Created on:2021-10-11 10:46

Aircraft hydraulic failure_ Fire switch failure

Aircraft hydraulic failure_ Fire switch failure

After MA60 aircraft was delivered to an airline for use, oil leakage of hydraulic system fire switch ydk-14 occurred many times. When the first oil leak occurred, civil aviation maintenance personnel replaced the fire switch. It will take several hours to replace the fire switch, which will delay the normal flight of the aircraft. The problem is that the fire switch is replaced soon, and similar faults occur again. Civil aviation has to send the faulty parts of the fire switch to the manufacturer for repair, but this repair takes longer. Therefore, during the period when the aircraft is grounded, civil aviation not only delays the passenger flight mission, but also causes serious losses to the economic benefits of airlines. The most important thing is that the reliability and safety of the aircraft can not be guaranteed.

(1) Fault analysis

① The functions of the pipe section accessories from the main to the hydraulic oil tank are shown in Figure F. the installation position of the main hydraulic pump on MA60 aircraft is about 130mm higher than the hydraulic oil tank. According to GJB 638a-97 (design and installation requirements for aircraft type I and II hydraulic systems), in order to ensure that the main hydraulic pump has good oil absorption capacity, oil backflow is not allowed when the aircraft is parked on the ground. Therefore, a check valve yxf-83 is installed at the outlet of the oil tank. At the same time, in order to meet the airworthiness requirements, considering that the hydraulic oil flowing to the engine area will be cut off in case of fire in the engine nacelle, a fire switch ydk-14 is set between the main hydraulic pump and the one-way valve.

F.jpg

② Cause analysis of oil leakage of fire switch during aircraft ground parking civil aviation maintenance personnel found that the oil leakage of fire switch occurred during aircraft parking the next day or short-term parking waiting for transportation. During this period, when the aircraft engine propeller is not fixed, the engine propeller rotates in the abnormal working direction under the influence of wind direction and wind force, so as to drive the main hydraulic pump connected with the engine transmission gearbox to reverse together, and suck the oil from the outlet pipeline of the hydraulic pump into the inlet suction pipeline. Due to the function of the one-way valve in the oil suction pipeline, the pressure in the oil suction pipeline of the main hydraulic pump in the hydraulic system increases. A high-pressure section (as shown in Figure g) is formed between the main hydraulic pump of the oil suction pipeline and the check valve, and the fire switch is just installed in this high-pressure section. When the wind in this direction is stronger, the pressure in the high-pressure section is higher. When the pressure exceeds the withstand voltage limit of the fire switch (0.9mpa), the external sealing ring of the fire switch is extruded, and a large amount of hydraulic oil leaks.

G.jpg

(2) Selection of improvement measures

Since the installation position of the main hydraulic pump and hydraulic oil tank cannot be changed, and the one-way valve in the oil suction pipeline must be retained, the following two measures can be taken to solve the oil leakage problem of the fire switch caused by the abnormal rise of the pressure in the oil suction pipeline.

① The manufacturer redesigned the one-way valve yxf-83 and added one side safety valve on the basis of the original design of the valve. When the pressure of the oil suction pipeline reaches 0.5MPa, the pipeline pressure will be released automatically.

② Install a safety valve between the oil suction pipeline of the main hydraulic pump and the oil return pipeline of the oil tank (as shown in Figure h). When the wind direction of the airport just drives the aircraft engine propeller and the main hydraulic pump to rotate abnormally, the oil pressure in the oil suction pipeline rises sharply. When the pressure reaches 0.5MPa, the safety valve opens to relieve the pressure, so that the pressure oil finally returns to the oil tank, so as to avoid the fault of large oil leakage caused by the extrusion of the sealing rubber ring of the fire switch. In order to avoid opening the safety valve when the main hydraulic pump works normally, the opening pressure of the safety valve is designed to be higher than that of the one-way valve.

Measure ① the manufacturer needs to re develop the one-way valve, which has long cycle and high cost. Measure ② self made accessories can be used for the required one-way valve. After parameter adjustment, some parts of a self-made one-way valve with mature technology and similar performance data can be changed into the required safety valve to meet the requirements, with short cycle and low cost. The specific design idea of the safety valve in measure ② is to change the original one-way valve y7-5822-0 of the hydraulic system of Yun 7 aircraft, mainly redesign its pressure regulating element spring and other components, so that its opening pressure is 0.5MPa, and change it into the required self-made accessory safety valve Y7 Ⅲ - 5862-0. This measure can not only ensure the normal operation of the fire switch, but also protect the system pipeline.

The improvement measures adopted ② firstly, the modification was carried out on the first outfield aircraft. After a period of flight verification, the oil leakage fault of the fire switch did not appear again. Subsequently, all aircraft were refitted with safety valves. Two years of field flight proved that the fire switch worked normally and the oil leakage fault was effectively eliminated. The improved design is easy to realize, the design cycle is short, the investment cost is low, and the reliability and safety can be guaranteed.

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