Undercarriage leg problem

[3wheeledtechie]

I'm hoping a Rigger with undercarriage bay experience can shed some light on this one:

Problem is flaking of chromium coating on fescalised portion of main u/c leg (large commercial transport a/c). Not only one a/c but on several. Not ladder marks, but great big chunks flaking off. Not discovered until the a/c is jacked as its the part inside the leg.

Also exact same main leg is fitted to another variant but no reports of flaking problem. Some evidence of brown marks presumed to be caused by heat on surface of u/c leg bearing, when disassembled.

Anyone know what temp chromium nickel coating what flake off at? Also when these bearings are packed with grease in reassembly, is there a measured quantity of grease? What would have to fail for the grease to escape - I'm thinking grease nipple or seals?

Anyone ever seen a similar problem? Any helpful replies appreciated.

 

[Shugster]

I'm hoping a Rigger with undercarriage bay experience can shed some light on this one:

Problem is flaking of chromium coating on fescalised portion of main u/c leg (large commercial transport a/c). Not only one a/c but on several. Not ladder marks, but great big chunks flaking off. Not discovered until the a/c is jacked as its the part inside the leg.

Also exact same main leg is fitted to another variant but no reports of flaking problem. Some evidence of brown marks presumed to be caused by heat on surface of u/c leg bearing, when disassembled.

Anyone know what temp chromium nickel coating what flake off at? Also when these bearings are packed with grease in reassembly, is there a measured quantity of grease? What would have to fail for the grease to escape - I'm thinking grease nipple or seals?

Anyone ever seen a similar problem? Any helpful replies appreciated.

Chrome is very hard and only flakes if you bend it too much. I don't think temperature would be an issue as the legs only move on take off and landing. (Unless the sit next to an exhaust somewhere).

You should strip the leg and see if all the seals were fitted properly or if there is any mechanical damage to the metal under the chrome.

It is possible that this was a batch of legs were not plated correctly, are they all from the same batch?

 

[Shugster]

Or maybe the wrong oil was used that perished the seals?

Does the other variant that has the same leg weigh the same? Maybe the leg is carrying more loads, (Including sideways loads during taxing etc), than it was designed for?

It could be a design issue if it's only one variant that suffers this problem.

 

[Obi Wan]

What A/C are you talking about. I had the same problem on an A300 B4 some years ago, ended up Cx the leg, unfortunately never got a strip report from messier. any more info would help, as im sure I can get an answer for you.

DT_Xtremez_30:

 

[Dirty Harry]

Sounds like possible corrosion under the chrome plating. I've seen it on an A300 and B757 leg. It manifests itself as bubbles or flaking material. Can be caused by poor bonding of the chrome to base metal during manufacture or scoring of the cylinder allowing moisture to get underneath. The scoring can be caused by inadequate lubrication of the lower shock strut bearing.

 

[The Masked Geek]

Have you tried re-booting it?

 

[Obi Wan]

Just spoke to a mate of mine who reckons its stress (landing Loads) and lack of lubrication. (and is quite common in the freight role)

 

[Harry B'Stard]

Fescalised Portion!

Fescalised Portion!

At a secret Training establishment in the West Midlands it is taught that any damage to the Chrome portion / Shiny Bit / Sliding Member requires checking IAW the Leg AP.

It will normally require replacement.

Although it only appears when jacked you must assume it happens during flight (as the Shock Absorber will be fully extended during flight and in the U/C bay).

As regards to why it's happening there could be a number of reasons and also a few as to why it's only happening on one type of aircraft. Most obvious ones (such as bad batch) have been discussed. Are the type that are affected used in a different way / role than the others (i.e. Tankers/ Freight, always turning the same way at the end of the runway to reach their Sqn dispersal)?

However, I remember a story about corrosion forming on Nimrod (I think) undercarriage legs. They traced it down to the wrong sort of Jointing Compound (yak sh1t) being used. The AP said to use the blue JC, but someone had been using the yellow JC. (Just remembered that this occurred on Jaguar Universal Joints!) It led to massively advanced corrosion of the CHROME in a short time frame. Is there any yak sh1t used in the vicinity?

Finally greasing of bearings is normally carried out by injecting grease to fill the bearing, wipe off the excess, fit the bearing. Then when final Lubrication is carried out, inject grease until you see clean/fresh grease and wipe off all the excess.

Hope some of this helps

HTB

P.S. For fescallising look here! http://www.airmech.co.uk/forums/showthread.php?t=7894 DT_Xtremez_30:

 

[MrMasher]

When I worked in a hyd bay I used to strip uc legs. The separating pistons were sealed via o rings and plastic u channels.

The corrosion, pitting and flakes we found were mostly on the fwd face of the fescalised portion. We put this down to the amount of fine grit picked up during take off, landing and taxiing. You just need a bit to sit right against the gland seal, then when you get small amounts of travel in the leg when the aircraft is moving it scrapes the fescalised portion leading to corrosion, pitting and flaking.

We never had issues with the fescalised coating being bonded incorrectly.

The legs I worked on were lubricated internally by it's own hydraulic fluid. The only greasing was to the scissor links and associated bolts on the exterior.

 

[I Look Like Kevin Costner]

A 737 NG undergoing a C check where I work required a NLG lower cylinder change due to brown discolouration of the fescalised portion. This was due to friction on the tube from the bronze bearing as the oleo was complety free of shock strut fluid oil. Nobody had ever seen that type of damage beforehand.

 

[Rigga]

Some legs are prone to corrosion "issues".

Different aircraft, even in the same airlines, can have different operating environments offering differing airflows.

Water can accumulate in the area behind the first o-ring and settle there for a while, between o-rings.

Landing Gears can sit "in situ" for 15 years in my experience, so corrosion does become a possibility.

Chrome plating will flake from anything imperfect under it.

 

[Dirty Harry]

the oleo was complety free of shock strut fluid oil

So that's why you kept putting a bit of wind in the leg every turnround to get it round the track.

 

[I Look Like Kevin Costner]

So that's why you kept putting a bit of wind in the leg every turnround to get it round the track.

Ha, ha! Not me mate, it was those ex B*T cowboys up north of the border!! That is exactly what they did, but what small operator wouldn't..

 

[Dirty Harry]

Ha, ha! Not me mate, it was those ex B*T cowboys up north of the border!! That is exactly what they did, but what small operator wouldn't..

I know mate, the result of having your line maintenance done on the cheap. Long term it doesn't pay though, particularly when you have to purchase a replacement NLG from Goodrich.

 

[3wheeledtechie]

What A/C are you talking about. I had the same problem on an A300 B4 some years ago, ended up Cx the leg, unfortunately never got a strip report from messier. any more info would help, as im sure I can get an answer for you.

DT_Xtremez_30:

Wow, thanks for quick response guys. It is an A300-600 main u/c leg we are talking about here, and Messier are involved. As they do the re-chroming of course its in the interests for it not to be down to them. I've yet to establish any differences in their process relating to these legs.

It unlikely to be a batch issue as these legs are from different batches as the a/c were constructed at different times. It is odd that its appeared on several a/c of this type but not on legs fitted to standard A300-600s.

 

[I Look Like Kevin Costner]

It unlikely to be a batch issue as these legs are from different batches as the a/c were constructed at different times. It is odd that its appeared on several a/c of this type but not on legs fitted to standard A300-600s.

Are you working on freight dogs (not that it should make any difference).

Does the companies daily include cleaning and lubing the fescilised portions?
Is it actually done?

 

[Dirty Harry]

3WT - The A300-600 messier MLG leg is prone to expelling excess grease out of the lower shock strut bearing, particularly after routine lubrication. The grease smothers the exposed portion of the inner cylinder which subsequently attracts debris (flies, grit, dirt etc.) If regularly cleaning of the fescalised portion with shock strut fluid is not adopted this crap gets up inside the leg between the seals and can cause damage. Ive seen a couple of instances on the A300-600 where this has led to corrosion and eventual flaking of the chrome plating.

 

[I Look Like Kevin Costner]

3WT - The A300-600 messier MLG leg is prone to expelling excess grease out of the lower shock strut bearing, particularly after routine lubrication. The grease smothers the exposed portion of the inner cylinder which subsequently attracts debris (flies, grit, dirt etc.) If regularly cleaning of the fescalised portion with shock strut fluid is not adopted this crap gets up inside the leg between the seals and can cause damage. Ive seen a couple of instances on the A300-600 where this has led to corrosion and eventual flaking of the chrome plating.

Somebody not cleaning and applying the shock strut fluid IAW the Daily. Scarebus Fluid 41 anybody?? Thats OM15 in E-Goat speakDT_Xtremez_30:

 

[BillyBunter]

Corrosion and fescalised issues I have seen on Shock Absorbers on the MR2, nothing major but you have to doubt where the metal is flaking if this is having a rubbing issue with movement of the shocks on the seals. As a rigger Im sure thats already been checked and all good.

The MR2 was heavily greased with PX6 every 30 days so there was always good lube in there and was never an issue only when in sandy places of melting away in hotter temps.

Don think its the same issues you are having mind , with regards to bearing grease I know of a new aircraft thats having issues retaining grease within a shock absorber housing but care not to discuss on here DT_Xtremez_42:

 

[3wheeledtechie]

Actually I am not working on these on a daily basis, I have been asked to facilitate a problem solving event. These a/c do a number of landaways each day before returning to MOB. Not sure what the civvy term is but is there an equivalent to a T/R servicing? If so I'll look into seeing if the various lubes are carried out (by the crew) on these servicings.

Dirty Harry do you know why it is prone to expelling grease? Is it due to design, and have other operators increased the frequency of injecting grease via grease nipple to compensate?