Created on:2021-10-11 10:46

Aircraft hydraulic fault diagnosis and maintenance

Aircraft hydraulic fault diagnosis and maintenance

In 2007, the frequency of fuel tank ventilation failure of a company exceeded that of pipeline failure, which became the primary cause of flight delay.

On June 15, 2007, when an aircraft was cruising, the pressure of the left hydraulic system instantly decreased to zero, while the hydraulic oil increased abnormally, and the system soon returned to normal. After the flight, it is considered that the vent valve of the hydraulic oil tank of the left system did not work normally at that time, resulting in the gas in the oil tank could not be discharged in time, resulting in virtual high oil volume and system air plug. After replacing the tank, check that the bleeder valve works normally.

In order to find out the real cause of the fault, the hydraulic system was repeatedly pressurized and tested, and no abnormality was found. Later, when preparing for the ground test run to test another troubleshooting result, it was accidentally found that the low pressure light of the reverse thrust accumulator of the left engine was on. After inspection, it is found that there is hydraulic oil in the inflation chamber of the accumulator, so it can be judged that it is an internal leakage fault. After replacing the faulty accumulator, the whole left hydraulic system works normally, and no similar fault has occurred so far.

What causes the system to suddenly produce a large amount of gas in a short time, resulting in the illusion of abnormal increase of hydraulic oil?

Through the analysis of the hydraulic system, main components and accessories, pressurization and ventilation of the hydraulic oil tank and the gas source in the hydraulic system of MD-82 aircraft, it is found that the volume of nitrogen in the accumulator will increase dozens of times after entering the hydraulic oil tank due to the failure of the diaphragm seal of the accumulator. Only take the accumulator with the lowest pressure and the smallest volume as an example. The pressure is 675 ~ 50 psi and the volume is about 3 quarts (1 Quart = 1.1365 L). If all the nitrogen in the accumulator enters the system and the hydraulic oil tank, the expanded volume of these gases in the oil tank can be obtained by the following formula:

Volume after expansion = accumulator volume × Accumulator absolute charging pressure / tank absolute pressure

=3 × (675 + 14.2) / (30 + 14.2) = 60.5 (quarts)

The volume of nitrogen will expand to more than 20 times of the original volume, greatly exceeding the total volume of the oil tank. At this time, the oil volume felt by the hydraulic system must be much more than the original oil volume. Moreover, such a large amount of gas entering the oil tank will make the hydraulic oil in the oil tank "boiling". It is inevitable that a large amount of gas will be mixed in the oil inlet pipeline of the hydraulic pump, which will further lead to unstable operation of the hydraulic pump, swing of system pressure, and even air plug. At this time, the temperature of the system will not change much and will not cause overtemperature warning of the system.

It can be seen that the failure of aircraft hydraulic system is caused by the failure of left engine reverse thrust accumulator diaphragm. This may indicate a trend for the McDonnell Douglas fleet with an average age of about 13 years, that is, with the increase of aircraft age, the aircraft components will gradually age, especially the performance of chemical and rubber products will decline significantly. Necessary preventive measures must be taken. The author believes that the work should be carried out from the following aspects.

① Technicians should deepen the research on the hydraulic system, find out the regularity of system faults, and provide a theoretical basis for taking targeted preventive measures.

② Maintenance personnel shall increase the work content of daily maintenance and find the fault precursor as soon as possible.

③ Formulate specific methods of system inspection to make them institutionalized, programmed, wired and visual.

④ Adjust the maintenance scheme according to the current situation of the fleet and the obtained reliability data, and strengthen the time limit control of components. At present, the pressure hose of MD-82 aircraft hydraulic system is a non time control, but it has been found to be invalid in practical work. The first reason is that the inner tube of the hose will gradually age over time. The second reason is that the outer layer of the hose is a metal braided net. Under the action of pressure pulsation, there will be small displacement between the metal wires, resulting in mutual friction and cutting, and then induce pipeline failure. There are chemical rubber products in the six accumulators on the aircraft, and there are rubber seals in each valve and actuator of the hydraulic system, which will age and fail. If we do not take corresponding inspection, test or time control for the above problems, it will inevitably lead to serious consequences.

⑤ If necessary, refit or improve the hydraulic system to overcome the inherent defects of the system. The MD-82 fleet managed by the company had concentrated on the disconnection of the hydraulic pipeline in the rear accessory cabin in previous years. The main reason was that the hydraulic pipeline in the rear accessory cabin was pre-stressed and poorly fixed, which was easy to vibrate during aircraft flight, resulting in pipeline nut loosening, pipeline failure and other faults. The plan given by the aircraft manufacturing company is to replace some hard pipes with hoses, adjust the direction of some pipelines, improve the fixation of pipelines, and temporarily improve the reliability of the hydraulic system. However, the hydraulic hoses in the rear accessory cabin have strong pulsation, the rubber rings of the fixing clips are often worn, and the hoses are often worn and leaked, At the same time, the pulsating hose will scratch any parts in contact with it and the structure of the aircraft, resulting in many secondary faults. If the original scheme given by the manufacturer is to adjust the pipeline direction to eliminate stress, improve pipeline fixation, reduce pipeline flutter, and add a fuse on the joint nut to prevent it from loosening, the effect may be better.

⑥ Re explore whether it is necessary to implement various service letters and technical bulletins. In the early stage, because the fleet was still very young, it was really unnecessary to implement the above letters and notifications, but implementation at an appropriate time would greatly improve safety and benefits.

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