Turboprop Operation and Troubleshooting

Turboprop engine operation is quite similar to that of a turbojet engine, except for the added feature of a propeller. The starting procedure and the various operational features are very much alike. The turboprop chiefly requires attention to engine operating limits, the throttle or power lever setting, and the torquemeter pressure gauge. Although torquemeters indicate only the power being supplied to the propeller and not the equivalent shaft horsepower, torquemeter pressure is approximately proportional to the total power output and, thus, is used as a measure of engine performance. The torquemeter pressure gauge reading during the takeoff engine check is an important value. It is usually necessary to compute the takeoff power in the same manner as is done for a turbojet engine. This computation is to determine the maximum allowable exhaust gas temperature and the torquemeter pressure that a normally functioning engine should produce for the outside, or ambient, air temperature and barometric pressure prevailing at the time.

Troubleshooting Procedures for Turboprop Engines

All test run-ups, inspections, and troubleshooting should be performed in accordance with the applicable engine manufacturer’s instructions. In Figure, the troubleshooting procedure for the turboprop reduction gear, torquemeter, and power section are combined because of their inter-relationships. The table includes the principal troubles, together with their probable causes and remedies.

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 turbine discharge 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.

Troubleshooting turboprop engines