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Wing comes off a PA28 during a checkride with an examiner (and wing spar structure discussion)

Peter wrote:

I can’t believe that walking up onto the wing is a significant stress, compared with the 1000-1200kg plane flying in turbulence which generates say 2g shocks (that’s 2x the aircraft weight, which is a lot more than a person’s weight).

A person walking up onto the wing is walking fairly close to the wing attachment point, which is also where flight stresses get transmitted.

But, regardless, if RH side failures are more common there must be a reason for it.

Peter, if I recall correctly, my local club had to have some damage on the right hand wing of a P28A (Archer III, 1999, 4000 hours) repaired because of a similar issue and we were told it was likely that the damage was caused by too many people standing on the wing at the same time hence a rule was introduced, one person on the wing at any time. I couldn’t believe what I was hearing because I also thought the same as you, that the stresses in turbulence would be significantly higher than (e.g.) someone (or multiple people) stepping onto the wing but hey, I’m not the guy who signed the aircraft off and gave his reasons for the repairs…..

The other really scary bit is that none of the external surfaces had any indication of corrosion/damage.

That is really interesting, because most people think that you can’t bend (permanently deform, i.e. exceed the yield strength of) the spar without wrinkling the skin.

I am not a structural engineer but it seems to point to long term fatigue. I bet the PA28s I flew in Arizona turbulence got the same; I was there in February and would not like to imagine what it would be like in the summer! OTOH those planes (Chandler Air Service) were doing ~700hrs/year and were diligently maintained, during the night.

Or maybe you can bend a PA28 spar without messing up the skin?

I couldn’t believe what I was hearing because I also thought the same as you, that the stresses in turbulence would be significantly higher

I still don’t buy it

There has to be some other reason… something to do with different loading on the two wing roots. Different fuel loadings?

Administrator
Shoreham EGKA, United Kingdom

Pure conjecture but perhaps the accident aircraft had been treated like this some time in the past:



Andreas IOM

Outstanding video – whether relevant to this crash or not, doesn’t matter. I have flown with some truly crazy instructors too.

Administrator
Shoreham EGKA, United Kingdom

Looking at all those wing bolts in the diagram causes me to wonder what they are all for, if a failure at the first one is catastrophic? What do the rest actually do, apart from conveying a sense of security? Serious question.

Last Edited by Aveling at 18 Apr 14:11
EGBW / KPRC, United Kingdom

You are right that no number of bolts can help you if the spar breaks at or outside the last bolt.

The purpose of the bolt is to attach the wing spar to the box in a way that the entire contact surface between the box and the spar caps takes the load, so they press the surface of the box to the surface of the spar cap, and more bolts make sure that this happens over the entire length of the spar that is inside the box.

So he bolts do not take the entire load of the aircraft, the box does. If there were absolutely no gap between the box and the spar, you would not need any bolts beyond one that keeps the spar from sliding out of the box – but that does not work in practice if you want to get the wings on and off the aircraft

Biggin Hill

The good thing (if one can find a positive here) is the striation lines are very distinct, so there is a lot to be learnt here.

ESMK, Sweden

Interesting. But 5 bolts? How can the last 2 or 3 do anything? In steam boilers, a single row of rivets for overlap plates sufficed into the 1860’s, when increasing bureaucracy led to double row riveting. But never more. Even with single row rivets, plates would tear before the riveted joints unless the rivets were defective. So are all the bolts there just because someone might forget to tighten one or more of them up?

EGBW / KPRC, United Kingdom

Structural engineers spend time designing joints where the loads transfer is progressive and smooth. This is for the benefit of the plate/spar cap/… not the bolts. It is especially important for fatigue. So, we play with bolt number bolt size and thickness/cross section area to avoid having peak of bearing load at a particular fastener.

Nympsfield, United Kingdom

Perhaps I can draw your attention to this video showing an A check being carried out on an Arrow…..

https://www.facebook.com/gold.standard.aviation.one/videos/1519540618154327/

to be noted it was only the right hand wing showing this issue, not the left hand wing (when viewed from inside the plane).

Last Edited by Steve6443 at 21 Apr 15:36
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