O-Ring Faiulure

plongson

plongson

Pre-Flight
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plongson
Interesting post-mortem...

Right main strut, Cherokee PA-28 160...not a perfect landing but not hard either. As soon as it was on the ground and all the weight shifted to the gear, it collapsed and the plane tried to make a HARD right turn. I was training and my instructor instinctively corrected to the left, retracted the flaps and got on the brakes. We didn't realize what had happened until we got out at the tiedown and saw all the strut oil puking all over the ground.

Not a difficult repair, had the kit in about 2 days from Aircraft Spruce and my good friend an ,A/P and I made the repair.

Interesting photo of the failed O-ring.

20230731_091339.jpg


20230731_091405.jpg
 
Does your strut have peeled chrome?
 
Did it have a minor leak before the catastrophic failure?
 
1.) Strut shaft is absolutely clean and smooth, upon CLOSE inspection, it is slick as a whistle including VERY little longitudinal wear lines as might be expected on a shaft.
2.) No oil leaks prior to the failure

My thought is the O-ring "rolled"...somehow in the groove upon landing tearing it in the spiral pattern...but it seems it would need a source of friction for that to happen.

What "IF"...the strut preload pressure was very low and the (hard) landing caused the strut shaft to compress completely, perhaps causing the O-ring to roll in the groove???
I'm just guessing because the strut looked to be in the correct extension (position) during pre-flight checklist but upon adding preload pressure after the repair, it does sit higher...

I dunno, I just would like to know what happened and avoid it happening again. Maybe it falls into the "Stuff happens category"
 
Wow Dana!! Great information!
Something caught my eye...it says sticky or gummy surface on the shaft...
Ya know how mil-h-5606 hydraulic fluid gets sticky and gummy when exposed to the air...What if there WAS a slight/minor leak and the shaft got gummy. The fast/rapid motion of the strut shaft being compressed MIGHT cause the O-ring to roll...
Dunno, just fishing
 
I took McFarlanes advise and keep my struts clean and slick with mineral spirits and a clean towel so my o rings slide nice on the strut. Yes 5606 gets sticky.
 
Last edited:
Place it in the “stuff happens” category. You can’t inspect the O rings without taking it apart. But keeping the shaft clean certainly helps with longevity. Most of the 5606 I’ve seen from leaky struts passed red a long time ago and went to anywhere between orange and black.
 
The last strutted airplane I had, I used to wipe the struts with a clean rag monthly, then shoot them with silicone spray.
Tried to not let them sit dry for lengthy periods.
This was after I popped a few wiper seals - worked for 20 years after that problem.
 
plongson said:
Wow Dana!! Great information!
Something caught my eye...it says sticky or gummy surface on the shaft...
Ya know how mil-h-5606 hydraulic fluid gets sticky and gummy when exposed to the air...What if there WAS a slight/minor leak and the shaft got gummy. The fast/rapid motion of the strut shaft being compressed MIGHT cause the O-ring to roll...
Dunno, just fishing
Click to expand...

Maybe, I feel the shaft being too clean more likely.

In another life we had considerable issues with strut seals failing. The fix from the oem was to sand the chrome of struts to scratch them up so they would hold some oil on the surface and lube the seal.
 
plongson said:
Ya know how mil-h-5606 hydraulic fluid gets sticky and gummy when exposed to the air...What if there WAS a slight/minor leak and the shaft got gummy. The fast/rapid motion of the strut shaft being compressed MIGHT cause the O-ring to roll...
Click to expand...

Gary Ward said:
I took McFarlanes advise and keep my struts clean and slick with a mineral spirits and a clean towel so my o rings slide nice on the strut. Yes 5606 gets sticky.
Click to expand...
This is why I disagree with the folks who wipe down their struts with 5606 to supposedly keep them clean and to lube them. It's a mistake. Besides turning gummy and sticky, it attracts dust that is carried into the scraper ring and O-ring, chewing both of them up and scratching the chrome. And sanding the chrome is not a good idea, either; that coating is unbelievably thin, and you just end up exposing the steel beneath it and corrosion sets in.

We often had spiral failures in the winter. The rubber shrinks in the cold and gets a bit looser in the O-ring groove, letting tiny amounts of oil escape, which then turns sticky real fast in the dry, cold air, and the ring gets spun in the groove and it fails. The answer was to replace that seal the instant any 5606 showed up, and to replace the nylon scraper ring along with it.

These are WW2-era Buna rubber O-rings. There are much more modern materials that would seal better, not take a set (permanent deformation) so easily, and suffer less damage in the heat and cold, yet the manufacturers haven't updated their parts manuals to permit the use of the more modern stuff. I suppose the research and testing costs just aren't worth it. McFarlane did do something about it, though. This kit is for the Cessna nose struts. I don't see anything yet for Piper struts:
https://www.mcfarlaneaviation.com/products/product/MCSK172-1F/

McFarlane's kit has the X-rings, and they have been tested and came out very well. Dave McFarlane's article:
https://www.mcfarlaneaviation.com/m...es-my-cessna-nose-strut-keep-leaking-down.pdf
 
Adding a small amount of STP to the strut fluid will do remarkable things to reduce strut friction. It even can be just be wiped onto the strut OD where it will be good for weeks.
DAMHIK.....
 
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