Broken MLG Bogie Pivot Pin
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
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To: Golden Rivet and Black Sheep
Any answer I give is based on theory and not actual first hand knowledge of the 767 landing gear. As previously indicated I am only familiar with the truck positioner but I don’t even know how it is installed.
From a design standpoint the hydraulic lines should be bonded to the structure every so many inches of line run. The electrical lines you referenced are grounded through the circuit and this will place the electrical lines at the same potential as the ground plane of the electrical system 28VDC system and AC systems. The hydraulic lines are at the same ground potential as the airframe. Whether it is from static build up due to precipitation static or from a lightning strike the airframe and everything connected to it by bonding straps will be at the same potential. The lightning strikes in flight are transitory and when the attachment breaks the airframe electrical charge will drop to the level of the precipitation static charge. If the aircraft is struck by lightning while on the ground and the aircraft is not grounded either through bonding to the earth ground plane or by a static discharge line on the landing gear the charge will most likely flash over to earth. However if it happens and the aircraft is earthed by the discharge cable on the landing gear the charge will pass through the bogy pin and either cause it to burn or it will weld the pin to the bushing and possibly to the bogy.
Black Sheep indicated in his Email to me that there was evidence of stress corrosion. Having had a bad experience with a German Stress engineer on the A-310 program I can tell you that sometimes that stress corrosion is sometimes confused with precipitation static generated electrostatic discharge corrosion. On the part in question he identified the problem as stress corrosion and I said it was electrostatic discharge corrosion. Eventually, I was proven correct.
Of course, I could be totally wrong in this particular matter.
Here is another point to consider and although not directly related you should be knowledgeable as it relates to other problems in the flight control system or any system powered by the hydraulic systems. Hydraulic fluid conducts electricity so bonding of the hydraulic lines helps to stabilize the electrical currents in the fluid. However when fluid passes through the servo valves the fluid in passing through the close tolerance opening of the servo valve will release small electrical charges. These charges will be in the form of microscopic sparks, which will attack the metal parts, eroding them over a long period. To minimize this action not only must the hydraulic delivery system be grounded the servos must be grounded.
From personal experience I have seen aircraft appliances which are anodized or not anodized attached with Cadmium plated bolts and separated from the airframe by plastic tape similar to scotch tape. Anodize coatings are non-conductive, Cad plate is non-conductive and the plastic tape is non-conductive and the appliance is not bonded to the structure.
Any answer I give is based on theory and not actual first hand knowledge of the 767 landing gear. As previously indicated I am only familiar with the truck positioner but I don’t even know how it is installed.
From a design standpoint the hydraulic lines should be bonded to the structure every so many inches of line run. The electrical lines you referenced are grounded through the circuit and this will place the electrical lines at the same potential as the ground plane of the electrical system 28VDC system and AC systems. The hydraulic lines are at the same ground potential as the airframe. Whether it is from static build up due to precipitation static or from a lightning strike the airframe and everything connected to it by bonding straps will be at the same potential. The lightning strikes in flight are transitory and when the attachment breaks the airframe electrical charge will drop to the level of the precipitation static charge. If the aircraft is struck by lightning while on the ground and the aircraft is not grounded either through bonding to the earth ground plane or by a static discharge line on the landing gear the charge will most likely flash over to earth. However if it happens and the aircraft is earthed by the discharge cable on the landing gear the charge will pass through the bogy pin and either cause it to burn or it will weld the pin to the bushing and possibly to the bogy.
Black Sheep indicated in his Email to me that there was evidence of stress corrosion. Having had a bad experience with a German Stress engineer on the A-310 program I can tell you that sometimes that stress corrosion is sometimes confused with precipitation static generated electrostatic discharge corrosion. On the part in question he identified the problem as stress corrosion and I said it was electrostatic discharge corrosion. Eventually, I was proven correct.
Of course, I could be totally wrong in this particular matter.
Here is another point to consider and although not directly related you should be knowledgeable as it relates to other problems in the flight control system or any system powered by the hydraulic systems. Hydraulic fluid conducts electricity so bonding of the hydraulic lines helps to stabilize the electrical currents in the fluid. However when fluid passes through the servo valves the fluid in passing through the close tolerance opening of the servo valve will release small electrical charges. These charges will be in the form of microscopic sparks, which will attack the metal parts, eroding them over a long period. To minimize this action not only must the hydraulic delivery system be grounded the servos must be grounded.
From personal experience I have seen aircraft appliances which are anodized or not anodized attached with Cadmium plated bolts and separated from the airframe by plastic tape similar to scotch tape. Anodize coatings are non-conductive, Cad plate is non-conductive and the plastic tape is non-conductive and the appliance is not bonded to the structure.
