Aircraft Landing Gear Shock Absorber Gland and Spare Seal

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When you are in the aircraft and as aircraft lands have you felt sudden impact of load on your body? May be not because these impacts load taken by aircraft landing gear. In some Situation you may felt impacts load due to hard landing of an aircraft. 

Aircraft Landing Gear act like legs of an aircraft similar to human body legs. As, we stand our body weight transmitted on ground through our legs. Similarly, aircraft weight  transmitted on ground through its landing gear. Landing gear also take load at time of  takeoff, landing and ground maneuvering.

But, How this load transmission take place on ground through human body and aircraft landing gear ? In Human body we have bone and viscoelastic muscle ( between leg joints) in our legs which take body weight and transmit on ground. Aircraft landing gear has shock strut and shock absorber. Shock strut work as bone and shock absorber act like viscoelastic muscles.

How the shock absorber of landing gear absorb the impact load ? Although, it is big topic of discussion. Briefly, Aircraft landing gear consists of two telescoping hollow cylinder and hollow parts of cylinders filled with air and oil. Telescope cylinder act like shock strut and mechanism  inside of cylinder which compress and expand air and meter the oil flow through internal passage
act like shock absorber.

Fig 1 Telescope Cylinder






  Fig 2 Shock Absorber With Upper Cylinder, Lower Cylinder ,oil and air with Internal passages

Landing gear consist of two telescope cylinder one attached with aircraft structure called upper cylinder and other provide attachment to wheel called lower cylinder.

When aircraft lands the load of aircraft transmitted to landing gear shock absorber through wheel. The Lower Cylinder of shock absorber moves inside of upper cylinder which cause movement of oil through small internal passages of cylinder and this will compressor air. When compression of air reached to its threshold limit the force of air act on oil and there will be movement of lower cylinder from upper cylinder. The movement of lower cylinder inside and outside of upper cylinder  continue till shock load minimized.

Air and Oil has very important role for taking impact load. Shock absorber unable to perform its function in absence of air and oil.

Consider the Situation where aircraft landing gear loosed its air and oil  in shock absorber  and as the aircraft lands, Due to absence of air and oil in shock absorber will act like solid rod which may damage the aircraft structure and landing gear itself and this situation may affect safety of aircraft and its occupant. 

Continue Retention of air and oil in shock abrader is important for operation of shock absorber but we know that air and oil has quality which can pass or leak through any minute hole or space. So, prevent leakage of oil or air we normally use seal which fills minute holes or spaces.

Seal is very important for operation of shock absorber and seal could be damage during aircraft operation. If seal damaged, there will be leakage of air and oil  from shock absorber and as a result of this aircraft will be unserviceable for operation. To make aircraft serviceable than shock absorber seal has to  replaced.

To carryout seal replacement maintenance task , it may take longer time and airline may have to cancel its flight which will affect its revenue and passenger convenience . Aircraft manufacturer always take these expected situation into consideration during aircraft design phase .Most of  Landing gear manufacturer  installed two seal in shock absorber one is called gland seal  and other one is spare seal. 

Gland seal is main seal which is always in activated condition until there is no damage in seal. If any damage occurred on gland seal there will be leakage of oil through seal and  this will be indication to flight crew  or Aircraft engineer which says the condition of seal. To prevent further leakage of oil aircraft mechanic will activate spare seal and allow aircraft to continue in operation without affecting airline revenue and passenger convenience.

                                                   
                                                            Fig 3 Spare Seal Activation Valve


There is Spare seal activation valve on aircraft landing gear which allow aircraft mechanic to activate spare seal. How to activate spare seal procedure described in aircraft maintenance manual.

After activation of spare seal we can not leave it as activated and forget because it may damage in operation or after certain time. If spare seal damaged then no option other than grounding of aircraft so every manufacture defines certain time limit after that  both seal of landing gear shock absorber should be replaced.

In further paragraph will see what scientific  principle used by manufacturer to  activate gland seal and spare seal.

Oil in shock absorber work as hydraulic fluid and this help to apply force on seals. Differences in pressure of  hydraulic fluid across the seal create the force which will apply on seal for activation. 

Please note marked  Area1,Area2 And Area3 in fig 4 and 5 and flow of oil though these areas.

                                                              Fig 4 Gland Seal In  Activated Condition

                                    


                           

                                                                Fig 5 Spare Seal In Activated Condition 

Gland Seal In Activated Condition

In this case gland seal in contact with lower cylinder and spare seal separate from lower cylinder.

In Fig 4, Oil flow from Area1 to Area3 through Area2.Spare Seal Activation Valve on landing gear form the path for Area2.At Area3, Oil pressure applies on upstream of gland seal and downstream of spare seal. At Area 1, oil pressure acting only upstream of spare seal. Same amount of pressure acting upstream and downstream of spare seal and due to equal pressure force not created which cause spare seal away from lower cylinder. Oil pressure at Area 3 acting only upstream of gland seal and there is zero oil pressure downstream of gland seal. Due Differnance in pressure across the gland seal create force  will move the gland seal and make the contact with lower cylinder.

Spare Seal In Activated Condition

In this case gland seal separate from lower cylinder and spare seal in contact with lower cylinder.

To Activate spare seal aircraft mechanic will move spare seal activation valve steam which will block the path of area 2.

In Fig 5,Oil Flow at Area3 is stop due path for area 2 is blocked by valve steam. Now, Zero oil pressure acting downstream of spare seal and upstream of gland seal. Oil pressure acting only upstream of spare seal at area 1.Differnance in pressure upstream and downstream of spare seal create force will move spare seal and make the contact with lower cylinder. Zero oil pressure acting on downstream and upstream of gland. Due to equal pressure, Force which were acting on gland seal removed and seal moved away from lower cylinder.

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