Created on:2021-10-11 10:46

Aircraft hydraulic fault diagnosis and maintenance_ Oil leakage of component pipeline

Aircraft hydraulic fault diagnosis and maintenance_ Oil leakage of component pipeline

The control surface control, landing gear retraction, braking, aircraft turning, aircraft engine reverse thrust device and other systems of B757-200 aircraft are inseparable from the action of hydraulic components. Because these components and pipelines usually work under high pressure and are located at the moving parts, the load of components and pipelines is large, and oil leakage occurs from time to time, which often leads to difficult aircraft operation, resulting in aircraft return and alternate landing, and flight delay.

(1) Oil leakage position and cause analysis

The hydraulic system of B757-200 aircraft is divided into three independent systems: left, middle and right. The left and right systems have an engine driven pump EDP and an AC motor pump ACMP to pressurize the system, and the middle system is pressurized by two ACMP pumps. After being pressurized by EDP pump or ACMP pump, the hydraulic oil of each system is transmitted to relevant system actuators to provide power for flight control, aircraft front wheel turning, braking and other systems. The power conversion unit PTU consists of a hydraulic motor and a booster pump. When the booster pump of the left system does not work, the right system hydraulic drive motor drives the hydraulic pump to pressurize the left system, which can realize the power exchange between the right system and the left system. The ram turbine rat is automatically released when the left and right engines stop in the air, and the turbine drives the hydraulic pump to pressurize the medium system and provide power in case of emergency. The normal operation of hydraulic components and pipelines is the basic guarantee to ensure the rapid transmission of aircraft power and realize flight control and control.

① Common oil leakage parts of hydraulic system components and pipelines (ata29)

a. Engine driven pump EDP, oil filter assembly and pipeline EDP pump is a pressure compensated variable displacement piston hydraulic pump, which is driven by engine accessory gearbox. The B757 fleet had many incidents of hydraulic oil leakage due to the rupture of the pump shell, resulting in difficult aircraft operation. Due to design reasons, the EDP pump shell is thin and easy to crack under high pressure. In addition, the damage of the sealing gasket between several joint surfaces of the EDP pump is the main cause of oil leakage of the EDP pump. The EDP pump oil filter assembly is located in the engine hanger. Due to the aging of the differential pressure sensor of the assembly, a large amount of hydraulic oil leakage also occurs from time to time, which is the key part of the inspection. Some airlines have included the inspection of differential pressure sensor of EDP oil filter assembly in regular inspection. Oil leakage often occurs at the connection between the EDP pump oil supply pipe and the oil filter assembly in the left and right engine hangers, which is caused by the pipeline itself and the aging of the sealing ring.

b. The pressure supply pipeline between the AC motor pump ACMP and the oil filter assembly, the pressure supply hose between the ACMP pump and the oil filter assembly is broken, resulting in oil leakage from time to time. The fault is characterized by the leakage of hydraulic oil in the system. The aging of hose during long-term use and the corrosion of hydraulic oil are the basic reasons for its rupture. Such failures occur in the left, middle and right systems. The failure rate of pressure supply hose rupture or oil leakage between the left ACMP pump and the oil filter assembly is higher than that of the other two systems. It is analyzed that the reason is that the pipeline working load of the left system is higher than that of other systems and the working time is longer than that of other systems.

② Common oil leakage parts of landing gear braking system (ATA32)

a. Main landing gear damping strut. The vibration damping strut of the main landing gear is often delayed due to the damage of dynamic and static sealing rings and oil leakage (non hydraulic system oil). The dynamic and static sealing rings are damaged for the following reasons: the sealing rings are normally worn; Quality problems of sealing ring; Assembly of sealing ring; The damping strut is not clean.

b. The main landing gear retraction and retraction actuator, hatch retraction and retraction actuator and anti-skid brake valve assembly are common oil leakage components of the landing gear braking system, in which the oil leakage of hatch retraction and retraction actuator often occurs on the left. In addition, the anti-skid brake valve assembly is also the part with more oil leakage. The oil leakage of the above components is mainly due to the damage of the sealing ring inside the assembly.

c. Landing gear and brake system hoses. The hydraulic hoses in some parts of the landing gear braking system are easy to break, resulting in a large amount of hydraulic oil leakage. In addition to the causes of hose aging and hydraulic oil corrosion, it is also related to its working position and working condition. The hose of landing gear lower lock actuator is the part with many cracks, which often leads to major safety events such as aircraft return, turning failure and so on. Due to the position of the hose, the hose will bend during the retraction and retraction of the landing gear, the curvature will change greatly, and the bending is unnatural. These factors are important reasons for its rupture. The hydraulic hose of the horizontal actuator of the landing gear four-wheel trolley and the brake hose on the upper part of the main wheel also burst from time to time. During the retraction and retraction of the landing gear, these hoses collided with the nearby structures, causing hose damage.

③ Oil leakage position of flight control system components (ata27)

The oil leakage of B757-200 aircraft flight control system components is mainly concentrated in the vertical tail rudder.

a. Yaw damper servo actuator. B757 aircraft has two yaw damper servo actuators, which are the parts with high oil leakage. Due to the frequent activities of yaw damper, its servo actuator is in a non-stop working state. The inner sealing ring of the actuator is easy to be worn. Boeing has issued a series of service announcements to reduce the working sensitivity of yaw damper and change the additives of actuator sealing ring materials to increase the wear resistance of sealing ring. The oil leakage of yaw damper servo actuator has been controlled to a certain extent. However, due to the working characteristics of this part, oil leakage still occurs from time to time.

b. The action of vertical tail rudder is simple. The vertical tail rudder integrates the ratio actuator, autopilot servo actuator and pea actuator. The common oil leakage parts include PCA and rudder ratio actuator, which are partly caused by the aging of the sealing ring.

(2) Measures

In order to reduce the occurrence of flight safety and flight delay caused by hydraulic component pipeline oil leakage, the following aspects should be done well.

① Carefully collect the data of oil leakage parts (bits) and analyze the fault characteristics. B757-200 aircraft has been introduced into China for many years, and airlines have accumulated certain experience in maintenance. It is necessary to collect the data of oil leakage hydraulic parts and pipelines, analyze the oil leakage characteristics and rules, find out the common oil leakage parts and oil leakage parts, and guide the inspection of workers. At the same time, the inspection interval shall be reasonably determined, and the inspection of some important parts shall be included in the regular inspection work. For the hose of landing gear lower lock actuator, the service time when it fails can be counted and replaced within a certain interval.

② Strengthening inspection and high-quality inspection is not only the guarantee of maintenance quality, but also the most basic and effective means to find oil leakage of hydraulic components in time. Different inspection methods shall be adopted for different parts. For example, detailed visual inspection shall be carried out for the hose, focusing on whether the hose is broken and whether it rubs with nearby structures. It is best to touch by hand during the inspection. If the external protective layer of the hose is broken, replace the hose in time to avoid hose rupture. At the same time, the inspection should be completed by personnel with sufficient work experience.

③ Install the landing gear seal ring correctly in strict accordance with the requirements of the AMM manual, and strengthen the cleaning of the damping strut, which can play a good role in reducing the oil leakage of the landing gear damping strut.

④ Actively consult the relevant service notice sb and service letter SL to obtain information, timely complete the corresponding modification, and reduce the oil leakage of hydraulic parts and pipelines from the source.

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