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.
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