Causes and Countermeasures for automatic landing of aircraft landing gear
Since an aircraft has been equipped with an army unit to perform engine test on the ground, a severe accident has occurred repeatedly on the landing gear. The common characteristics are: the landing gear handle (the handle) are in the neutral position; the engine speed to idle speed, the hydraulic system pressure close to the maximum value of 21MPa. Because of this failure has occurred many times, the consequences are particularly serious, and the reasons for the barrier is not clear. In order to prevent such failures, the army requires that the handle be pulled down before the test run. Although this can prevent the occurrence of such failures, but will make the electromagnetic switch because of long power on and easy to burn. Also, because the landing gear is good, also requires the handle back to the position, and according to the principle of landing gear that, when the handle is in the median and the retraction system is working properly, the front landing gear is not retracted, which makes maintenance personnel in the test, will often forget the lever to the down position. A serious accident of the automatic landing gear of the landing gear occurred during the test run without pulling the handle from the middle to the lower position. Therefore, only by making a clear understanding of the causes of the failure and taking specific measures, can some accidents be effectively prevented.
1. landing gear working principle
The landing gear adopts hydraulic principle to carry out the principle of retracting and retracting, as shown in figure 23-4. By the handle control solenoid valve 1, when the handle to the left down position, the solenoid valve coil L is energized to make oil and put down the pipeline connected, seven oil return pipeline and communicated with the front landing gear hydraulic cylinder piston rod of the 3 indent, before the gear down; when in place, the handle in return, solenoid valve power, to cut off oil, collect and put down the pipeline and oil is the down position hydraulic overflow valve lock 2 and lock down (not shown in figure 23-4) maintain. When the handle to the retracted position when the coil is energized, right, up to the oil pipeline, first open the hydraulic lock, then the oil into the hydraulic cylinder, the piston rod, open the lock down, up the front landing gear; when up and handle to the mid position, the front landing gear are locked to keep the position in charge. The one-way valve 4 in the pipeline is used to prevent other parts from working, and the return pressure is transmitted to the undercarriage to cause the automatic operation of the part.
2. automatic retraction reason
From the landing gear of the front landing gear, you must first open the hydraulic lock and put down the lock, and then open the 2 locks and close the line with a certain amount of hydraulic fluid. When the handle is in the middle position, only when the solenoid valve fails or the one-way valve is in the open position and the return pressure is higher, the hydraulic oil which receives the pipeline will have a certain pressure. Therefore, through the analysis of the actual test environment and the take-off and landing mechanism of the landing gear, the reasons for the automatic landing of the landing gear are as follows.
(1) return oil pressure is high and the one-way valve is not sealed from the working principle of the system, when the landing gear lever in the neutral position, and if oil pressure is high, and the one-way valve is not sealed, the oil will have a certain pressure on the landing gear in the pipeline, when the oil return pressure raised to a certain extent will open before the main landing gear, hydraulic lock (according to the experiment, the hydraulic lock opening pressure is generally not more than 1MPa). For the front landing gear, as far away from the center of gravity of the aircraft, the aircraft under gravity is small, resulting in the front wheel and the ground friction is small, easy to make the front landing gear retracted; in addition, the action area of the hydraulic cylinder piston on both sides are not equal, the pressure under the action of the piston will produce a force of the piston rod extends out, open before the landing gear lock, even if there is no oil pressure test in the plane under the action of gravity and engine thrust will before retracting landing gear. While the main landing gear from near the center of gravity of the aircraft, by force, the main wheel and the ground friction, coupled with the hydraulic lock open, but also must have a 5MPa about the oil pressure in the hydraulic cylinder to open the card ring lock can be the main landing gear retract. Therefore, the return pressure is generally insufficient to retract the main landing gear.
The one-way valve 4 is not sealed, after the analysis of the actual use, there are spring fatigue, ball indentation, scratch, pitting and oil contamination of the ball valve in the open position of the card. The system back to the oil surface caused by high pressure, one is due to the oil filter blockage, long-term use of some accessories make the gap is too large and the damage caused by the sealing device; on the other hand, the same aircraft test state, from a statistical point of view, the fault plane is generally a few days no parking then, under the gear plate and 2 side plates are all open reduction. So, in flight test, with the pressure increases to a certain value, the 3 reduction board will quickly put away, resulting in the phenomenon of excessive instantaneous return oil pressure; in addition, the hydraulic system at the beginning of the work of oil temperature is low, the gap between the various sealing device is relatively large, from the surface caused by the increase of leakage, but also will the oil return pressure.
(2) electromagnetic valve work is not normal, the aircraft landing gear hydraulic system using YDF-12 electromagnetic valve, its structure as shown in figure 23-5. It is characterized by the use of ball valve instead of column plug to control the oil distribution of the plunger, to avoid the defects of small piston seizure and easy; the wet electromagnet, namely 2 electromagnetic core chamber group communication and oil return connector, and a groove is arranged on the movable iron core, core to ensure flexible movement and cooling, the with the plunger is a flat type (i.e. on the two sides of the piston and the inner wall of the casing is flat and contact), good sealing. Although these characteristics improve the performance of the solenoid valve, but in the case of hydraulic oil pollution, it will make its work is not normal and cause trouble.
1) movable core difficult to move. The 2 group of electromagnetic solenoid valve core chamber are in communication, and the return oil line, once the hydraulic oil contamination, especially contains water and solid particles, is easy to corrosion and friction core activities more difficult to move. By the electromagnetic valve working principle can be seen, open the ball valve by electromagnetic suction F oil, and closed only by the ball valve oil pressure F oil, due to F oil is smaller, when the friction F friction larger, will make the ball valve can not be closed. In the face of this type of aircraft, the design of the hydraulic system, the lack of consideration of pollution prevention system, make its anti pollution ability is poor, and the maintenance work of pollution prevention of hydraulic system is not doing enough, the hydraulic system is vulnerable to pollution, the activities of iron corrosion; and the magnetic material particles in the oil can be adsorbed on the moving core on such core moving friction will increase, which will be stuck. It is understood that the aircraft before each test, the crew to check the power of the electromagnetic valve, is often the undercarriage of the handle to pull down and retracted position, around the coils, such as solenoid valve working voice heard, that it is working, and then pulled a handle, at this time around because no reset spring and ball valve in the open position. When the movable iron core corrosion, although the test with the engine speed increases the oil pressure gradually increased, but when the oil pressure is not enough to overcome the friction to the right when the ball valve is closed, it will make the oil distribution right plunger, the oil path through high pressure oil and will automatically retract landing gear. A recent failure of an automatic landing gear to retract automatically. When the solenoid valve is broken down, it is found that the movable core is seriously rusted.
In addition, when the presence of solid pollutants or ball deformation, scratches and pitting in the oil caused by the right valve is not sealed, also has a similar fault. Of course, when the movable core is corroded, the left ball valve will not be closed, but only the landing gear will be lowered.
2) the plunger of oil distribution is stuck. As the solenoid valve with oil distribution piston plane type, compared with cylindrical piston friction greater, and the gap between the valve and bushing gap is small, only 0.006~0.015mm. When the oil pollution of particulate pollutants, with the oil plunger end into a high-pressure oil, a gap could make the smaller diameter particles into the valve core and bushing, thus destroying the liquid friction between piston and liner with oil, increases the friction force when the piston moves oil distribution. On the other hand, it will lead to deformation, resulting in the gap between the plunger and bushing increases, and because the particle contamination is usually irregular shape, which makes the particles larger than the gap was also squeezed into them. When the end of the oil return, the elastic deformation is reduced, the oil distribution between the plunger and sleeve gap basically returned to normal, so some particles larger than the gap in the gap, serious when some high hardness particles will be embedded into the internal part of the bushing, causing oil plunger is sluggish or stuck, caused by automatic landing gear system. If the distribution plunger is in the right position, the automatic landing of the landing gear occurs during the test run.
Although the activity of core and oil piston card will make the main landing gear collapsed down, but because the front landing gear ratio of the main landing gear easy, and in special environment test, once found before the landing gear retracted, the first reaction is to rapidly test personnel will receive the throttle handle parking position, stop the engine however, when the engine after parking, landing gear hydraulic system without pressure. Therefore, the automatic landing of the landing gear is often occurred in the test run. The clamping of movable core and oil distribution plunger has greater randomness, so the occurrence of barrier is difficult to analyze and judge. The best way to prevent the failure is to prevent the pollution of the hydraulic system.
3. Countermeasures
According to the above analysis of the fault mechanism, taking into account the economic and feasibility of the demolition, the following measures can be taken:
1 check the check valve of the landing gear solenoid valve and return oil connector regularly, and list the work in the work.
2) periodically check the tightness of the hydraulic retraction system and check the oil return pressure of the return line. This work is appropriate 2 times a year, and the inspection contents and procedures shall be based on the maintenance procedures.
3) strictly control oil pollution. Always add, and focus, test ", we should strengthen the study on anti pollution technology and management. Regular cycle cleaning hydraulic system and hydraulic filtration (especially oil return filter), regular replacement of the aircraft hydraulic oil, this work may be carried out 1 times a year according to the situation.
4) improve the design of the hydraulic system of the aircraft to improve its reliability. The improvement of electromagnetic valve structure, increase in the reset spring activity around the core, when the power is switched off, the ball valve can be closed in the action of reset spring and oil pressure, thus improving the working reliability; in the electromagnetic valve to add a filter oil pipeline, to reduce oil pollution effect on the electromagnetic valve the installation and system; the oil return valve buffer. In the United States F-4 aircraft radar hydraulic system and hydraulic system of the same type "Yi machine similar F-8 aircraft hydraulic system, the oil return valve to reduce the impact of hydraulic buffer and reduce the pressure of oil return. Such as fighter aircraft in the simulation bench test, when no loaded oil return buffer valve; measuring system for oil return oil pressure: pressure 8.5MPa to put brakes, oil pressure 5.4MPa recovery reduction plate; when the adjusting cone, gas discharging door and deceleration plate composite work, the maximum return oil pressure in 1/100 up to 11.2MPa. When the return buffer is installed, the oil return pressure is 2.1 MPa; the oil return pressure is 1.8MPa when the speed reducer is stopped. From the above data, it can be seen that the oil return buffer valve can reduce the oil return pressure. In order to reduce the oil return pressure effectively, it is suggested that the oil return buffer valve is installed between the oil inlet and return line of the electromagnetic valve of the deceleration plate, as shown in figure 23-6.
Take the above measures and by the demonstration and test, confirmed in the hydraulic system return oil pressure has dropped to the allowable range, and the reliability of the system significantly improve the situation, can cancel the ground test should be specified and handle at "put down" position.
