The landing gear is the structure that supports the entire weight of the aircraft on the ground. It lets the aircraft takeoff, land, and taxi on its wheels. The landing gear structure has to absorb the greatest impact during landing and has to be robust enough to do it many times daily throughout the life of the aircraft. It is designed to absorb all the energy and dissipate it harmlessly so that the passengers and the cargo in the aircraft remain safe. It has to tolerate extreme acceleration and deceleration routinely as part of its daily operating conditions. Its shock-absorbing capabilities are its most important feature. The principal component that acts as a shock absorber in the landing gear is the shock strut.
Shock struts, which are also called oleo or air struts, use a combination of compressed nitrogen or air and a specific hydraulic fluid which helps to reduce the impact of landing. Most of the large aircraft and passenger airliners are equipped with shock struts. You may find shock struts on some models of small airplanes as well.
A combination of nitrogen and specified hydraulic fluid are used in a standard shock strut which helps to absorb the impact of the aircraft load on the tarmac and dissipate it. The design of the shock strut is rather simple with two telescoping cylinders that are closed on either end. The upper cylinder is firmly fixed to the aircraft airframe and does not move at all but the lower cylinder provides all the cushion as it moves in and out of the upper cylinder and it is fixed to the wheels of the aircraft via the axles.
The lower cylinder is filled with hydraulic fluid and the upper cylinder is filled with nitrogen. There is an orifice placed in between these two cylinders. The fluid can pass from the lower cylinder to the upper one through this orifice when the strut gets compressed. Most shock struts use a metering pin to control the fluid flow rate from the lower cylinder to the top cylinder.
As the aircraft comes to land, and the wheels touch the tarmac shock strut goes into compression mode. With the weight of the aircraft pushing it down, the strut compresses even more as the lower piston is pushed into the upper cylinder. The metering pin moves up through the orifice. The rate of fluid flow from the bottom cylinder to the top cylinder is controlled by the taper of the pin during this compression. The kinetic energy is converted into heat and is dissipated through the strut in the atmosphere.
When the downward stroke reaches the end, the gas in the upper cylinder is compressed even more which finally stops the compression stroke of the strut without any significant impact. Now as the aircraft is fully on the ground and as it taxis, the tires and the strut combine to act together to smooth out any bumps.
If there is insufficient fluid or gas in the strut, the compression stroke will not be effective. The strut could hit the end and this will impact the airframe directly, which is to be avoided. If the strut is properly maintained and kept in good condition the extension stroke of the shock strut operation will occur at the end of the compression stroke. The energy stored in the compressed gas in the upper cylinder will cause the aircraft to move slightly upward as the strut gradually returns to returns its normal position. The fluid goes back into the lower cylinder through the orifice.
When the aircraft takes off the shock strut extends quickly which may cause an impact at the end of the stroke and damage the strut. To avoid it the shock struts are fitted with a recoil valve to prevent this impact. This valve controls the fluid flow during the extension stroke and thereby acts to slow the motion of the cylinder and avoid any impact damage.
To keep the nose wheel aligned its gear shock struts are provided with a locating cam assembly. A cam extension is attached to the lower cylinder of the strut, and a matching cam recess is fixed to the upper cylinder. When the landing gear is down and the nose strut is fully extended these cams line up the axle assembly so that the wheel in the straight position. This also ensures that when the nose wheel is retracted it enters the wheel well correctly.
The wheel axle is attached to the strut, so the struts are an integral part of the landing gear system of an aircraft. If the axle is not part of the strut then some other mechanism is provided so that the struts provide a connection between the wheels and the airframe of the aircraft.
It is essential that optimum fluid level and gas pressure is maintained in the struts for their proper functioning. However, during the servicing of the struts, to check the fluid level, the struts have to be in a fully compressed and deflated state. It is dangerous to deflate the shock strut without taking adequate precautions during maintenance. The technician must be fully trained in this procedure and it should be followed strictly when opening the high-pressure service valve of the strut’s upper cylinder.
There are various key steps while servicing a landing gear shock strut. This includes deflating the air from the strut and removing the top strut service valve to top up the hydraulic fluid to the cylinder. The strut must always be serviced when it is vertically placed, in the landing gear down position.
Following is the procedure to service the landing gear strut:
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