Aircraft Turboprop Engine Operation

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.

Trouble Probable Causes Remedy
Power unit fails to turn over during attempted start. • No air to starter. • Check started air valve solenoid and air supply.
• Propeller brake locked. • Unlock brake by turning propeller by hand in direction of normal rotation.
Power unit fails to start. • Starter speed low because of inadequate air supply to starter. • Check starter air valve solenoid and air supply.
• If fuel is not observed leaving the exhaust pipe during start, fuel selector valve may be inoperative because of low power supply or may be locked in “OFF.” • Check power supply or electrically operated valves. Replace valves if defective.
• Fuel pump inoperative. • Check pump for sheared drives or internal damage. Check for air leaks at outlet.
• Aircraft fuel filter dirty. • Clean filter and replace filtering elements if necessary.
• Fuel control cutoff valve closed. • Check electrical circuit to ensure that actuator is being energized. Replace actuator or control.
Engine fires, but will not accelerate to correct speed. • Insufficient fuel supply to control unit. • Check fuel system to ensure all valves are open and pumps are operative.
• Fuel control main metering valve sticking. • Flush system. Replace control.
• Fuel control bypass valve sticking open. • Flush system. Replace control.
• Drain valve stuck open. Starting fuel enrichment pressure switch setting too high. • Replace drain valve. Replace pressure switch.
Acceleration temperature too high during starting. • Fuel control bypass valve sticking closed. • Flush system. Replace control.
• Fuel control acceleration cam incorrectly adjusted. • Replace control.
• Defective fuel nozzle. • Replace nozzle with a known satisfactory unit.
• Fuel control thermostat failure. • Replace control.
Acceleration temperature during starting too low. • Acceleration cam of fuel control incorrectly adjusted. • Replace control.
Engine speed cycles after start. • Unstable fuel control governor operation. • Continue engine operation to allow control to condition itself.
Power unit oil pressure drops off severely. • Oil supply low. • Check oil supply and refill as necessary.
• Oil pressure transmitter or indicator giving false indication. • Check transmitter or indicator and repair or replace if necessary.
Oil leakage at accessory drive seals. • Seal failure. • Replace seal or seals.
Engine unable to reach maximum controlled speed of 100 percent. • Faulty propeller governor. • Replace propeller control assembly
• Faulty fuel control or air sensing tip. • Replace faulty control. If dirty, use air pressure in reverse direction of normal flow through internal engine passage and sensing tip.
Vibration indication high. • Vibration pickup or vibration meter malfunction. • Calibrate vibration meter.
• Start engine and increase power gradually.
• Observe vibration indicator. If indications prove pickup to be at fault, replace it. If high vibration remains as originally observed, remove power unit for overhaul.
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