Troubleshooting: Taking a look at the PT6A-27 engine

When troubleshooting you have to look at a situation from every angle. How do you find the most efficient method to eliminate the problem? This article discusses how engineers used troubleshooting to resolve the lower power trouble of the PT6A-27 turbo-prop engine in Y12 (II) aircraft.

Background on the Y12

When running the engines of the Y12 on the ground shortly after the aircraft returned from flight test, the pilots complained that the torque of the left engine could not reach maximum take-off torque of 1,480 lb.-ft. although Ng had obtained 100 percent. After an engine performance run and simultaneous recording of engine torque (TQ), inter-turbine temperature (ITT), fuel flow rate (Wf), propeller speed (Np), oil temperature (Oilt), oil pressure (OilP) while keeping right and left engine Ng identical, they admitted that the left-hand engine lost power. Results of the ground test are shown in Table 1 below.

The Y12 (II), is a 19-seat light general-purpose aircraft. Maximum take-off weight of the Y12 (II) aircraft is 5,300kg. It is a twin-engine high monoplane with a single vertical tail and a non-retractable tricycle landing gear. Two PT6A-27 turboprop engines are mounted in LH and RH engine nacelles. The PT6A-27 turboprop engine has three-stage axial compressors and a one-stage centrifugal compressor. Maximum engine power output is 680 shp. In the Y12 (II) aircraft, engine power output is limited to 620 shp. The propeller is a three-bladed constant speed, featherable, and reversible. The pitch control of the propeller is achieved by varying the oil pressure in the pitch-change mechanism with the propeller speed governor.

PT6A-27 engine parameters

Ng: Gas generator rotation speed indication. Ng is an indication of the power output of the engine. The pilots and technicians reference the Ng speed to determine the engine power setting. In PT6A-27 turbo-prop engine, gas generator speed is 37,500 rpm at Ng = 100 percent. Maximum Ng = 101.5 percent corresponding to 38,000 rpm. Ng = 52+1 percent at ground idle condition.

ITT: Inter-turbine temperature, the temperature taken between compressor turbine and free turbine by thermocouples and probes. ITT is extremely important because it indicates if an over temperature occurs at engine.

Torque: Indication of propeller power output. Maximum take-off torque of PT6A-27 turbo-prop engine is restricted to 1,480 lb.-ft. in Y12 (II) aircraft.

Nf: Indication of power turbine (free turbine) rotation speed.

Np: Indication of propeller rotation speed. Constant speed of the propeller in Y12 (II) aircraft is 2,200 rpm. Usually, propeller speed does not attain constant speed of 2,200 rpm until Ng >86-89 percent.

P3: Axial compressor discharge pressure. This is an air pressure reading taken at the exit of axial compressor and the entrance of the centrifugal compressor (at pressure station 3).

P2.5: Axial compressor discharge pressure. This is a pressure reading taken between the second-stage compressor and the third-stage compressor.

Bleed valve: The bleed valve is located between the second-stage compressor and the third-stage compressor. The valve modulates open and closed to prevent stage stalls and engine surges.

Oilt: Oil temperature indication. Maximum oil temperature is 99 degrees C in PT6A-27 turbo-prop engine.

Oilp: Oil pressure indication. Maximum oil pressure is 100 psi in PT6A-27 turbo-prop engine.

Wf: Fuel flow rate taken with the fuel flow meter that is installed in the engine nacelles.