Broken MLG Bogie Pivot Pin

[Volume]

Does anyone know more about broken or cracked bogie pivot pins on Boeing 756/767 main landing gears ?. .Looks like one broke early this month and an uplock fitting assy (about 30 lbs heavy) which is connected to the pin has fallen into a swimming pool on final for RWY 09L in FLL.. .Failure of the pin can cause the bogie to separate from the gear leg and subsequent lead to an catastrophic event. Luckily the pin broke at a position where the remaining part was able to hold the bogie in position this time.. .Roumors say this is not the first time this happens, Boeing changed the pin design 3 times at least.. .Does anyone know more about these pin failures ?

[None]

What should I be looking for on my walkarounds?

[trapped off]

The 767 has a know problem of brake to brake rod attachment pins fracturing as such they are being replaced regularly. I would imagine by now modified pins have been installed on most a/c.I can't see the connection of the bogie and the uplock though.

[Volume]

I´m not talking about a break rod pin, I asked about the 5.5 inch diameter (140 mm) pin which connects the MLG bogie beam to the gear leg.

Cracks probably can´t be found during walkaround, kicking the uplock fitting won´t do. Best you can do is landing very smoooooth and avoid drift landings :)

[snaphead]

this is a well known problem on b757 a/c caused by lack of lubrication and moisture ingress resulting in corrosion and seizure of the pin and the bronze bushing, it is normally inspected during routine heavy maintenance checks, there is i think a boeing service bullitin on the problem ;sometimes witness marks are painted on the bushing flange and the leg housing to give an indication of migration of the bush, apart from this it not possible to detect the problem without stripping the bogie assy.[the only a/c which i know for a fact have these witness lines are 757 operated by air 2000]

[Blacksheep]

Just a small point but Boeing 'bogies' are called trucks. (Bogies get up their nose) <img src="tongue.gif" border="0">

The problem is related to wear causing loss of the plating on the pin, followed by moisture ingress and corrosion. If this condition remains undetected, the corrosion leads to cracking and ultimately failure of the pin. It may manifest as rotating or migrating pins, but heavy corrosion can freeze the pin and hamper detection. The problem is being addressed by redesigned pins and bushings, samples of which are already being service tested on selected aircraft (or airplanes as Boeing calls them) among the world fleet. In the meantime, and despite the redesign, the existing pins shouldn't fracture as described so long as they are lubricated exactly as specified in the MM, using the correct grease and inspected at the recommended intervals. If you can't get fresh grease out of the joint when greasing, then you may already have a problem... But there's been nothing new about any of that since bushings and pins were first invented. All LAEs know that. Don't we?

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[slingsby]

Blacksheep, can you remember if it was the downed banana 767 in situ with John that had a MLG cracked pin problem on landing in Dubai. May have been 'H' but to be FRANK with you I can't remember.

Compliments from S.N.

Switch, what switch, it's on your side not mine. <img src="wink.gif" border="0">

[Blacksheep]

I think the bent banana was a Ffyfes - something with an 'F' in it anyway. I didn't mention it in my previous post, but the premature wear that leads to the corrosion seems to be related to harmonic vibrations and a resulting breakdown of the lubricating properties of the grease. The problem is not related directly to time in service or the number of landings, it is more related to the area of operations - some runways are in better shape than others. Also, observant LAEs will have noticed that Boeing have removed a particular grease from the list of those approved for use on Landing Gears.

Meanwhile, John's having a new skin put on his banana, I hope its not too painful. They want to do a Hi-Blow test when they've finished - now that sounds like an interesting experience. <img src="eek.gif" border="0">

**********************************. .Through difficulties to the cinema

[ 02 February 2002: Message edited by: Blacksheep ]</p>

[DoctorA300]

Does the B767/757 have the uplock pin fitted to the bogie ?. .Brgds. .Doc

[Lu Zuckerman]

Can anyone tell me what grease is used to lubricate these pins and if the grease contains Molybdenum Di Sulfide?

[Golden Rivet]

The 757 truck pivot pin uses Royco 11MS which equates to MIL-G-7711A+MoS2 for those of you with an inquisitive nature.

According to the Royco web site it is a general purpose extreme pressure lubricant. Don't think it contains any Molybdenum Di Sulfide.

Of course other operators may use something different.

The 757 has uplock attached to oleo and 767 has indeed got an uplock fitting attached to the truck pivot

. .[ 03 February 2002: Message edited by: Golden Rivet ]

[ 03 February 2002: Message edited by: Golden Rivet ]</p>

[Lu Zuckerman]

To: Golden Rivet and all

Golden Rivet said:

“The 757 truck pivot pin uses ROYCO 11MS which equates to MIL-G-7711A+MoS2 for those of you with an inquisitive nature. According to the ROYCO web site it is a general-purpose extreme pressure lubricant. Don't think it contains any Molybdenum Di Sulfide”.

Response: Here is the spec you referenced, taken directly from the ROYCO web site and it is exactly as you stated. However it does contain Molybdenum Di Sulfide which has a chemical symbol of MoS2.

(ROYCO 11MS MIL-G-7711A+MoS2 Aircraft general-purpose extreme pressure grease).

This may be the cause of the corrosion on the subject pin. The US Military banned the use of Mo S2 from, use on their air vehicles. Molybdenum Di Sulfide when in contact with a warm saline environment will degenerate into Hydrochloric and Sulfuric acid. The major lubrication suppliers re-compounded the Mo S2 to contain an inhibitor to keep the degeneration from happening. The inhibitor can weaken and not prevent the decomposition process. It is imperative that you and all users of this lubricant check to see if the inhibitor is added and if not discontinue the use of the product. If the inhibitor is added then check to see if it is in fact losing efficacy and if so then consult Boeing to see if the lubrication schedule can be changed to minimize the degradation of the inhibitor by replacing the grease at a higher frequency than specified in the maintenance manual.

Does anyone have a teapot as I believe this will cause a tempest?

[Golden Rivet]

Well spotted Lu.

regards

[Golden Rivet]

Lu,

Done a bit more research and Boeing apparently changed to Royco11MS in 1997 from Aeroshell 7

"The truck beam pivot pin and the brake rod should only be lubricated with Royco 11MS grease. Royco 11MS grease provides better lubricity than the current MIL–G–23827 grease (Aeroshell 7) at these locations.

Royco 11MS grease contains molybdenum disulfide which is desirable for highly loaded movable Joints."

The greasing inteval has also been reduced from 2A to 1A.

??

Regards

[Blacksheep]

Lu,

Boeing are already looking at the Royco grease, but as to discontinuing the use of Royco 11MS, we maintenance people have our hands tied. We must use the recommended lubricants and are not free to switch on our own authority. Frequent greasing following the generally accepted method of pumping in fresh grease until only fresh grease comes out of the joint ought to flush out any decomposing grease. Also, the greasing interval is now reduced to 1A Check (generally 500-600 hours or about 130 cycles) to ensure constant change of the grease in the joint. An important factor in all this, is that not all operators are affected. There are high time/high cycle aircraft with no sign of trouble and comparatively low time aircraft with severe problems. The difficulty lies in isolating the operating conditions that are common to all those aircraft that exhibit excessive wear on the pin. In the case of worn pins there are definite signs of heat damage, which of course contributes to break down of the grease. Selected operators are flying with a new design in the right gear and the old original pins in the left. All pins are removed for inspection during "C" check, and the results reported back to Seattle. As the database develops, the final fix will become clearer.

There is a theory that operation on certain runways leads to a particular vibration frequency, which in turn causes heating of the pivot pin/bushing. The subsequent break down of the grease then causes corrosion and eventual cracking of the pivot pins. In this case, it makes little difference which grease is used, any type will eventually decompose and result in pin failure.

And to think we've only just fixed the problem of vibrating carbon brakes shaking the gears to pieces. Now this bit of achemy - ain't engineering wonderful?...

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[Lu Zuckerman]

To: Blacksheep

If you have access to the failure data check which airlines are experiencing the failure and which are not. If you have access to this data then check to see which of these airlines operate in or near salt water and warm or hot temperatures. There is a possibility that in this case air temperature may not be a factor and that the heat rejected from the brakes is the heat portion of the salt environment / heat equation.

Also if you have access to the data sheet for the grease check to see if the MoS2 is inhibited.

To: Golden Rivet

If memory serves me correctly ROYCO is or was a division of shell as their products were listed in the Shell data book.

Speaking of the AEROSHELL book I would strongly advise all of you to contact Shell in London and request a copy of this excellent handbook. It is available in hardcopy and CD. The Address is :

Shell Aviation Limited. .OIAM/3. .Shell Mex House. .Strand. .London. .WC2R OZA

[Blacksheep]

Lu,

The signs of overheating that I referred to are localised hot spots on the rejected pivot pin. They occur in a random pattern and are associated directly with worn areas. It isn't clear if they precede or are the result of the wear. So far the redesigned pins and bushings appear to be effective although the trial is still in the early stages.

It occurs to me that some readers may find this discussion alarming. Please be aware that although this subject concerns a serious problem, the problem is identified, under control and the follow up described is just normal engineering activity. The daily routine for those working in Tech Support and Maintenance, in fact.

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[Lu Zuckerman]

To: Blacksheep

The only part of the 767 landing gear that I am familiar with is the truck positioner as I did the RMS on it. I am familiar somewhat with large landing gear on large aircraft but there is one point that I do not know and that is if there is a cable grounding strap that makes contact with the ground slightly before the wheels touch. This strap grounds the static charge built up on the airframe. I do know that on the A-310 the flaps can build a charge of 1400 volts so I assume that the airframe of the 767 has a significantly higher charge. The point I am making is that this charge unless diverted must pass through the truck pivot point in order to reach the ground and this high voltage can cause electrostatic erosion or in some cases, very high temperatures.

Just a theory.

[Blacksheep]

Lu,

I e-mailed you some more info but I like your static discharge idea. We also occasionally see arc spots from lightning strikes on the main gear sleeves, so it could be lightning rather than mere static that initiates the hot spots. You can't ground an aircraft parked on the ramp in these parts and we do get some fearsome electrical storms. I'll feed that idea back into the data that we exchange with Boeing.

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[Golden Rivet]

Interesting theory

"The point I am making is that this charge unless diverted must pass through the truck pivot point in order to reach the ground and this high voltage can cause electrostatic erosion or in some cases, very high temperatures."

Would'nt the static charge also cross via the truck tilt actuator,electrical conduits and hydraulic lines, also ditto for the other leg ?

Brgds

[ 05 February 2002: Message edited by: Golden Rivet ]</p>

[Lu Zuckerman]

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.