On turbine engines that have a thrust reverser, retarding the aircraft throttle to idle or power lever to OFF cuts the fuel supply to the engine and shuts down the engine. On engines equipped with thrust reversers, this is accomplished by means of a separate fuel shutoff lever or switch. When an engine has been operated at high power levels for extended periods of time, a cool down time should be allowed before shut down. It is recommended the engine be operated at below a low power setting, preferably at idle for a period of 5 minutes to prevent possible seizure of the rotors. This applies, in particular, to prolonged operation at high rpm on the ground, such as during engine trimming. The turbine case and the turbine wheels operate at approximately the same temperature when the engine is running. However, the turbine wheels are relatively massive, compared with the case, and are not cooled so readily. The turbine case is exposed to cooling air from both inside and outside the engine. Consequently, the case and the wheels lose their residual heat at different rates after the engine has been shut down. The case, cooling faster, tends to shrink upon the wheels, that are still rotating. Under extreme conditions, the turbine blades may squeal or seize; thus a cooling period is required if the engine has been operating at prolonged high speed. Should the turbine wheels seize, no harm normally results, provided no attempt is made to turn the engine over until it has cooled sufficiently to free the wheels. In spite of this, every effort should be made to avoid seizure.
To ensure that fuel remains in the lines and that the engine driven fuel pumps are not starved for fuel that lubricates the pumps, the aircraft fuel boost pump must be turned off after, not before, the throttle or the fuel shutoff lever is placed in the OFF position.
Generally, an engine should not be shut down by the fuel shutoff lever until after the aircraft throttle has been retarded to idle. Because the fuel shutoff valve is located on the fuel control discharge, a shutdown from high thrust settings results in high fuel pressures within the control that can harm the fuel system parts.
When an accurate reading of the oil level in the oil tank is needed following an engine shutdown, the engine should be operated and shut down with the oil check taking place within not more than 30 minutes after shutdown. Check the engine manuals for the specific procedure.
Aircraft Turbine Engines Troubleshooting
Included in this page are typical guidelines for locating engine malfunctions on most turbine engines. Since it would be impractical to list all the malfunctions that could occur, only the most common malfunctions are covered. A thorough knowledge of the engine systems, applied with logical reasoning, solves most problems that may occur.
Figure enumerates some malfunctions that may be encountered. Possible causes and suggested actions are given in the adjacent columns. The malfunctions presented herein are solely for the purpose of illustration and should not be construed to have general application. For exact information about a specific engine model, consult the applicable manufacturer’s instructions.
| Indicated Malfunction | Probable Causes | Suggested Action |
| Engine has low rpm, exhaust gas temperature, and fuel flow when set to expected engine pressure ratio. | • Engine pressure ratio indication has high reading error. | • Check inlet pressure line from probe to transmitter for leaks. |
| • Check engine pressure ratio transmitter and indicator for accuracy. | ||
| Engine has high rpm, exhaust gas temperature, and fuel flow when set to expect engine pressure ration. | • Engine pressure ratio indication has low reading error due to: | |
| – Misaligned or cracked turbine discharge probe. | • Check probe condition. | |
| – Leak in turbine discharge pressure line from probe to transmitter. | • Pressure-test turbine discharge pressure line for leaks. | |
| – Inaccurate engine pressure ratio transmitter or indicator.- Carbon particles collected in turbinedischarge pressure line or restrictor orifices. | • Check engine pressure ratio transmitter and indicator for accuracy. | |
| Engine has high exhaust gas temperature, low rpm, and high fuel flow at all engine pressure ratio settings. | • Possible turbine damage and/or loss of turbine efficiency. | • Confirm indication of turbine damage by:- Checking engine coast-down for abnormal noise and reduced time.- Visually inspect turbine area with strong light. |
| NOTE: Engines with damage in turbine section may have tendency to hang up during starting. | • If only exhaust gas temperature is high, other parameters normal, the problem may be thermocouple leads or instrument. | • Re-calibrate exhaust gas temperature instrumentation. |
| Engine vibrates throughout rpm range, but indicated amplitude reduces as rpm is reduced. | • Turbine damage. | • Check turbine as outlined in preceding item. |
| Engine vibrates at high rpm and fuel flow when compared to constant engine pressure ratio. | • Damage in compressor section. | • Check compressor section for damage. |
| Engine vibrates throughout rpm range, but is more pronounced in cruise or idle rpm range. | • Engine-mounted accessory such as constant-speed drive, generator, hydraulic pump, etc. | • Check each component in turn. |
| No change in power setting parameters, but oil temperature high. | • Engine main bearings. | • Check scavenge oil filters and magnetic plugs. |
| Engine has higher than normal exhaust gas temperature during takeoff, climb, and cruise. Rpm and fuel flow higher than normal. | • Engine bleed-air valve malfunction. | • Check operation of bleed valve. |
| • Turbine discharge pressure probe or line to transmitter leaking. | • Check condition of probe and pressure line to transmitter. | |
| Engine has high exhaust gas temperature at target engine pressure ratio for takeoff. | • Engine out of trim. | • Check engine with jetcal. Re-trim as desired. |
| Engine rumbles during starting and at low power cruise conditions. | • Pressurizing and drain valve malfunction. | • Replace pressurizing and drain valves. |
| • Cracked air duct. | • Repair or replace duct. | |
| • Fuel control malfunction. | • Replace fuel control. | |
| Engine rpm hangs up during starting. | • Subzero ambient temperatures. | • If hang-up is due to low ambient temperature, engine usually can be started by turning on fuel booster pump or by positioning start leverto run earlier in the starting cycle. |
| • Compressor section damage. | • Check compressor for damage. | |
| • Turbine section damage. | • Inspect turbine for damage. | |
| High oil temperature. | • Scavenge pump failure. | • Check lubricating system and scavenge pumps. |
| • Fuel heater malfunction. | • Replace fuel heater. | |
| High oil consumption. | • Scavenge pump failure. | • Check scavenge pumps. |
| • High sump pressure. | • Check sump pressure as outlined in manufacturer’s maintenance manual | |
• Gearbox seal leakage. | • Check gearbox seal by pressurizing overboard vent. | |
| Overboard oil loss. | • Can be caused by high airflow through the tank, foaming oil, or unusual amounts of oil returned to the tank through the vent system. | • Check oil for foaming.• Vacuum-check sumps.• Check scavenge pumps. |
Troubleshooting Turbojet Engines
Aircraft Turbine Engine Calibration and Testing
Some of the most important factors affecting turbine engine life are EGT, engine cycles (a cycle is generally a takeoff and landing) and engine speed. Excess EGT of a few degrees reduces turbine component life. Low EGT materially reduces turbine engine efficiency and thrust. So, to make the engine highly efficient, the exhaust temperatures need to be as high as possible, while maintaining an EGT operating temperature that does not damage the turbine section of the engine. If the engine is operated at excess exhaust temperatures, engine deterioration occurs. Since the EGT temperature is set by the EGT temperature gauge, it is imperative that it is accurate. Excessive engine speed can cause premature engine wear and, if extreme, can cause engine failure.
One older type of calibration test unit used to analyze the turbine engine is the jetcal analyzer. [Figure 1] A jetcal analyzer is a portable instrument made of aluminum, stainless steel, and plastic. The major components of the analyzer are the thermocouple, rpm, EGT indicator, resistance, and insulation check circuits, as well as the potentiometer, temperature regulators, meters, switches, and all the necessary cables, probes, and adapters for performing all tests.
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| Figure 1. Jetcal analyzer instrument compartment |
Turbine Engine Analyzer Uses
Many different types of analyzers are used each with its own function, including onboard systems that use computers to test aircraft systems. Depending upon the specific analyzer used, procedures vary somewhat, but the basic test are outlined here. Always refer to the specific instructions associated with the analyzer being used.
Most analyzers may be used to:
- Functionally check the aircraft EGT system for error, without running the engine or disconnecting the wiring.
- Check individual thermocouples before placement in a parallel harness.
- Check each engine thermocouple in a parallel harness for continuity.
- Check the thermocouples and parallel harness for accuracy.
- Check the resistance of the EGT circuit.
- Check the insulation of the EGT circuit for shorts to ground, or for shorts between leads.
- Check EGT indicators , either in or out of the aircraft, for error.
- Determine engine rpm accuracy during engine testing. Added to this is the checking and troubleshooting of the aircraft tachometer system.
- Establish the proper relationship between the EGT and engine rpm during engine run-up.
Analyzer Safety Precautions
Observe the following safety precautions while operating the engine analyzer or other types of test equipment:
- Never use a voltammeter to check the potentiometer for continuity. If a voltammeter is used, damage to the galvanometer and standard battery cell results.
- Check the thermocouple harness before engine run-up. This must be done because the circuit must be correct before the thermocouples can be used for true EGT pickup.
- For safety, ground the jetcal analyzer when using an AC power supply. Any electrical equipment operated on AC power and utilizing wire-wound coils, such as the probes with the jetcal analyzer, has an induced voltage on the case that can be discharged if the equipment is not grounded. This condition is not apparent during dry weather, but on damp days the operator can be shocked slightly. Therefore, for the operator’s protection, the jetcal analyzer should be grounded using the pigtail lead in the power inlet cable.
- Use heater probes designed for use on the engine thermocouples to be tested. Temperature gradients are very critical in the design of heater probes. Each type of aircraft thermocouple has its own specially designed probe. Never attempt to modify heater probes to test other types of thermocouples.
- Do not leave heater probe assemblies in the exhaust nozzle during engine run-up.
- Never allow the heater probes to go over 900 °C (1,652 °F). Exceeding these temperatures results in damage to the jetcal analyzer and heater probe assemblies.
Continuity Check of Aircraft EGT Circuit
To eliminate any error caused by one or more inoperative aircraft thermocouples, a continuity check is performed. The check is made by heating one heater probe to between 500 and 700 °C and placing the hot probe over each of the aircraft thermocouples, one at a time. The EGT indicator must show a temperature rise as each thermocouple is checked. When large numbers (eight or more) of thermocouples are used in the harness, it is difficult to see a rise on the aircraft instrument because of the electrical characteristics of a parallel circuit. Therefore, the temperature indication of the aircraft thermocouples is read on the potentiometer of the analyzer by using the check cable and necessary adapter.
Functional Check of Aircraft EGT Circuit
During the EGT system functional test and the thermocouple harness checks, the analyzer has a specific degree of accuracy at the test temperature, which is usually the maximum operating temperature of the turbine engine. [Figure 2] Each engine has its own maximum operating temperature, that can be found in applicable technical instructions.
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| Figure 2. EGT analyzer |
The test is made by heating the engine thermocouples in the exhaust nozzle or turbine section to the engine test temperature. The heat is supplied by heater probes through the necessary cables. With the engine thermocouples hot, their temperature is registered on the aircraft EGT indicator. At the same time, the thermocouples embedded in the heater probes, which are completely isolated from the aircraft system, are picking up and registering the same temperature on the test analyzer.
The temperature registered on the aircraft EGT indicator should be within the specified tolerance of the aircraft system and the temperature reading on the temperature analyzer. When the temperature difference exceeds the allowable tolerance, troubleshoot the aircraft system.
EGT Indicator Check
The EGT indicator is tested after being removed from the aircraft instrument panel and disconnected from the aircraft EGT circuit leads. Attach the instrument cable and EGT indicator adapter leads to the indicator terminals, and place the indicator in its normal operating position. Adjust the analyzer switches to the proper settings. The indicator reading should correspond to the readings of the analyzer within the allowable limits of the EGT indicator.
Correction for ambient temperature is not required for this test, as both the EGT indicator and analyzer are temperature compensated. The temperature registered on the aircraft EGT indicator should be within the specified tolerance of the aircraft system and the temperature reading on the analyzer readout. When the temperature difference exceeds the allowable tolerance, troubleshoot the aircraft system.
Resistance and Insulation Check
The thermocouple harness continuity is checked while the EGT system is being checked functionally. The resistance of the thermocouple harness is held to very close tolerances, since a change in resistance changes the amount of current flow in the circuit. A change of resistance gives erroneous temperature readings. The resistance and insulation check circuits make it possible to analyze and isolate any error in the aircraft system. How the resistance and insulation circuits are used is discussed with troubleshooting procedures.
Tachometer Check
To read engine speed with an accuracy of ±0.1 percent during engine run, the frequency of the tachometer-generator (older style) is measured by the rpm check analyzer. The scale of the rpm check circuit is calibrated in percent rpm to correspond to the aircraft tachometer indicator, which also reads in percent rpm. The aircraft tachometer and the rpm check circuit are connected in parallel, and both are indicating during engine run-up. The rpm check circuit readings can be compared with the readings of the aircraft tachometer to determine the accuracy of the aircraft instrument.
Many newer engines use a magnetic pickup that counts passing gear teeth edges, which are seen electrically as pulses of electrical power as they pass by the pickup. [Figure 3] By counting the amount of pulses, the rpm of the shaft is obtained. This type of system requires little maintenance, other than setting the clearance between the gear teeth and the magnetic pickup.
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| Figure 3. Magnetic pickup and gear |
Aircraft Turbine Engine EGT System and Tachometer System Troubleshooting
Troubleshooting EGT System
An appropriate analyzer is used to test and troubleshoot the aircraft thermocouple system at the first indication of trouble, or during periodic maintenance checks. The test circuits of the analyzer make it possible to isolate the troubles listed below. Following the list is a discussion of each trouble mentioned.
- One or more inoperative thermocouples in engine parallel harness
- Engine thermocouples out of calibration
- EGT indicator error
- Resistance of circuit out of tolerance
- Shorts to ground
- Shorts between leads
One or More Inoperative Thermocouples in Engine Parallel Harness
This error is found in the regular testing of aircraft thermocouples with a hot heater probe and is a broken lead wire in the parallel harness, or a short to ground in the harness. In the latter case, the current from the grounded thermocouple can leak off and never be shown on the indicator. However, this grounded condition can be found by using the insulation resistance check.
Engine Thermocouples Out of Calibration
When thermocouples are subjected for a period of time to oxidizing atmospheres, such as encountered in turbine engines, they drift appreciably from their original calibration. On engine parallel harnesses, when individual thermocouples can be removed, these thermocouples can be bench-checked, using one heater probe. The temperature reading obtained from the thermocouples should be within manufacturer’s tolerances.
EGT Circuit Error
This error is found by using the EGT and comparing the reading of the aircraft EGT indicator with the analyzer temperature reading. [Figure] The analyzer and aircraft temperature readings are then compared.
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| EGT analyzer |
Resistance of Circuit Out of Tolerance
The engine thermocouple circuit resistance is a very important adjustment since a high-resistance condition gives a low indication on the aircraft EGT indicator. This condition is dangerous, because the engine is operating with excess temperature, but the high resistance makes the indicator read low. It is important to check and correct this condition.
Shorts to Ground/Shorts Between Leads
These errors are found by doing the insulation check using an ohmmeter. Resistance values from zero to 550,000 ohms can be read on the insulation check ohmmeter by selecting the proper range.
Troubleshooting Aircraft Tachometer System
A function of the rpm check is troubleshooting the aircraft tachometer system. The rpm check circuit in the analyzer is used to read engine speed during engine run-up with an accuracy of ±0.1 percent. The connections for the rpm check are the instrument cable and aircraft tachometer system lead to the tachometer indicator. After the connections have been made between the analyzer rpm check circuit and the aircraft tachometer circuit, the two circuits, now classed as one, are a parallel circuit. The engine is then run-up as prescribed in applicable technical instructions. Both systems can be read simultaneously. If the difference between the readings of the aircraft tachometer indicator and the analyzer rpm check circuit exceeds the tolerance prescribed in applicable technical instructions, the engine must be stopped, and the trouble located and corrected.
