Aircraft Fuel Tank Access Door

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 Aircraft Fuel Tank Access Door

Aircraft carries fuel in the wing fuel tank .There are number of component in the fuel tank to control refuelling/defueling operation,measure fuel quantity and supply fuel to the engine.To carry out maintenance task such as tank cleaning,inspection,repair or modification work on fuel system components access to fuel tank is required.Thus,access door provided on the lower surface of wing which allow entry into tank  for necessary maintenance work. 

The design of fuel tank access door compliance with number of requirement some of them are no fuel leak,fire resistance,resistance to a tyre impact, resistance to impact resulting from an uncontained engine rotor failure ( UERF),lightning strike,seal friction and wing bending.



Fig 01 Fuel Tank Access Door  (Unload Bearing Panel)





Fig 02 Fuel Tank Access Door ( Load Bearing Panel )



How fuel tank access door comply with design requirements let's discuss step by step 


Fuel Leak:


                                                   Fig 03 Fuel Tank Access Door Seal


The seal on the access door prevent fuel leakage.Fuel tank access door has groove on the inner surface which allow installation of seal.When fuel tank access door is in place, the seal on door inner surface seal the area around fuel tank opening. 


Wing Bending:

The wing of an aircraft deflect upward and downward due aerodynamic load and accordingly  there will be  some flexing of the wing structural components .The flexing of wing affect the attachment of fuel tank access door to the wing structure.Fuel tank access doors are load bearing and unload bearing type.Load bearing access door carry wing flexing load and they are bolted to the wing skin to facilitate load transfer.Due to wing flexing intense effect at the wing tip,  load bearing access door are installed  outboard of wing.Unload bearing access door as the name indicate do not carry the load and due its design the door tend to be clamped either side of the wing lower skin.Unload bearing access door installed inboard of wing.


Impact Resistance:



Fig 04 New Version Unload Impact Panel 



Fig 05 Old Unload Impact Panel


Aircraft landing gear and engines are mounted inboard of wing.During an aircraft operation if landing tyre burst or uncontained engine rotor failure occured the fragment of tyre or engine rotary parts may damage the fuel tank access panel.Thus, there is requirement of access door which should be  strong to take these impact loads.Fuel tank access door inboard of wing particularly near to landing gear and engine must have high impact resistance quality.Unload bearing panel inboard of wing are impact resistance panel.Impact resistance panel consists of two panels called inner panel and outer panel.Inner panel are inside of fuel tank and outer panel located outside of fuel tank flushed with lower surface of wing.These two panels clamped on either side of doors.The older aircraft were uses high thickness single impact panel with clamp ring.The biggest drawback of  older panel is fuel sometimes enter the hollow structure of panel.The elimination of clamp ring and use of two panels in new aircraft unload panel allow easier installation and save the weight.  Modern aircraft impact resistance panel made from superplastic forming titanium alloy material which carry impact load effectively without affecting panel seal integrity.


Fire and lightning strike Protection:

When aircraft struck with lightning,the aircraft structure should conduct lightning electrostatic charge effectively throughout its body.Differnance in electrostatic potential between body causes electrostatic discharge or arcing.Thus, every parts of an aircraft bonded to keep electrostatic potential equal.To prevent electrostatic discharge or arcing at fuel tank access door, there must be continuous path for electrostatic charge.Load bearing access door directly bolted to wing surface so it provides continuous path for electrostatic charge.As we know the unload bearing panel clamped on wing skin there is possibility of discontinuation between panel and wing skin which may cause electrostatic discharge or arcing and this will lead to fire hazard condition.To prevent such occurrence manufacture placed metal knitted gasket between wing skin and panel which ensure continuous path for electrostatic charge.



Fig 06 Unload Impact Panel Drain Hole


Unload bearing panel( Impact resistance panel) consists of two panel inner and outer panel and there is void area between these two panel.If seal on inner panel damage it will cause fuel leakage in void area.Drainage of fuel from void area is necessary to prevent fuel vapour formation which may cause fire hazard condition.Drain hole provided on outer panel to drain the fuel from void area.This two holes on panel ensure ventilation of void area. 


Seal Wear and Corrosion:

When aircraft wing flex there will be relative movement between  knitted gasket and wing skin or access door, this may damage the protective treatment.The ingress of water between access door and wing skin interface area cause corrosion.Thus, It is recommended that whenever access door opened for maintenance  knitted gasket should be thoroughly check for wear and any sign of degradation and before installing access door lubricate gasket with grease for corrosion prevention.


Access Door Mismatch Prevention:



Fig 07 Access Door Mismatch Indicator


In large aircraft a significant number of wing access doors are present, many with similar or identical size and planform. These wing access doors are designed with different strengths depending on location of panel on the wing.The fuel tank unload bearing access door inboard of wing must have impact resistance quality and load bearing access door outboard of wing efficiently transfer wing load thus load bearing access door will not be as strong as unload bearing access door.Both access door have different strength.To prevent certain doors are not installed at incorrect location some type of physical marking arrangement provided on door.In picture radial protrude part is indicator  for panel mismatch prevention.

 


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