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 .
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