The mysterious X dimension

Flying Safety, August, 2002 by Chad Hogan, Craig Pessetto

In the past six years the USAF has had at least seven mishaps where improperly serviced landing gear shock struts have played either a direct or contributing role. This article is to help field maintenance crews understand the importance of properly servicing shock struts. Improper servicing can cause damage to the shock strut, adjacent landing gear and airframe. In the worst case, it can cause the shock strut to fail completely, resulting in the loss of aircraft and crew.

The Basic Design Of A Shock Strut

Most Air Force aircraft use an oleopneumatic (oil/gas) shock strut. The main purpose of the shock strut is to alleviate load on the airframe and to cushion impact during landing. The oleo-pneumatic shock strut is the most efficient of all shock absorbers and is the best in energy dissipation.

These topics are very general and apply to all struts, but there are some distinct differences in various aircraft types. Some struts, like those on the A10 and C-5, have oil over air, but the principle is the same. Oil is still forced from one area to the other through a metering system. All struts on the F-15 and F-16 have dual gas chambers in addition to a single low-pressure oil/gas chamber. This is also the case in the B-1, C-5 and B-2 nose gear. Dual chambered struts are slightly more difficult to service properly.

Most oleo-pneumatic shock struts have an upper and lower chamber. The shock strut is filled with fluid (oil such as MIL-PRF-5606 or MIL-PRF-83282). Shock struts are designed such that filling a strut to the top servicing port with oil while it is fully compressed sets the oil volume to the correct level before it's pressurized. The strut is then extended and the space above the fluid is pressurized with gas (nitrogen or dry air). An oil metering system is used to control the rate at which oil moves from one area of the strut to the other on compression. On extension, another system (termed a snubbing system) controls the rate of extension by metering the return flow. The metering system and strut are designed to accomplish two things. First, the strut is designed to convert some of the aircraft's kinetic energy into heat energy (a brake does the same thing). This way, the aircraft structure does not see full landing loads and the absorbed energy dissipates as heat energy after the aircraft has landed. Second, struts can effectively reduce the peak load an airframe experiences by spreading out the total landing energy over the time it takes to complete a compression stroke.

The Operation Of A Strut

In order for the shock strut to function properly, it must have the correct gas-to-fluid ratio. Physical laws dictate that when a strut is serviced with the correct volume of gas/oil, it will follow a specific air curve while being compressed. When a strut has the wrong fluid-to-gas ratio, the following can happen.

1. Not enough fluid:

If the fluid level is extremely low, it will also have reduced snubbing (rebound dampening) capabilities. This can result in a violent extension. When the aircraft takes off or bounces on landing, the piston assembly will slam against the gland nut, which could result in damage to the shock strut. A common effect of snubbing loss is for internal strut components to deform, causing struts to bind in a certain position. Another result is a separation of the piston, wheel and brake a ssembly from the shock strut. An important fact to remember is that in even the smallest struts there is enough explosive pressure energy to equal one to three sticks of dynamite.

When the fluid level is significantly low due to improper servicing or leaks, the shock strut will not be able to absorb landing energy effectively. The airframe will experience higher peak loads because of a shortened stroke interval. Increased loads represent a greater chance of damage to the airframe and strut. Occasionally, depending upon strut geometry, this can allow the strut to bottom out with metal-to-metal contact during normal landings. This impactive loading can lead to failure of the shock strut or supporting structure.

This condition is exacerbated when continued "re-servicing" at preflight to meet the X-dimension requirement at that given weight yields an improper gas to fluid ratio. Doing this can increase gas pressure to a dangerous level. The topic of 're-servicing" is discussed in depth below.

2. Too much fluid:

When fluid levels are high, the gear becomes too stiff. This results in a decreased ability to absorb energy and may cause the strut to rupture under heavy aircraft gross weights or hard landings. High fluid levels also decrease the allowable stroke of the strut, reducing its ability to dampen peak loading. This condition is less common and is usually a result of confusing or inadequate technical data for initial strut servicing.

The methods used to determine proper X-dimension values vary between aircraft. X-dimension can be generally defined as the amount of chrome showing on the strut. Every maintainer who works with landing gear should be familiar with the strut servicing instruction decal on the outside of each shock strut. There are three basic types of instruction decals: