Piper PA-28 wing falls off

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jimdavis
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Re: Piper PA-28 wing falls off

Unread post by jimdavis » Sat Apr 21, 2018 10:44 am

Come folks - I think there is a bit of overreaction here.

Look at the three most popular light aircraft in the world How many Cessnas have come apart due to flutter? (I can think of 4 in South Africa) How many Bonanzas have lost their tail feathres in the air? (over 200!) and how many Cherokees have we heard of with this wing root failure? (almost none).

And consider that with low-wing aircraft the wing roots don't only take stress in the air, but also on landing!

You can be pretty sure that all of the above were badly maintained or badly flown.

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Re: Piper PA-28 wing falls off

Unread post by jimdavis » Sat Apr 21, 2018 10:47 am

TOFFS wrote:The PA 28s main spar slots tightly into the fuselage in a spar box and is bolted through the upper and lower spar caps. This is a very strong attachment (as demonstrated by a million hard landings).

Inspection in this area during MPI is difficult compounded by upholsterers taking the easy road and gluing their brand new carpet over the spar box instead of the hassle of putting press studs as Piper intended. As per normal, the lightening holes would never be adjacent the bolts as this would make life to easy. So, pulling the owners brand new carpet off the spar cap, standing on your head with torch and mirror to inspect the spar often gets ignored by some.

In South Africa RSA AD 97-02 has you x-ray in this area every five years. It does take a very well trained eye to pick up cracks or corrosion, even on an x-ray.

From viewing the picture of the affected wing, there is no corrosion seen. There are old cracks that weaken the structure to the point of total failure.

Unfortunately, no pre-flight inspection gains access to where this failure occurred and if this accident makes you scared of flying a Cherokee you had then better be very wary of flying just about any other aircraft and I am not the biggest Piper fan but that wing attachment is as strong as you can possibly make it without any huge weight penalties.
Thanks Eric - a great post, as usual!

I was transmitting at the same time as you. :D

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Re: Piper PA-28 wing falls off

Unread post by Volo » Sat Apr 21, 2018 1:32 pm

This is quite interesting -speaks of issues with spars in Pipers as far back as 1999.
https://bsd-box.net/~mikeg/N8031W/SB_SL/SB_0978.pdf
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Re: Piper PA-28 wing falls off

Unread post by StressMerchant » Sat Apr 21, 2018 3:56 pm

I haven't had much to do with the later build Pipers, but over the years I have had to look into a few spar repairs on the earlier Piper singles and light twins. The structures are very similar (no doubt same senior people in the design team, and common basis), and have a few problems.

The main problem is that the structure is difficult to inspect. Eric alluded to this in his post - even with an X-ray damage can be hard to spot.The main extrusion is sandwiched between doublers and skins, so if a crack develops in the extrusion it is basically invisible until it breaks through to an edge. The doublers have some ability to carry the load after failure, but in this case they are having to carry higher loads than the original design intent. The doublers will then obviously fatigue quicker than expected. Since the original crack can easily go undetected, the aircraft operator may not be aware that the primary load path has failed and that the redundant load path is now working overtime.

The bolts in this area are close tolerance types, which can be sensitive to installation defects. I think Chalkie wrote a piece on the care needed when installing such bolts. This accident may well be the "perfect storm" of a defect introduced at manufacture or major maintenance, undetected even when it caused failure of the primary element, which eventually broke through the secondary element as well.
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Re: Piper PA-28 wing falls off

Unread post by dollar » Mon Apr 23, 2018 1:21 pm

Forgive my ignorance but what material is this spar made of?
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Re: Piper PA-28 wing falls off

Unread post by Volo » Mon Apr 23, 2018 4:19 pm

dollar wrote:Forgive my ignorance but what material is this spar made of?
Probably Extruded Aluminium Alloy section 6061 - T6 condition - may also be 7075 -T6 ? - about 6 x times stronger than common commercial aluminium sheet .
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Re: Piper PA-28 wing falls off

Unread post by dollar » Tue Apr 24, 2018 10:59 am

Thanks Volo :D
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Re: Piper PA-28 wing falls off

Unread post by StressMerchant » Wed Apr 25, 2018 8:49 am

I can't recall the exact alloys used in the wing spar, but I suspect that the construction uses 2024-T35xx extrusions. The Service Manual lists the wing skins as 2024-T3.

Given the age of the design, 2024 was the standard alloy used by most light plane manufacturers of the time. 7075 alloys are stronger, but at the expense of fatigue strength. 7075 is often used for fittings associated with the upper wing, as the top surface of the wing is predominantly in compression.

One of the difficulties dealing with Pipers is the lack of a Structural Repair Manual, where these things would normally be detailed.
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Re: Piper PA-28 wing falls off

Unread post by Orthin Opter » Wed Apr 25, 2018 3:29 pm

TOFFS wrote:The PA 28s main spar slots tightly into the fuselage in a spar box and is bolted through the upper and lower spar caps. This is a very strong attachment (as demonstrated by a million hard landings).

Inspection in this area during MPI is difficult compounded by upholsterers taking the easy road and gluing their brand new carpet over the spar box instead of the hassle of putting press studs as Piper intended. As per normal, the lightening holes would never be adjacent the bolts as this would make life to easy. So, pulling the owners brand new carpet off the spar cap, standing on your head with torch and mirror to inspect the spar often gets ignored by some.
Personally, if I owned a Piper I would have the AMO remove the wings at the next MPI and check the spar root for cracks. Toffs says it is almost impossible to check the area due to the spar attach design, complicated by other issues.
Jim says the aircraft is strong. Perhaps this is so. I wonder what his oldest (EKE) spar attachment looks like?
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Re: Piper PA-28 wing falls off

Unread post by Raffles » Sat Apr 28, 2018 5:57 pm

In the good old days we used to full spins with Cherokees. And stall turns.
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Re: Piper PA-28 wing falls off

Unread post by bluesky2 » Wed May 16, 2018 5:59 pm

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Re: Piper PA-28 wing falls off

Unread post by whiskeyflyer1 » Mon May 21, 2018 12:49 pm

here is link to article on the just release NTSB report, from the Piper Owners Society
http://piperowner.org/features/piper-cr ... -534420069
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Re: Piper PA-28 wing falls off

Unread post by flysouth » Tue May 22, 2018 3:15 pm

Can't imagine flying around in any aircraft type which has ever had a wing fall off and a clear cause not identified and remedied! Like something out of a nightmare.

Below is a wing-spar however which has been tested in a test rig to 16.8g in all directions to the point of permanent deformation. None have ever failed in use. I often wonder why all aircraft do not use this configuration.

One such aircraft entered a thunderstorm in the US and was thrown around like a ping-pong ball, being spat out inverted, recovering and then finally landing safely after the pilot continued the flight for many miles to the nearest airfield, wondering why the dihedral on the wings seemed to have increased! The aircraft was written off though being economically unrepairable.
Wingtanks.jpg
2009_0724bdn0010.jpg
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Re: Piper PA-28 wing falls off

Unread post by Aquila » Tue May 22, 2018 4:53 pm

Deanw wrote:
pilotC wrote:This is why I think ballistic chutes are the future.
At what height did the wing fall off?

If low level, would a chute have made any difference, given that to would take quite a few seconds for the pilot to activate?
Yes but if at a higher altitude a ballistic chute gives you a much better chance than not having one...
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Re: Piper PA-28 wing falls off

Unread post by Volo » Wed May 23, 2018 12:34 pm

flysouth wrote:Can't imagine flying around in any aircraft type which has ever had a wing fall off and a clear cause not identified and remedied! Like something out of a nightmare.

Below is a wing-spar however which has been tested in a test rig to 16.8g in all directions to the point of permanent deformation. None have ever failed in use. I often wonder why all aircraft do not use this configuration.

One such aircraft entered a thunderstorm in the US and was thrown around like a ping-pong ball, being spat out inverted, recovering and then finally landing safely after the pilot continued the flight for many miles to the nearest airfield, wondering why the dihedral on the wings seemed to have increased! The aircraft was written off though being economically unrepairable.


Wingtanks.jpg

2009_0724bdn0010.jpg
While I am apt to agree with you regarding the fact that when more than one aircraft of a type sheds a wing I would not want to fly in that type unless an inspection protocol is followed that should be put in place for high time aircraft used in a training regime. In fact the FAA mandated that nearly 20 years ago on certain Piper models but then recinded it.

The failures in this case are not about fundamental strength but about fatigue failure which sets in as a result of long term high loading that in this case might have been caused by high landing cycles on hard surfaces . The nature of fatigue failures is that they will ultimately fail at any loading , even 1 g loading or the first small peak just above that . Not like in the case you have outlined which was a single case of a loading beyond the elastic limit .

I am still of the opinion that the Piper design is poor . They did not even bother to stagger the bolt attachment placement which would probably made the attachment flange 20% stronger . A better design would have been a tongue and pin arrangement which does not take any strength away from the spar flange at all , however that would cost more to produce .

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