Created on:2021-10-14 13:35

Aircraft hydraulic fault diagnosis and maintenance_ Landing gear retraction failure

Aircraft hydraulic fault diagnosis and maintenance_ Landing gear retraction failure

The failure of landing gear retraction and retraction hydraulic system is a common failure in the process of aircraft use. Due to the different composition and structure of hydraulic system of different models, their fault modes also have their own characteristics.

(1) System composition and basic working conditions

TB20 aircraft is a small tonnage light aircraft. Its landing gear retraction and retraction hydraulic system is mainly composed of hydraulic power components (including motor and hydraulic pump), hydraulic distributor, pressure switch, emergency control valve, retraction and retraction actuator and system pipeline. The basic working condition is that the hydraulic power assembly generates the hydraulic pressure required for the retraction and retraction of the landing gear, and the pressurized hydraulic oil flows into the "retraction" or "lowering" Chamber of the actuator through the system pipeline to push the piston of the actuator to move, so as to retract or lower the landing gear.

① After the landing gear is retracted in place, when the hydraulic pressure of the retraction pipeline reaches (1400 ± 70) psi, the pressure switch cuts off the motor circuit, the motor stops working, the one-way valve of the distributor cuts off the retraction pipeline, and the pressure oil is sealed in the pipeline to keep the landing gear in the retraction position. When the pressure in the retraction pipeline drops below (1120 ± 56) psi, the pressure switch turns on the circuit, the motor works again to pressurize the pipeline, and the landing gear continues to remain in the retraction position.

② After the landing gear is put down in place, the end switch of the landing gear lock cuts off the motor circuit and the motor stops working.

③ When the landing gear is in the retracted position, if the hydraulic power components cannot work normally and the landing gear cannot be lowered normally, manually open the emergency control valve to connect the retracted pipeline with the lowered pipeline, retract the pipeline to relieve the pressure and return oil, and the landing gear is lowered and locked by its own gravity and auxiliary spring force.

TB20 aircraft maintenance manual stipulates that the time for normal landing gear retraction or lowering shall be less than 12s, and the time for emergency landing gear lowering shall be less than 8s; After the landing gear is retracted, the hydraulic pump motor shall not work again within 5min.

(2) Fault analysis

There are three common faults in the landing gear retraction and retraction hydraulic system of TB20 aircraft during actual flight and maintenance test.

① System air plug system air plug is a common fault of landing gear retraction and retraction system of TB20 aircraft. It can be divided into two types: retracting (high pressure) pipeline air plug and lowering (low pressure) pipeline air plug. When the emergency control valve is closed, the upper and lower pipelines of the hydraulic system are isolated from each other at the actuator piston. Therefore, the hydraulic oil in the pipeline cannot circulate at full flow. If there is air in the pipeline and an air plug occurs, the system cannot discharge the bubbles in the pipeline through the self circulation of the hydraulic oil. Therefore, repeated operation of retracting and lowering the landing gear cannot discharge the bubbles in the pipeline.

According to the maintenance experience, the normal time of the system is about 6S. After the air plug occurs in the pipeline, the working process of retracting the landing gear is obviously slow, and the retraction time exceeds 6S or even 12s; When serious air jam occurs in the retracted pipeline, the landing gear cannot be retracted in place. At the same time, the hydraulic pressure in the pipeline cannot reach (1400 ± 70) psi, the pressure switch cannot cut off the motor circuit, and the motor works continuously. The air plug of the lowering pipeline has no obvious influence on the lowering process, because the boost spring on the landing gear strut has the auxiliary force to help the landing gear put down and lock in place.

In view of the above characteristics, when troubleshooting the air plug fault, the key is how to discharge the bubbles in the pipeline. According to different situations, the following methods can be adopted.

a. Partial circulation exhaust method. Open the emergency control valve to connect the upper pipeline with the lower pipeline, and then open the hydraulic pump to make it work, so as to realize the circulation of some hydraulic oil in the hydraulic oil tank, part of the upper pipeline and lower pipeline, so as to make this part of the hydraulic oil flow back to the hydraulic oil tank and discharge the bubbles in the pipeline. Since the emergency control valve is located on the side of the pipeline close to the hydraulic pump, it can only remove the air plug in the pipeline close to the hydraulic pump, while the pipeline close to the retraction actuator can not realize circulation and can not eliminate the bubbles in the pipeline on this side.

b. Sectional exhaust method. Through the retraction and retraction of the landing gear, judge whether the air plug occurs in the retracted pipeline or the lowered pipeline. According to the maintenance experience, the air plug mostly occurs in the retracted pipeline. The troubleshooting method is: disconnect the joint of the retracted pipeline in sections, manually open the landing gear lock, pull the front landing gear to retract it in place (before connecting the disengaged joint, keep the landing gear in the retracted position, otherwise lowering the landing gear immediately will make more air enter the pipeline), During the retraction process, the actuator is compressed to discharge the hydraulic oil in the retraction pipeline, and the bubbles are discharged. Then immediately connect the disconnected joint, manually pull the front landing gear and main landing gear to put them down in place, and the actuator extends. At this time, the clean hydraulic oil of the hydraulic oil tank enters the pipeline. Repeat the operation for several times to discharge the hydraulic oil with bubbles in the pipeline.

② Internal or external leakage of the system according to the structural characteristics of the system, the parts prone to internal leakage of the system are the emergency control valve (sealing rubber ring), the actuator (piston rubber ring) and the high-pressure one-way valve in the hydraulic pump distributor. The part prone to system leakage is the actuating cylinder (actuating rod sealing rubber ring).

In case of internal or external leakage of the system, it takes a long time to retract or lower the landing gear. After the landing gear is retracted and locked in place, the hydraulic pump motor works frequently. This is because the pressure in the pipeline cannot be maintained due to internal or external leakage. After the pressure induced by the pressure switch decreases, the motor circuit is frequently connected, resulting in frequent operation of the motor. The troubleshooting method is to replace the corresponding faulty rubber ring. For example, after the landing gear was retracted in place, the motor worked frequently, working once per second on average. After disassembly and inspection of the left landing gear retraction and retraction actuator, it was found that the piston sealing rubber ring of the actuator was obviously scratched, resulting in oil leakage between the upper chamber and the lower chamber of the actuator, so that the pressure of the retraction pipeline could not be maintained. After replacing the actuator, the fault is eliminated.

③ The pressure switch fails. The pressure switch is located in the upper pipeline and is a pressure capsule switch. After the landing gear is retracted in place, when the pressure of the retraction pipeline reaches (1400 ± 70) psi, the pressure switch disconnects the motor circuit, the motor stops working, the one-way valve of the distributor cuts off the retraction pipeline, the pressure oil is closed in the pipeline, and the pressure oil keeps the landing gear in the retraction position.

If the capsule is deformed and cracked due to fatigue, the action pressure of the switch will change. In one case, when the action pressure decreases and the pressure of the retracted pipeline has not reached (1400 ± 70) psi, the pressure switch cuts off the motor circuit and the motor stops working. The fault phenomenon is that the motor continues to work during the retraction of the landing gear, and the retraction time is too long, or even cannot be retracted to the position. Another situation is that the action pressure increases, and the pressure of the closing pipeline has exceeded (1400 ± 70) psi, but the pressure switch has not cut off the motor circuit. It is necessary to increase the pressure to cut off the motor circuit and stop the motor. The solution is to replace the pressure switch.

For example, during the landing gear retraction and retraction test of an aircraft on the ground, the hydraulic pump motor worked intermittently during the landing gear retraction, and the landing gear retracted intermittently. After analysis, the action pressure of the pressure switch decreases, and the pressure switch cuts off the motor circuit prematurely. After replacing the pressure switch, the fault is eliminated.

(3) System maintenance precautions

Correct maintenance plays an important role in preventing landing gear hydraulic system failure. Pay attention to the following aspects.

① Keep the hydraulic oil volume at the gauge position. Due to the small capacity of the hydraulic oil tank of the system, too little oil should be prevented to cause air to enter the system pipeline and cause system air plug.

② When maintaining the hydraulic system, keep the system and oil clean to prevent impurities from entering the system, resulting in excessive wear of system parts and sealing rubber rings, resulting in internal or external leakage.

③ Keep the actuator rod clean to prevent impurities from scratching the sealing rubber ring of the actuator rod.

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