usually distorted by the pressure of other local magnetic fields, it is the most widely used directional reference. The gyrocompass uses as its datum an arbitrary fixed point in space determined by the initial alignment of the gyroscope axis. Compasses of this type are widely used today and may eventually replace the magnetic compass entirely.
Magnetic Compass
The magnetic compass indicates direction in the horizontal plane with reference to the horizontal component of the earth’s magnetic field. This field is made up of the earth’s field in combination with other magnetic fields in the vicinity of the compass. These secondary fields are caused by the presence of ferromagnetic objects.
Magnetic compasses may be divided into two classes:
- The direct-indicating magnetic compass in which the measurement of direction is made by a direct observation of the position of a pivoted magnetic needle; and
- The remote-indicating gyro-stabilized magnetic compass.
Magnetic direction is sensed by an element located at positions where local magnetic fields are at a minimum, such as the vertical stabilizer and wing tips. The direction is then transmitted electrically to repeater indicators on the instrument panels.
Direct-Indicating Magnetic Compass
Basically, the magnetic compass is a magnetized rod pivoted at its middle, with several features incorporated to improve its performance. One type of direct-indicating magnetic compass, the B-16 compass (often called the whiskey compass), is illustrated in Figure 3-2. It is used as a standby compass in case of failure of the electrical system that operates the remote compasses. It is a reliable compass and gives good navigational results if used carefully.
Figure 3-2. Magnetic compass. [click image to enlarge]Magnetic Variation and Compass Errors
The earth’s magnetic poles are joined by irregular curves called magnetic meridians. The angle between the magnetic meridian and the geographic meridian is called the magnetic variation. Variation is listed on charts as east or west. When variation is east, magnetic north (MN) is east of true north (TN). Similarly, when variation is west, MN is west of TN. [Figure 3-3] Lines connecting points having the same magnetic variation are called isogonic lines. [Figure 3-4] Compensate for magnetic variation to convert a compass direction to true direction.
Figure 3-3. Effects of variation.
Compass error is caused by nearby magnetic influences, such as magnetic material in the structure of the aircraft and its electrical systems. These magnetic forces deflect a compass needle from its normal alignment. The amount of such deflection is called deviation which, like variation, is labeled “east” or “west” as the north-seeking end of the compass is deflected east or west of MN, respectively.
Figure 3-4. Isogonic lines show same magnetic variation. [click image to enlarge]The correction for variation and deviation is usually expressed as a plus or minus value and is computed as a correction to true heading (TH). If variation or deviation is east, the sign of the correction is minus; if west, the sign is plus. A rule of thumb for this correction is easily remembered as east is least and west is best.
Aircraft headings are expressed as TH or magnetic headings (MH). If the heading is measured in relation to geographical north, it is a TH. If the heading is in reference to MN, it is a MH; if it is in reference to the compass lubber line, it is a compass heading (CH). CH corrected for variation and deviation is TH. MH corrected for variation is TH.
Figure 3-5. Find true heading by working backwards.
This relationship is best expressed by reference to the navigator’s log, where the various headings and corrections are listed as TH, variation (var), MH, deviation (dev), and CH. [Figure 3-5] Thus, if an aircraft is flying in an area where the variation is 10° E and the compass has a deviation of 3° E, the relationship would be expressed as follows, assuming a CH of 125°:
TH var MH dev CH
138 – 10 = 128 – 3 = 125
Variation
Variation has been measured throughout the world and the values found have been plotted on charts. Isogonic lines are printed on most charts used in aerial navigation so that, if the aircraft’s approximate position is known, the amount of variation can be determined by visual interpolation between the printed lines. At high altitudes, these values can be considered quite realistic. Conversely, at low altitudes, these magnetic values are less reliable because of local anomalies.
Variation changes slowly over a period of years and the yearly amount of such change is printed on most charts. Variation is also subject to small diurnal (daily) changes that may generally be neglected in air navigation.
Deviation
Because deviation depends upon the distribution of magnetic forces in the aircraft itself, it must be obtained individually for each magnetic compass on each aircraft. The process of determining deviation, known as compass swinging, should be discussed in the technical order for each compass.
Deviation changes with heading are shown in Figure 3-6. The net result of all magnetic forces of the aircraft (those forces excluding the earth’s field) is represented by a dot located just behind the wings of the aircraft. If the aircraft is headed toward MN, the dot attracts one pole of the magnetic compass (in this case, the South Pole) but, on this heading, does not change its direction. The only effect is to amplify the directive force of the earth’s field. If the aircraft heads toward magnetic east, the dot is now west of the compass, and attracts the South Pole of the compass, causing easterly deviation. Figure 3-6 also shows that the deviation is zero on a south heading, and westerly when the aircraft is heading west.
Figure 3-6. Deviation changes with heading.
Deviation can be reduced (but not eliminated) in some direct-indicating magnetic compasses by adjusting the small compensating magnets in the compass case. Remaining deviation is referred to as residual deviation and can be determined by comparison with true values. This residual deviation is recorded on a compass correction card showing actual deviation on various headings or the compass headings. From the compass correction card illustrated in Figure 3-7, the navigator knows that to fly a magnetic heading (MH) of 270°, the pilot must steer a CH of 268°.
Figure 3-7. Compass correction card.
Errors in Flight
Unfortunately, deviation is not the only error of a magnetic compass. Additional errors are introduced by the motion of the aircraft itself. These errors have minimal effect on the use of magnetic compasses and come into play normally during turns or changes in speed. They are mentioned only to bring awareness of the limitations of the basic compass. Although a basic magnetic compass has some shortcomings, it is simple and reliable. The compass is very useful to both the pilot and navigator and is carried on all aircraft as an auxiliary compass. Because compass systems are dependent upon the electrical system of the aircraft, a loss of power means a loss of the compass system. For this reason, an occasional check on the standby compass provides an excellent backup to the main systems.
Remote-Indicating Gyro-Stabilized Magnetic Compass System
A chief disadvantage of the simple magnetic compass is its susceptibility to deviation. In remote-indicating gyrostabilized compass systems, this difficulty is overcome by locating the compass direction-sensing device outside magnetic fields created by electrical circuits in the aircraft. This is done by installing the direction-sensing device in a remote part of the aircraft, such as the outer extremity of a wing or vertical stabilizer. Indicators of the compass system can then be located throughout the aircraft without regard to magnetic disturbances.
Several kinds of compass system are used in aircraft systems. All include the following five basic components: remote compass transmitter, directional gyro (DG), amplifier, heading indicators, and slaving control. Though the names of these components vary among systems, the principle of operation is identical for each. Thus, the N-1 compass system shown in Figure 3-8 can be considered typical of all such systems.
Figure 3-8. N-1 compass system components. [click image to enlarge]The N-1 compass system is designed for airborne use at all latitudes. It can be used either as a magnetic-slaved compass or as a DG. In addition, the N-1 generates an electric signal that is used as an azimuth reference by the autopilot, the radar system, the navigation and bombing computers, and various compass cards.
Compasses (Part Two)
Remote Compass Transmitter
The remote compass transmitter is the magnetic-direction sensing component of the compass system when the system is in operation as a magnetic-slaved compass. The transmitter is located as far from magnetic disturbances of the aircraft as possible, usually in a wing tip or the vertical stabilizer. The transmitter senses the horizontal component of the earth’s magnetic field and electrically transmits it to the master indicator. The compensator, an auxiliary unit of the remote compass transmitter, is used to eliminate most of the magnetic deviation caused by the aircraft electrical equipment and ferrous metal when a deviation-free location for the remote compass transmitter is not available.
Directional Gyro (DG)
The DG is the stabilizing component of the compass system when the system is in magnetic-slaved operation. When the compass system is in DG operation, the gyro acts as the directional reference component of the system.
Amplifier
The amplifier is the receiving and distributing center of the compass system. Azimuth correction and leveling signals originating in the components of the system are each received, amplified, and transmitted by separate channels in the amplifier. Primary power to operate the compass is fed to the amplifier and distributed to the systems components.
Master Indicator
The master indicator is the heading-indicating component of the compass system. The mechanism in the master indicator integrates all data received from the directional gyro and the remote compass transmitter, corrects the master indicator heading pointer for azimuth drift of the DG due to the earth’s rotation, and provides takeoff signals for operating remote indicators, radar, navigation computers, and directional control of the autopilot.
The latitude correction control provides a means for selecting either magnetic-slaved operation or DG operation of the compass system, as well as the proper latitude correction rate. The latitude correction pointer is mechanically connected to the latitude correction control knob and indicates the latitude setting on the latitude correction scale at the center of the master indicator dial face.
The synchronizer control knob at the lower right-hand corner of the master indicator face provides a means of synchronizing the master indicator heading pointer with the correct MH when the system is in magnetic-slaved operation. It also provides a means of setting the master indicator heading pointer on the desired gyro heading reference when the system is in DG operation. The annunciator pointer indicates the direction in which to rotate the synchronizer control knob to align the heading pointer with the correct MH.
Gyro-Magnetic Compass Indicators
The gyro-magnetic compass indicators are remote-reading, movable dial compass indicators. They are intended for supplementary use as directional compass indicators when used with the compass system. The indicators duplicate the azimuth heading of the master indicator heading pointer. A setting knob is provided at the front of each indicator for rotating the dial 360° in either direction without changing the physical alignment of the pointer.
Slaving Control
The slaving control is a gyro control rate switch that reduces errors in the compass system during turns. When the aircraft turns at a rate of 23° or more per minute, the slaving control prevents the remote compass transmitter signal from being transmitted to the compass system during magnetic-slaved operation. It also interrupts leveling action in the DG when the system is in magnetic-slaved or DG operation.
Gyro Basics
Any spinning body exhibits gyroscopic properties. A wheel designed and mounted to use these properties is called a gyroscope or gyro. Basically, a gyro is a rapidly rotating mass that is free to move about one or both axes perpendicular to the axis of rotation and to each other. The three axes of a gyro (spin, drift, and topple) shown in Figure 3-9 are defined as follows:
- In a DG, the spin axis or axis of rotation is mounted horizontally;
- The topple axis is that axis in the horizontal plane that is 90° from the spin axis;
- The drift axis is that axis 90° vertically from the spin axis.
Figure 3-9. Gyroscope axes.
Gyroscopic drift is the horizontal rotation of the spin axis about the drift axis. Topple is the vertical rotating of the spin axis about the topple axis. These two component drifts result in motion of the gyro called precession.
A freely spinning gyro tends to maintain its axis in a constant direction in space, a property known as rigidity in space or gyroscopic inertia. Thus, if the spin axis of a gyro were pointed toward a star, it would keep pointing at the star. Actually, the gyro does not move, but the earth moving beneath it gives it an apparent motion. This apparent motion is called apparent precession. [Figure 3-10] The magnitude of apparent precession is dependent upon latitude. The horizontal component, drift, is equal to 15° per hour times the sine of the latitude, and the vertical component, topple, is equal to 15° per hour times the cosine of the latitude.
Figure 3-10. Apparent precession.
These computations assume the gyro is stationary with respect to the earth. If the gyro is to be used in a high-speed aircraft, however, it is readily apparent that its speed with respect to a point in space may be more or less than the speed of rotation of the earth. If the aircraft in which the gyro is mounted is moving in the same direction as the earth, the speed of the gyro with respect to space is greater than the earth’s speed. The opposite is true if the aircraft is flying in a direction opposite to that of the earth’s rotation. This difference in the magnitude of apparent precession caused by transporting the gyro over the earth is called transport precession.
A gyro may precess because of factors other than the earth’s rotation. When this occurs, the precession is labeled real precession. When a force is applied to the plane of rotation of a gyro, the plane tends to rotate, not in the direction of the applied force, but 90° around the spin axis from it. This torquing action, shown in Figure 3-11, may be used to control the gyro by bringing about a desired reorientation of the spin axis, and most DGs are equipped with some sort of device to introduce this force. However, friction within the bearings of a gyro may have the same effect and cause a certain amount of unwanted precession. Great care is taken in the manufacture and maintenance of gyroscopes to eliminate this factor as much as possible, but, as yet, it has not been possible to eliminate it entirely. Precession caused by the mechanical limitations of the gyro is called real or induced precession. The combined effects of apparent precession, transport precession, and real precession produce the total precession of the gyro. The properties of the gyro that most concern the navigator are rigidity and precession. By understanding these two properties, the navigator is well equipped to use the gyro as a reliable steering guide.
Figure 3-11. By applying an upward pressure on the gyro spin axis, a deflective force is applied to the rim of the gyro at point A (plane of force). The resultant force is 90° ahead in the direction of rotation to point B (plane of rotation), which causes the gyro to precess (plane of precession). [click image to enlarge]
Directional Gyro (DG)
The discussion thus far has been of a universally mounted gyro, free to turn in the horizontal or vertical or any component of these two. This type of gyro is seldom, if ever, used as a DG. When the gyro is used as a steering instrument, it is restricted so that the spin axis remains parallel to the surface of the earth. Thus, the spin axis is free to turn only in the horizontal plane (assuming the aircraft normally flies in a near-level attitude), and only the horizontal component (drift) affects a steering gyro. In the terminology of gyro steering, precession always means the horizontal component of precession.
The operation of the instrument depends upon the principle of rigidity in space of the gyroscope. Fixed to the plane of the spin axis is a circular compass card, similar to that of the magnetic compass. Since the spin axis remains rigid in space, the points on the card hold the same position in space relative to the horizontal plane. The case, to which the lubber line is attached, simply revolves about the card.
It is important at this point to understand that the numbers on the compass card have no meaning within themselves, as on the magnetic compass. The fact that the gyro may indicate 100° under the lubber line is not an indication that the instrument is actually oriented to magnetic north (MN), or any other known point. To steer by the gyro, the navigator must first set it to a known direction or point. Usually, this is MN or geographic north, though it can be at any known point. If, for example, MN is set as the reference, all headings on the gyro read relative to the position of the magnetic poles.
The actual setting of the initial reference heading is done by using the principle discussed earlier of torque application to the spinning gyro. By artificially introducing precession, the navigator can set the gyro to whatever heading is desired and can reset it at any time, by using the same technique.
Gyrocompass Errors
The major error affecting the gyro and its use as a steering instrument is precession. Apparent precession causes an apparent change of heading equal to 15° per hour times the sine of the latitude. Real precession, caused by defects in the gyro, may occur at any rate, but is typically very small in current gyros. Apparent precession is a known value depending upon location and can be compensated for. In some of the more complex gyro systems, apparent precession is compensated for by setting in a constant correction equal to, and in the opposite direction to, the precession caused by the earth’s rotation.
Altitude and Altimeters
Altitude may be defined as a vertical distance above some point or plane used as a reference. Knowledge of the aircraft altitude is imperative for terrain clearance, aircraft separation, and a multitude of operational reasons. There are as many kinds of altitude as there are reference planes from which to measure them. Only six concern the navigator: indicated altitude, calibrated altitude, pressure altitude (PA), density altitude (DA), true altitude (TA), and absolute altitude. There are two main types of altimeters: the pressure altimeter, which is installed in every aircraft, and the absolute or radar altimeter. To understand the pressure altimeter’s principle of operation, a knowledge of the standard datum plane is essential.
Standard Datum Plane
The standard datum plane is a theoretical plane where the atmospheric pressure is 29.92 inches of mercury (“Hg) and the temperature is 15 °C. The standard datum plane is the zero elevation level of an imaginary atmosphere known as the standard atmosphere. In the standard atmosphere, pressure is 29.92 “Hg at 0 feet and decreases upward at the standard pressure lapse rate. The temperature is 15 °C at 0 feet and decreases at the standard temperature lapse rate. Both the pressure and temperature lapse rates are given in Figure 3-12. The standard atmosphere is theoretical. It was derived by averaging the readings taken over a period of many years. The list of altitudes and their corresponding values of temperature and pressure given in the table were determined by these averages. The height of the aircraft above the standard datum plane (29.92 “Hg and 15 °C) is the PA. [Figure 3-13]
Figure 3-12. Standard lapse rate table.Figure 3-13. Depiction of altimetry terms. [click image to enlarge]
Pressure Altimeter Principles of Operation
The pressure altimeter is an aneroid barometer calibrated to indicate feet of altitude instead of pressure. As shown in Figure 3-14, the pointers are connected by a mechanical linkage to a set of aneroid cells. These aneroid cells expand or contract with changes in barometric pressure. In this manner, the cells assume a particular thickness at a given pressure level and, thereby, position the altitude pointers accordingly. On the face of the altimeter is a barometric scale that indicates the barometric pressure (expressed in inches of mercury) of the point or plane from which the instrument is measuring altitude. Turning the barometric pressure set knob on the altimeter manually changes this altimeter setting on the barometric scale and results in simultaneous movement of the altitude pointers to the corresponding altitude reading. Like all measurements, an altitude reading is meaningless if the point from which it starts is unknown. The face of the pressure altimeter supplies both values. The position of the pointers indicates the altitude in feet, and the barometric pressure appearing on the barometric scale is that of the reference plane above which the measurement is made.
Figure 3-14. Altimeter mechanical linkage.
Altimeter Displays
Counter-Pointer Altimeter
The counter-pointer altimeter has a two-counter digital display unit located in the 9 o’clock position of the dial. The counter indicates altitude in 1,000 foot increments from zero to 80,000 feet. [Figure 3-15] A single conventional pointer indicates l00s of feet on the fixed circular scale. It makes one complete revolution per 1,000 feet of altitude change and, as it passes through the 900- to 1,000-foot area of the dial, the 1,000-foot counter is actuated. The shaft of the 1,000- foot counter in turn actuates the 10,000-foot counter at each 10,000-feet of altitude change. To determine the indicated altitude, first read the 1,000-foot counter and then add the 100-foot pointer indication.
Figure 3-15. Counter-pointer.
It is possible to misinterpret the counter-pointer altimeter by 1,000 feet immediately before or after the 1,000-foot counter moves. This error is possible because the 1,000-foot counter changes when the 100-foot pointer is between the 900- and 1,000-foot position.
Counter-Drum-Pointer Altimeter
Aside from the familiar circular scale and 100-foot pointer, the counter-drum-pointer presentation differs somewhat in appearance from the present three-pointer altimeter. Starting at the left of the instrument illustrated in Figure 3-16 and reading from left to right, there are two counter windows and one drum window (white). The numerals presented in the counter windows indicate 10,000s and 1,000s of feet, respectively. The drum window numbers always follow the pointer number, thereby indicating 100s of feet.
Figure 3-16. Counter-drum pointer altimeter.
Two methods may be used to read indicated pressure altitude on the counter-drum-pointer altimeter:
- Read the counter-drum window, without referring to the 100-foot pointer, as a direct digital readout of both thousands and hundreds of feet;
- Read the two counter indications, without referring to the drum, and then add the 100-foot pointer indication. The 100 foot pointer serves as a precise readout of values less than 100 feet.
The differential air pressure that is used to operate the counter-drum-pointer altimeter is processed by an altitude transducer where it is converted to electrical signals that drive the indicator. The transducer is also used to send digital signals to a transponder for purposes of automatic altitude reporting to Air Route Traffic Control Centers (ARTCC). A standby system is available for use if an electrical malfunction occurs. In the standby system, the altimeter receives static air pressure directly from the pitot-static system. When the instrument is operating in the standby system, the word STANDBY appears on the instrument face. A switch in the upper right-hand corner of the instrument is provided to return the instrument to its normal mode of operation. This switch may also be used to manually place the instrument in the STANDBY mode.
Altimeter Errors
The pressure altimeter is subject to certain errors that fall in five general categories.
Mechanical Error
A mechanical error is caused by misalignment of gears and levers that transmit the aneroid cell expansion and contraction to the pointers of the altimeter. This error is not constant and must be checked before each flight by the setting procedure.
Scale Error
A scale error is caused by irregular expansion of the aneroid cells and is recorded on a scale correction card maintained for each altimeter in the instrument maintenance shop.
Installation or Position Error
An installation, or position, error is caused by the airflow around the static ports. This error varies with the type of aircraft, airspeed, and altitude. The magnitude and direction of this error can be determined by referring to the performance data section in the aircraft technical order. An altimeter correction card is installed in some aircraft that combines the installation or position and scale errors. The card indicates the amount of correction required at different altitudes and airspeeds. Installation, or position, error may be considerable at high speeds and altitudes. Apply the corrections as outlined in the technical order or on the altimeter correction card.
Reversal Error
A reversal error is caused by inducing false static pressure in the static system. It normally occurs during abrupt or large pitch changes. This error appears on the altimeter as a momentary indication in the opposite direction.
Hysteresis Error
A hysteresis error is a lag in altitude indication caused by the elastic properties of the material within the altimeter. This occurs after an aircraft has maintained a constant altitude for an extended period of time and then makes a large, rapid altitude change. After a rapid descent, altimeter indications are higher than actual. This error is negligible during climbs and descents at slow rate or after maintaining a new altitude for a short period of time.
Setting the Altimeter
The barometric scale is used to set a reference plane into the altimeter. Rotating the barometric pressure set knob increases or decreases the scale reading and the indicated altitude. Each .01 change on the barometric scale is equal to 10 feet of altitude. The majority of altimeters have mechanical stops at or just beyond the barometric scale limits (28.10 to 31.00). Attempting to adjust outside this range may cause damage to the instrument. Altimeters not equipped with mechanical stops near the barometric scale limits can be set with a 10,000 foot error. Therefore, when setting the altimeter, ensure the 10,000 foot pointer is reading correctly. Check the altimeter for accuracy before every flight.
To check and set the altimeter:
- Set the current altimeter setting on the barometric scale.
- Check the altimeter at a known elevation and note the error in feet.
- Set the reported altimeter setting on the barometric scale and compare the indicated altitude to the elevation of a known cockpit.
Nonstandard Atmospheric Effects
The altimeter setting is a correction for nonstandard surface pressure only. Atmospheric pressure is measured at each station and the value obtained is corrected to sea level according to the surveyed field elevation. Thus, the altimeter setting is the computed sea level pressure and should be considered valid only in close proximity to the station and the surface. It does not reflect nonstandard temperature nor distortion of atmospheric pressure at higher altitudes.
Types of Altitude
Indicated Altitude
Indicated altitude is the value of altitude that is displayed on the pressure altimeter.
Calibrated Altitude
Calibrated altitude is indicated altitude corrected for installation or position error.
Pressure Altitude (PA)
The height above the standard datum plane (29.92 “Hg and 15 °C) is PA. [Figure 3-13]
Figure 3-13. Depiction of altimetry terms. [click image to enlarge]Density Altitude (DA)
Density is mass per unit volume. The density of the air varies with temperature and with height. Warm air expands and is less dense than cold air. Normally, the higher the PA, the less dense the air becomes. The density of the air can be expressed in terms of the standard atmosphere. DA is the PA corrected for temperature in the density altitude window of the DR computer. This calculation converts the density of the air to the standard atmospheric altitude having the same density. DA is used in performance data and true airspeed (TAS) calculations.
True Altitude (TA)
TA is the actual vertical distance above mean sea level (MSL), measured in feet. It can be determined by two methods:
- Set the local altimeter setting on the barometric scale of the pressure altimeter to obtain the indicated true altitude (ITA). The ITA can then be resolved to TA by use of the DR computer. [Figure 3-17]
- Measure altitude over water with an absolute altimeter.
Figure 3-17. Finding true altitude.
The height above the terrain is called absolute altitude. It is computed by subtracting terrain elevation from TA, or it can be read directly from a radar altimeter.
The two altitudes most commonly accomplished on the computer are TA and DA. Nearly all DR computers have a window by which DA can be determined; however, be certain that the window is labeled density altitude.
True Altitude (TA) Determination
In the space marked FOR ALTITUDE COMPUTATIONS are two scales: a centigrade scale in the window and a PA scale on the upper disk. When a PA is placed opposite the temperature at that height, all values on the outer (miles) scale are equal to the corresponding values on the inner (minutes) scale, increased or decreased by 2 percent for each 5.5 °C that the actual temperature differs from the standard temperature at that PA, as set in the window. Although the PA is set in the window, the ITA is used on the inner (minutes) scale for finding the TA, corrected for difference in temperature lapse rate.
PA = 8,500 feet
ITA = 8,000 feet
Air Temperature = –16 °C
Place PA (8,500 feet) opposite the temperature (–16) on the FOR ALTITUDE COMPUTATIONS scale. Opposite the ITA (8,000 feet) on the inner scale, read the TA (7,600 feet) on the outer scale. [Figure 3-l7]
Density Altitude (DA) Determination
DA determination on the computer is accomplished by using the window just above FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS and the small window just above that marked DENSITY ALTITUDE.
PA = 9,000 feet
Air temperature = 10 °C
Place pressure altitude (9,000 feet) opposite air temperature (10) in window marked FOR AIRSPEED AND DENSITY ALTITUDE, read DA (10,400 feet). [Figure 3-18]
Figure 3-18. Finding density altitude.
Absolute Altimeter
Accurate absolute altitude is an important requisite for navigation, as well as for safe aviating. It is particularly important in pressure pattern navigation. Absolute altitude may be computed from the PA readings if the position of the aircraft is known, but the results are often inaccurate. Under changing atmospheric conditions, corrections applied to PA readings to obtain TAs are only approximate. In addition, any error made in determining the terrain elevations results in a corresponding error in the absolute altitude.
Radar Altimeter High-Level
A typical high-level radar altimeter is designed to indicate absolute altitude of the aircraft up to 50,000 feet above the terrain, land, or water. This altimeter does not warn of approaching obstructions, such as mountains, because it measures altitude only to a point directly below the aircraft. [Figure 3-19]
Figure 3-19. High-level radar altimeter.
A typical set consists of the radar receiver-transmitter, height indicator, and antenna. The transmitter section of the receiver-transmitter unit develops recurring pulses of radio frequency (RF) energy that are delivered to the transmitter antenna located either flush mounted or on the underside of the aircraft. The transmitter antenna radiates the pulsed energy downward to reflect off the earth and return to the receiver antenna on the aircraft. The time consumed between transmission and reception of the RF pulse is determined only by the absolute altitude of the aircraft above the terrain since the radio wave velocity is constant.
The receiver antenna delivers the returned pulse to the receiver section of the receiver-transmitter unit where it is amplified and detected for presentation on the indicator unit. The radar altimeter indicator displays absolute altitude, which is used in pressure pattern navigation, terrain clearance, or as a backup for the PA.
This type altimeter provides a dial or digital indication of the altitude of the aircraft above the terrain. It is designed to eliminate the necessity of adding antennas or any other equipment external to the surface of the aircraft. This equipment may also be used in conjunction with automatic pilot or other devices requiring altitude limit data. [Figure 3-20]
Figure 3-20. Low-level radar altimeter.
Systems vary widely, but typically include a receivertransmitter, height indicator, and electronic control amplifier. The height indicator contains the only operating control on the equipment. This instrument normally gives altitude readings up to 35,000 feet. If the instrument has an analog scale, the markings are usually logarithmic, graduated for the low altitude portion of its range. A variable altitude limit indicator system is included to provide an indication of flight below a preset altitude.
To operate the equipment, turn the ON-LIMIT control to on. After warmup, the terrain clearance of the aircraft within the range of 0–20,000 is read directly from the single pointer on the indicator. [Figure 3-20] This pointer can be preset to any desired altitude by the ON-LIMIT control and is used as a reference for flying at fixed altitudes. The altitude can be maintained by observing the position of the pointer with respect to the small triangular marker instead of the actual altitude scale. In addition, a red light on the front of the indicator lights up when the aircraft is at or below the preset altitude. To turn off the equipment, it is only necessary to turn off the ON-LIMIT control on the indicator.
Airspeed (Part One)
Airspeed is the speed of the aircraft in relation to the air mass surrounding that aircraft.
Pitot-Static System
Accurate airspeed measurement is obtained by means of a pitot-static system. The system consists of:
- A tube mounted parallel to the longitudinal axis of the aircraft in an area that is free of turbulent air generated by the aircraft, and
- A static source that provides still, or undisturbed, air pressure.
Ram and static pressures may be taken from a single pitotstatic tube or from completely separate sources. A pitot-static tube usually has a baffle plate [Figure 3-21] to reduce turbulence and to prevent rain, ice, and dirt from entering the tube. There may be one or more drain holes in the bottom of the tube to dispose of condensed moisture. A built-in electrical heating element, controlled by a switch inside the aircraft, prevents the formation of ice in the tube.
Figure 3-21. Expansion and contraction of the diaphragm is transmitted to the pointer of the airspeed indicator.
Reasonable care should be taken with the pitot-static system to ensure continuous, reliable service. The drain holes should be checked periodically to ensure they are not clogged. At the completion of each flight, a cover is placed over the intake end of the tube to prevent foreign objects and moisture from collecting in the tube.
Principles of Operation of Airspeed Indicators
The heart of the airspeed indicator is a diaphragm that is sensitive to pressure changes. Figure 3-21 shows it located inside the indicator case and connected to the ram air source in the pitot tube. The indicator case is sealed airtight and connected to the static pressure source. The differential pressure created by the relative effects of the impact and static pressures on the diaphragm causes it to expand or contract. As the speed of the aircraft increases, the impact pressure increases, causing the diaphragm to expand. Through mechanical linkage, the expansion is displayed as an increase in airspeed. This principle is used in the IAS meter, the TAS meter, and the Machmeter.
Airspeed Definitions
There are many reasons for the difference between IAS and TAS. Some of the reasons are the error in the mechanical makeup of the instrument, the error caused by incorrect installation, and the fact that density and pressure of the atmosphere vary from standard conditions.
Indicated Airspeed (IAS)
IAS is the uncorrected reading taken from the face of the indicator. It is the airspeed that the instrument shows on the dial.
Basic Airspeed (BAS)
Basic airspeed (BAS) is the IAS corrected for instrument error. Each airspeed indicator has its own characteristics that cause it to differ from any other airspeed indicator. These differences may be caused by slightly different hairspring tensions, flexibility of the diaphragm, accuracy of the scale markings, or even the effect of temperature on the different metals in the indicator mechanism. The effect of temperature introduces an instrument error due to the variance in the coefficient of expansion of the different metals comprising the working mechanisms. This error can be removed by the installation of a bimetallic compensator within the mechanical linkage. This bimetallic compensator is installed and properly set at the factory, thereby eliminating the temperature error within the instrument. The accuracy of the airspeed indicator is also affected by the length and curvature of the pressure line from the pitot tube. These installation errors must be corrected mathematically. Installation, scale, and instrument errors are all combined under one title called instrument error. Instrument error is factory-determined to be within specified tolerances for various airspeeds. It is considered negligible or is accounted for in technical order tables and graphs.
Calibrated Airspeed (CAS)
Calibrated airspeed (CAS) is basic airspeed corrected for pitot-static error or attitude of the aircraft. The pitot-static system of a moving aircraft has some error. Minor errors are found in the pitot section of the system. The major difficulty is encountered in the static pressure section. As the flight attitude of the aircraft changes, the pressure at the static inlets changes. This is caused by the airstream striking the inlet at an angle. Different types and locations of installations cause different errors. It is immaterial whether the status source is located in the pitot-static head or at some flush mounting on the aircraft. This error is essentially the same for all aircraft of the same model, and a correction can be computed by referring to tables in the appendix of the flight manual.
Equivalent Airspeed (EAS)
Equivalant airspeed (EAS) is CAS corrected for compressibility. Compressibility becomes noticeable when the airspeed is great enough to create an impact pressure that causes the air molecules to be compressed within the impact chamber of the pitot tube. The amount of the compression is directly proportionate to the impact pressure. As the air is compressed, it causes the dynamic pressure to be greater than it should be. Therefore, the correction is a negative value.
The correction for compressibility error can be determined by referring to the performance data section of the aircraft flight manual or by using the F-correction factor on the DR computer.
Density Airspeed (DAS)
Density airspeed (DAS) is calibrated airspeed corrected for PA and TAT. Pitot pressure varies not only with airspeed but also with air density. As the density of the atmosphere decreases with height, pitot pressure for a given airspeed must also decrease with height. Thus, an airspeed indicator operating in a less dense medium than that for which it was calibrated indicates an airspeed lower than true speed. The higher the altitude, the greater the discrepancy. The necessary correction can be found on the DR computer. Using the window on the computer above the area marked FOR AIRSPEED DENSITY ALTITUDE COMPUTATIONS, set the PA against the TAT. Opposite the CAS on the minutes scale, read the DAS on the miles scale. At lower airspeeds and altitudes, DAS may be taken as true airspeed with negligible error. However, at high speeds and altitudes, this is no longer true and compressibility error must be considered. (Compressibility error is explained in the equivalent airspeed section.) When DA is multiplied by the compressibility factor, the result is true airspeed.
True Airspeed (TAS)
TAS is equivalent airspeed that has been corrected for air density error. By correcting EAS for TAT and PA, the navigator compensates for air density error and computes an accurate value of TAS. The TAS increases with altitude when the IAS remains constant. When the TAS remains constant, the IAS decreases with altitude. CAS and EAS can be determined by referring to the performance data section of the aircraft flight manual.
Airspeed (Part Two)
Computing True Airspeed
ICE-T Method
To compute TAS using the ICE-T method on the DR computer, solve, for each type of airspeed, in the order of I, C, E, and T; that is, change IAS to CAS, change CAS to EAS, and change EAS to TAS. This process is illustrated by the following sample problem. (Refer to definitions as necessary.)
Given:
PA = 30.000′
Temperature = –37 °C
IAS = 253 knots
Flight Manual Correction Factor = 2 knots
Find: CAS, EAS, and TAS
CAS is determined by algebraically adding to IAS the correction factor taken from the chart in the flight manual. (This correction is insignificant at low speeds but can be higher than 10 knots near Mach 1.) To correct CAS to EAS, use the chart on the slide of the computer entitled F-CORRECTION FACTORS FOR TAS. [Figure 3-22] Enter the chart with CAS and PA. The F factor is .96. Multiply CAS by .96 or take 96 percent of 255 knots. To do this, place 255 knots on the inner scale under the 10 index on the outer scale. Locate 96 on the outer scale and read EAS on the inner scale: 245 knots.
Now, we need to correct EAS for temperature and altitude to get TAS. As shown in Figure 3-22, in the window marked FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS, place temperature over PA. Locate the EAS of 245 knots on the inner scale and read TAS on the outer scale. The TAS is 408 knots.
Figure 3-22. ICE-T method. [click image to enlarge]Alternate TAS Method
There is an alternate method of finding TAS when given CAS. The instructions for alternate solution are printed on the computer directly below the F factor table (Multiply F factor by TAS obtained with computer to obtain TAS corrected for compressibility). Mathematically, the answer should be the same regardless of the procedure being used, but the ICE-T method is used most often because the computation can be worked backwards from TAS. If the desire is to maintain a constant TAS, determine what IAS to fly by working the ICE-T method in reverse (also known as reverse ICE-T). [Figure 3-23]
Figure 3-23. ICE-T in reverse.
Machmeters
Machmeters indicate the ratio of aircraft speed to the speed of sound at any particular altitude and temperature during flight. It is often necessary to convert TAS to a Mach number or vice versa. Instructions are clearly written on the computer in the center portion of the circular slide rule. Locate the window marked FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS and rotate the disk until the window points to the top of the computer (toward the l0 index on the outer scale). Within the window is an arrow entitled MACH NO. INDEX. [Figure 3-24] To obtain TAS from a given Mach number, set air temperature over the MACH NO. INDEX and, opposite the Mach number on the MINUTES scale, read the TAS on the outer scale.
Figure 3-24. Finding true airspeed from Mach number.
Example: If you are planning to maintain Mach 1.2 on a crosscountry flight, place the air temperature at flight altitude over the MACH NO. INDEX. Read the TAS on the outer scale opposite 1.2 on the inner scale. If the temperature is –20 °C, the TAS is 742 knots.
Airspeed Indicators
The combined airspeed-Mach indicator, shown in Figure 3-25, is usually found in high-performance aircraft or where instrument panel space is limited. It simultaneously displays IAS, indicated Mach number, and maximum allowable airspeed. It contains a differential pressure diaphragm and two aneroid cells. The diaphragm drives the airspeed-Mach pointer. One aneroid cell rotates the Mach scale, permitting IAS and Mach number to be read simultaneously. The second aneroid cell drives the maximum allowable airspeed pointer. This pointer is preset to the aircraft’s maximum IAS. Unlike the maximum IAS and unlike the maximum allowable airspeed, Mach number increases with altitude. An airspeed marker set knob positions a movable airspeed marker. This marker serves as a memory reference for desired airspeed.
Figure 3-25. Combined airspeed Mach indicator.
Air Data Computer
The air data computer is an electro-pneumatic unit that uses pitot and static pressures and total air temperature to compute outputs for various systems. These output parameters of voltage and resistance represent functions of altitude, Mach number, TAS, computed airspeed, and static air temperature. Air data computer outputs are used with the flight director computers, automatic flight controls, cabin pressurization equipment, and normal basic indicators. The air data computer provides extreme accuracy and increased reliability.
Doppler
Doppler radar provides the navigator with continuous, instantaneous, and accurate readings of groundspeed (GS) and drift angle in all weather conditions, both over land and water. It does this automatically with equipment that is of practical size and weight. Its operation makes use of the Doppler effect.
Two basic Doppler radar systems exist: the four-beam and the three-beam. Both types use either continuous-wave (CW) or pulse-wave (PW) transmission. CW transmission requires one antenna for transmission and a second antenna for reception. Both systems use an X-shaped beam configuration. Groundspeed is computed by comparing Doppler shift between front and rear beams, and drift angle is computed by comparing the shift between the left and right beams.
Doppler is not the only source of drift angle and GS. The same basic information is available on virtually all inertial navigation systems (INS), and now global positioning system (GPS) computers can also give accurate information under a wider range of conditions.
