Cherokee Landing Gear Bolts -AGAIN!

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Magman

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Magman
I’ve found a fair amount of Lower Landing Gear attach bolts that were loose ,

stretched or broken.

SB 1375A addresses this but I don’t agree that Time Change is the answer.

Current patient had a broken bolt previously.

Had it analyzed and failure due to fretting.

All bolts replaced with AN.

100 hrs later we are finding the nuts loose again.

One is so loose can spin it with fingers but Torque Indicator paint is not broken.

It must be stretched.

Hole measures .250 Bolts .247

Oddly ; some old bolts are fine on other side.

The plan is to install 4 new NAS bolts on the bottom.

Some concerns:

NAS 6604-11 comes in both Shear and Tension with the same p/n

Is the Torque Spec for a NAS bolt the same as equivalent AN?

Friction Torque + Applied Torque = Desired Torque ( Standard 1/4 - 28 torque)?

Or?

This is not the only aircraft with issues like this.

Many years ago a Pawnee was breaking wheel bolts.

Switching to NAS eliminated the problem.


Could the issues be poor hardware? Incorrect torque or other procedure?
 
Counterfeit parts?

That has been a problem lately for high strength hardware.
 
geezer said:
Counterfeit parts?

That has been a problem lately for high strength hardware.
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Wow. That's all we'd need. Sub-strength AN hardware. Once in a while there's a problem with some particular AN or MS number, and service bulletins advise removal and replacement of affected batch numbers, or ADs force the replacement. A few years ago I had to check to make sure we hadn't installed any MS21042-4 nuts on any aircraft we'd had in the shop. A batch of those had turned to to be hydrogen embrittle and were cracking. Not nice, when they're holding the airplane together.
 
Thanks Dom ; I’ll check out D.

Some bolts are original and some were replaced.

It would seem with the higher strength NAS bolts selling for S0.75 each

that there would be more lucrative fish to fry.

This condition has been found on numerous fixed gear Cherokees.

I have considered measuring and recording individual bolt length

to confirm stretch. The torque paint is not broken so the nut didn’t turn.

I always check every inspection which requires pulling the “ cuff”

down to gain access to 2 in the middle. They can easily be checked

with just a 7/16 open end. Occasionally; one might be turned with

only fingers!

Does this happen from 1 hard landing?

When 1 fastener fails the others are now likely to be overloaded

and more prone to failure.

I would welcome comments on install procedures etc.

For now I’m recommending a check at mid - inspection points.
 
The landing gear casting fits in between the spar flanges. Sometimes the dimensions are a little off. If after tightening the top bolts there's just a little gap between the landing gear casting flange and the spar flange a shim should be made and inserted. Use a feeler gauge and measure that gap and fabricate a shim. Don't tighten the two longitudinal bolts in the middle until last.

These planes are built one part at a time by people using jigs and calipers. CNC machines were not common place back then. Sometimes things need a little shimming and adjusting to get a tight fit.
 
Magman said:
Does this happen from 1 hard landing?
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Doubtful. More than likely its a design flaw that has been aggravated by lack of corrective action or inspection. Hence the SB and the liberal repair allowances. The initial release of the SB (1375) had you change the AN bolts with new AN bolts due to bolt metal fatigue and corrosion. The subsequent revisions (1375A-D) call out for NAS bolts as part of a repair procedure as I believe only NAS bolts come in oversize diameters and AN bolts do not.
Magman said:
I would welcome comments on install procedures etc.
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Follow SB 1375D which added an eddy current check in revision C. Given the SB addresses the exact issue you have been fighting, I would follow it to the tee.
 
Klaus may be onto something .

The problem would be a lot of work to verify the need for a shim.

Is a bolt that has torque indicator intact but is loose presumed to

be stretched and is that the “damaged” Piper refers to?

That would trigger an Eddy Current Inspection.

This could be an annual event in some cases.

Since the EC will not fix anything my focus is on Prevention.

I see nothing in the SB addressing torque.

There is a tightening procedure referred to as “ torque to yield” that

I have minimum knowledge of. Could this be a place where it would apply?

If you torque a stronger NAS bolt to standard AN bolt torque is the

fastener actually undertorqed?

Does anyone have an approved torque chart forNAS bolts?

I do see a lot of opinions but no actual backing.
 
Magman said:
Is a bolt that has torque indicator intact but is loose presumed to be stretched and is that the “damaged” Piper refers to?
Click to expand...
Maybe. But it can be caused by other issues as well like a crack in the clamped material or worn mount hole.
Magman said:
Since the EC will not fix anything my focus is on Prevention.
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But the EC is preventative. If the underlying material is cracked or has other anomalies, you'll never correct the problem.
Magman said:
I see nothing in the SB addressing torque.
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Because when a torque is not specifically called out in a mx instruction it reverts to standard torque. So with no specific tq listed in the SB check the standard torque chart in the Piper mx manual, a standard practices manual, AC 43.13-1B, etc. whichever is applicable.
Magman said:
If you torque a stronger NAS bolt to standard AN bolt torque is the fastener actually undertorqed?
Click to expand...
No. Clamping torque is determined by the installation and is either a specified or a standard value. As noted, the only reason the NAS bolts are called for in the SB is because they come in oversize diameters. If AN provided the same oversize options it would have been replaced with an AN oversize bolt. Regardless, whether NAS or AN both would be torqued to standard torque per the SB.

As a side note, also keep in mind the AN spec has been discontinued and was replaced by the MS and NAS standards. So when you look at current airframe OEM parts manuals you will see a major shift in calling out NAS spec hardware in places that in previous manuals were AN spec hardware. However, if your aircraft originally came with AN hardware and the current IPC calls for NAS hardware, even by your S/N, you can still legally use the AN hardware. Eventually the stock of AN hardware will run out but I dont see that happening for many decades into the future like zinc chromate spray paint.
Magman said:
Does anyone have an approved torque chart forNAS bolts?
Click to expand...
Never seen one. You'll find with most steel bolts the limiting factor for torque is thread type and nut type. Have seen NAS and 20 series bolts installed from finger tight to torques well over listed standard torque values. By not following the acceptable guidance in relation to hardware torques you will also run the risk of causing additional damage or even violating the performance standards of Part 43.13.
 
Thanks Bell.

I’m pursuing this with several Non-Piper agencies.

Previously I said that if I inspect 10 Cherokees there will be a loose

bolt on 3 of them. That’s the first time around.

Inspecting the same aircraft at their next scheduled inspections

will yield similar results. Occurs with new and old fasteners.

It would have been good to track individual bolt locations etc.

My belief is if all fasteners are kept tight the possibility of Spar damage

is diminished. Ideally a conclusion that incorrect torquing procedures is the root cause.

That would be easy to rectify.


As a SB compliance in most cases is not mandatory. If it were to become an AD I’m sure

most owners would grumble a lot. Some may even just replace the bolt like “ It never Happened”.


Still looking for a Piper definition of “ damaged”.
 
Magman said:
As a SB compliance in most cases is not mandatory. If it were to become an AD I’m sure most owners would grumble a lot.
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I guess I'm not following the issue here. You have a reoccurring discrepancy with these L/G mount bolts. Piper came out with a SB to correct the issue and provided a repair procedure if needed. So comply with the SB and move on. If the owner doesn't want you to comply with it and fix the problem, sign off the inspection with a disc list. If he doesn't want that either perhaps tell him to find another mechanic? Or what am I missing?
Magman said:
Still looking for a Piper definition of “ damaged”.
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It will be the same as any other definition of bolt damage. The initial reason for the SB was bolt metal fatigue hence the reason to replace the bolts on a regular schedule. So if fatigue is an underlying issue any damage to the bolt should require replacement. Right?
 
A broken bolt is obviously “damaged”.

Initially my belief was a loose bolt was due to the nut backing off.

The last few showed the nut has not moved via torque indicator.

Does that mean the bolt is damaged?

I’ve enlisted some aircraft metallurgy folks to assist with this.

So far the one failure he had was determined to be caused by fretting.

That means slight movement to me.

Presumably from loose bolts.

It should be interesting checking the next time or two.


It looks like no one else is finding this?

Or just keeping their mouth shut?
 
Magman said:
It looks like no one else is finding this? Or just keeping their mouth shut?
Click to expand...
FYI: the SB mentioned throughout this thread was the result of many people contacting Piper about... loose landing gear mount bolts. No metallurgists needed.;)
 
Klaus. Any particular things to address the issues?

Bell. I like to utilize resources when I can.

To my knowledge Piper never stated the reason for the condition.

As far as I’m concerned it’s not “ High Time” in some cases.
 
Magman said:
To my knowledge Piper never stated the reason for the condition.
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Because they didn't need to. But if you had talked to them they would have told you. The root cause was people weren't checking the hardware properly at each required 100hr and annual. And once it loosened it would deform the mount holes. After a certain point simply retorquing the hardware would only lead to further wear. This is why SB1375 started with only bolt replacement and then progressed to repairing the holes. Without addressing the hole issues as per the SB you are basically spinning your wheels and the problem will persist.

There's been several PoA threads on this. To see how the SB progressed I made the short summary below:

SB1375: Replace the L/G bolts
SB1375A: Replace L/G bolts and allow for repair of worn holes
SB1375B: Replace bolts and fix FU in Rev. A for repair of worn holes
SB1375C: Replace bolts, allow for repair, and add eddy current procedure to aid in inspecting worn holes
SB1375D: Replace bolts, allow for repair, eddy current check, and revised worn hole calculation to cover a greater range of situations to include reevaluation of worn holes performed under previous revisions.
 
Magman said:
Klaus. Any particular things to address the issues?

Bell. I like to utilize resources when I can.

To my knowledge Piper never stated the reason for the condition.

As far as I’m concerned it’s not “ High Time” in some cases
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Service Bulletin (SB) 1375D page 8 shows the gear casting in a graphic and calls it a 'Cylinder'. The distances between the spar flanges and the cylinder flanges need to be the same. Every time the plane lands it pushes up on the upper spar flange and 'Pulls' on the bolts going through the bottom spar flange. If there's a gap in there, even with the bolts tight, the bolts are under more stress then their design.

Quite often the gear will be so tight that it takes a rubber mallet to free it up for removal and that's good. If all the bolts are removed and the gear just comes loose easy it's too loose. That usually means the cylinder flanges where machined just a fraction too much and the gear needs to be shimmed up until it takes a mallet and a couple punches to line up the holes during install.

Piper didn't figure their planes would still be operating 60 years later and these little imperfections would really matter.
 
Using Applied Torque = Friction ( Running) Torque + Desired torque

Shimming does make sense but looks like quite a task.

So is Piper saying that if one bolt is slightly low on a torque check that

it is considered ( damaged ) and an EC is needed?

Again; to me aircraft time is not significant but time on a fastener

might be.

There will be one coming due for inspection soon that had all

Lower bolts replaced not long ago.
 
Magman said:
Using Applied Torque = Friction ( Running) Torque + Desired torque
Click to expand...
Magman said:
So is Piper saying that if one bolt is slightly low on a torque check that it is considered ( damaged ) and an EC is needed?
Click to expand...
Interesting. So after reading the SB instructions this is your conclusion? Perhaps if I summarize them a bit, you’ll have a better understanding of it?

>Remove and discard all landing gear mount screws, bolts, nuts, and washers.
>Measure all bolt hole dimensions.
>Eddy current all bolt holes.
>Repair any serviceable bolt hole that exceeds acceptable dimensions with a proper oversized bolt.***
>Install new bolts and torque ¼” bolts to 65−100 in·lb.

*** NOTE: Any bolt hole outside acceptable dimensions or found unserviceable will require alternate repair procedure.
 
Torque Procedure?

IIRC Piper specifies an EC only if the bolt is damaged.

True?

SB 1375A is less than 2 years old and is on rev 4.

It’s not my intention to be argumentative on this but Piper ,

Boeing and others do not instill the same degree of confidence as was in

the past.

I have been finding loose and broken bolts for many years.

Many years ago I requested and received a reply from FAA re

Lycoming prop strikes. They were to be ignored if they occurred

prior to AD 91-14-22 that was the topic. Many aircraft I have been

inspecting for many years. I doubt if there is a single Cherokee that

has not had at least 1 loose bolt in that time. If retroactive before

Nov 2022 it could be a long line waiting for EC.


Prior to 1375 I would just check torque and Re torque if slightly loose.

Very loose or a second time loose was a new AN bolt.

Piper has already switched from AN to NAS . Will they specify

Tension or Shear on next revision? Oddly; they have the same P/N

with different specs .
 
Magman said:
SB 1375A is less than 2 years old and is on rev 4. It’s not my intention to be argumentative on this
Click to expand...
Well, don’t know what else to add. I tried to simplify the SB the best I could, yet you still seem to struggle with it in one form or another. In my opinion, the SB is a straightforward maintenance procedure that will correct the MLG issues you’ve been discussing. Perhaps find another mechanic to walk you through the SB steps and perform it on one landing gear assembly?
 
I’m not faulting you on this at all but I have found not all mfg

instructions are correct. 2 areas of concern are;

1. Fasteners in tension are torque critical.

With self locking nuts Running or Friction Torque is a factor.

My guess is many were field replaced w/o this consideration.


2. IMHO - Piper has some things backward.

EC is required when a “damaged” bolt is encountered but no

mention of this for a Spar that is “ damaged” with oversize holes!


We have a sort of plan developed that I’ll submit next time.
 
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