Taking a look at the PT6A-27 engine
By Ma Hui
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.
A logical plan is needed to solve any problem. Here is a overview of the typical troubleshooting logic used to solve this problem:
1. Verify that there is a problem.
From Table 1, we could determine that the parameters of the left and right engines were almost identical when Ng = 52 percent and Ng = 80 percent. But, the torque of the left engine only reached 1,400 lb.-ft. when the torque of the right engine had achieved 1,480 lb-ft at Ng = 95 percent.
In order to identify root causes of the trouble, engineers consulted previous engine performance records. They found that the parameters of both engines were previously identical. It verified that the failure occurred after the engines were delivered to Harbin Aircraft Manufacturing Corporation.
2. Check the instrumentation system.
They swapped and tested Ng and ITT gauges to see if the problem was caused by a faulty indicator.Fortunately, results showed that Ng and ITT indicators were in all readiness.
3. Trouble analysis of the fuel control system.
The fuel control system of PT6A-27 turbo-prop engine consists of an oil-fuel heat exchanger, fuel pump, fuel control unit, fuel starting flow control unit, fuel-spraying nozzles, and fuel and pneumatic pipelines, etc. The fuel control unit is installed at the rear of the engine with its rotation speed directly proportional to the gas generator rotation speed Ng. The FCU senses any changes in Ng and adjusts fuel delivery. In general, the FCU functions to determine fuel delivery to the combustion chamber automatically, and thereby satisfies the power demands of the propeller.
The FCU is made up of three parts: a metering section, a computing section, and a gas generator speed governor. The main component of metering section is a metering valve shaped like a needle. The area of metering orifice is changed with the movement of the metering valve. The metering valve is connected to the computing section by the torque tube. The computing section consists of an acceleration bellow and a speed governor bellow. Px and Py are modified air pressure bleeding from axial compressor. Px acts on the lower side of the bellow, and Py acts on the upper side. When Py increases and Px keeps constant, the bellow moves downward with the torque tube, leading to the increment of orifice area in the metering valve. As a result, fuel flow increases. On the contrary, with reduced Py and unchanged Px, the bellow moves upward with the torque tube, the orifice area shrinks and fuel flow decreases. Py is connected to Ng speed governor in FCU through an external pressure air pipeline.
Propeller speed governor
The propeller speed governor consists of three parts: constant speed unit, beta valve and Nf speed governor. Here it is concerned with the Nf speed governor. The power turbine speed governor functions to prevent the power turbine from overspeeding in case of propeller speed governor failure. The reset arm of the Nf speed governor is connected to Py through an external pressure air pipe. A person on the ground can adjust the reset arm to regulate Py .
Trouble analysis of fuel control systems
Some engineers thought that the trouble was due to a failure of fuel schedule. They deemed that it was the result of improper adjustment of the reset arm in the Nf speed governor. Improper adjustment of reset arm affected Py.
In normal conditions, Py - Px differential causes the metering valve to maintain the required Ng. If improper adjustment of the reset arm is decreased, the Py, bellow would move upward to decrease Py and keep Px constant. Movement of the torque tube with the bellow decreased the orifice area of the metering valve, and reduced fuel flow and output engine power accordingly. Engine power output is the product of engine torque (TQ), propeller speed (Np), and coefficient (K). Np and K are constant values. The less power the engine produces, the lower torque the propeller generates. Therefore, the ground crew thought that the improper adjustment of the reset arm caused Py leakage and decreased engine power keeping the aircraft from achieving maximum take-off torque.
However, both engines torques were not identical and the left engine's torque did not achieve 1,480 lb.-ft. Subsequently, they checked the pressure air pipeline connected to Py, no air leakage occurred. They proved that the failure had nothing to do with fuel control system of the engine.
Trouble analysis of compressor bleed valve
Other engineers thought air leakage of components or the pressure air pipeline led to the loss of engine power and decreased torque.
The engines were run again and the engine parameters were recorded. The new data was compared with the data obtained previously, and it was found that the parameters of both engines were identical at Ng 90 percent. It was concluded that failure of the bleed valve caused the trouble.
The bleed valve in PT6A-27 engine functions to guarantee that no surge occurs in engine compressor at the low axial compressor speed Ng, especially at ground idle condition. In the PT6A-27 engine, the bleed valve starts to close at Ng = 86 percent and closes completely at Ng = 91 percent. The bleed valve is installed at the 7-o'clock position on the gas generator case. A piston is sustained in the house with the guide pin to guarantee that the piston opens or closes smoothly in the reciprocal movement. Pressure operates piston sliding on the guide pin. The rolling diaphragm mounted on the valve piston prevents leakage between the P2.5 and P3 chamber.
The bleed valve is installed on the gas generator with four bolts. A guide tube ensures that outlet of compressor P3 air bleeding from the gas generator case is in proper alignment with the P3 passage in the bleed valve. P3 air flows through a primary metering orifice and is directed to the bottom of the piston and to the atmosphere via a convergent divergent orifice. The convergent-divergent orifice functions to restrict the airflow to specified range to obtain fixed ratio of P3/Px. Here Px refers to modified air pressure between primary metering orifice in the lower chamber and the venture. Two forces act on the bleed valve piston. Px pressure pushes to close the valve and P2.5 air pressure, from the interstage compressor area, pushes to open it. The valve closing point is achieved during engine acceleration when the pressure acting on the valve diaphragm (Px) is sufficient to overcome the compressor interstage pressure within limits (P2.5). At lower engine power, P2.5> Px, bleed valve opens completely. With increasing engine power, P3 rises faster than P2.5, thus increasing the pressure acting on the piston to gradually close it. The speed (Ng) at which the valve closes is a function of the primary and convergent divergent orifice sizes. The bleed valve cannot abruptly close tightly due to the increase of P2.5. Instead, it closes gradually at the constant ratio of P3 /Px. At high engine power, the bleed valve closes completely and Px > P2.5.
Trouble analysis of bleed valve and elimination of trouble
At lower engine power, P2.5 is much greater than Px, and the bleed valve opens completely. The condition of the bleed valve in the right engine is the same as that in the left engine. As a result, the parameters on both engines are identical. With increasing engine power, P3 pressure rises and leads to the increase of Px. Therefore, resultant acting force on the rolling diaphragm increases with it, making the piston move upward gradually. At Ng = 91 percent, the piston is engaged in the base tightly. If the piston of the left engine compressor can not be engaged in the base properly inter-stage pressure air will flow into atmosphere, which results in power loss in the left engine and a decrease in the left engine's torque. The trouble was eliminated after the bleed valve of the left engine compressor was replaced with a new one.
Tips and hints
Previous experience indicated that the bleed valve usually had trouble after extended flight time. Some engineers observed that the performance of the aircraft met the specifications in previous ground tests, so they did not believe that the bleed valve could fail. Instead, they focused on the fuel control system. Many component failures could have had an effect. It will save time if different tests are done in accordance with different component characteristics under varying circumstances to distinguish failures.
It increases efficiencies to resolve problems as quickly and efficiently as possible. Hopefully, these troubleshooting methods and analysis will help bring success to other aircraft maintenance engineers.