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
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Probable
Causes
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Suggested
Action
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Engine has low rpm, exhaust gas
temperature, and fuel flow when set to expected engine pressure ratio.
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• Engine
pressure ratio indication has high reading error.
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• Check
inlet pressure line from probe to transmitter for leaks.
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• Check
engine pressure ratio transmitter and indicator for accuracy.
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Engine has high rpm, exhaust gas temperature,
and fuel flow when set to expect engine pressure ration.
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• Engine
pressure ratio indication has low reading error due to:
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- Misaligned
or cracked turbine discharge probe.
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• Check
probe condition.
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- Leak
in turbine discharge pressure line from probe to transmitter.
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• Pressure-test
turbine discharge pressure line
for leaks.
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- Inaccurate
engine pressure ratio transmitter or indicator.
- Carbon
particles collected in turbine discharge pressure line or restrictor orifices.
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• Check
engine pressure ratio transmitter and indicator for accuracy.
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Engine has high
exhaust gas temperature, low rpm, and high fuel flow at all engine
pressure ratio settings.
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• Possible
turbine damage and/or loss of turbine efficiency.
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• Confirm
indication of turbine damage by:
- Checking engine coast-down for abnormal
noise and reduced time.
- Visually
inspect turbine area with strong light.
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NOTE: Engines with
damage in turbine section may have tendency to hang up during starting.
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• If
only exhaust gas temperature is high, other parameters normal, the problem
may be thermocouple leads or instrument.
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• Re-calibrate
exhaust gas temperature instrumentation.
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Engine vibrates throughout rpm range, but
indicated amplitude reduces as rpm is reduced.
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• Turbine
damage.
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• Check
turbine as outlined in preceding item.
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Engine vibrates at
high rpm and fuel flow when compared to constant engine pressure ratio.
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• Damage
in compressor section.
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• Check
compressor section for damage.
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Engine vibrates
throughout rpm range, but is more pronounced in cruise or idle rpm range.
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• Engine-mounted
accessory such as constant-speed drive, generator, hydraulic pump, etc.
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• Check
each component in turn.
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No change in power
setting parameters, but oil temperature high.
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• Engine
main bearings.
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• Check
scavenge oil filters and magnetic plugs.
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Engine has higher
than normal exhaust gas temperature during takeoff, climb, and cruise. Rpm
and fuel flow higher than normal.
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• Engine
bleed-air valve malfunction.
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• Check
operation of bleed valve.
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• Turbine discharge pressure probe or
line to transmitter leaking.
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• Check
condition of probe and pressure line to transmitter.
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Engine has high exhaust gas temperature at
target engine pressure ratio for takeoff.
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• Engine
out of trim.
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• Check
engine with jetcal. Re-trim as desired.
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