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Is the PA-28 Wing Spar really weaker compared to other designs

Is the PA-28 Wing Spar structure really weaker compared to other designs? I’m lost in information. On the one side I read about insane tensile strength of wing spar attach bolts, on the other side I read people would avoid flying through some „chop“ in a PA-28.

The spar consists of two I-beams, attached to a spar carry through via upper/lower caps with bolts. Like on all metal planes, corrosion, and lately, fatigue is a factor as the fleet ages.

From another thread:

In a PA28, I would not risk it. [Flying through convection/build up/tcu] The structure is very uninspiring when you see one opened up – compared to the tank-like spar of a TB20. But not everyone has a TB20…

Interested in engineering points of view as well as anecdotal input (for example „I only fly my PA-28 in calm weather etc..).

There are two „confirmed“ cases of 28s losing a wing (Archer 181 and Arrow) due to fatigue. Then there are a couple others, like this one

dft_avsafety_pdf_025533_pdf

and this one (came apart due to structural overload)

Report_CEN20LA201_101348_5_13_2022_3_09_51_PM_pdf

The original 1980‘s AD for wing removal and inspection was receded. The present „new“ one calling for an eddy current inspection (if „factored service hours“ are reached) applies to all the 151/161/181 + Arrow models.

@Peter Kindly not merge this thread for reasons of clarity. Thanks.

Last Edited by Snoopy at 13 May 19:10
always learning
LO__, Austria

As a Warrior owner I take comfort from the small number of incidents. (The Thruxton one was an outboard downward failure and the other a recovery from a spiral dive, so really there only the two cases, one a pipeline inspection task and the other a high cycle trainer). But I do wonder about the design thinking – why are there so many bolts? The recent failure happened at the first of five (Yes, five) bolts. What do the other four do?

Could anyone explain this please? It may be obvious to a stress engineer, but to me as a simple airplane owner it’s confusing. After all, the design can’t be that bad for the type to have outlived virtually every car on the road.

BTW, to answer your question, I don’t fly through TS/CB, not because I’m afraid of the plane breaking, but because it’s not very nice. Having people mess with the wing bolts to inspect the holes on the other hand is a concern.

EGBW / KPRC, United Kingdom

Both the Arrow 2 and the Cherokee 235 are slab wing design. The Warrior is taper wing, and while similar wing design to the Florida Embry Riddle Arrow 3, and also subject to the NDT eddy inspection AD, I don’t think there has been a wing structural failure for the Warrior. It usually rates amongst the safest, if not the safest, or lowest fatal accident per 100,000 hours type in GA.

Oxford (EGTK)

The NTSB supported the inclusion of Models PA-28-235, all PA-28R-series, PA-32-260, and PA-32-300 airplanes in the proposed AD, and requested that the FAA reconsider whether the proposed AD should include all PA-28-series models (other than Model PA-28-235).

„An NTSB member responded to the FAA Proposed Piper Wing Spar AD and said it would like some clarifications and changes made. A summary of the letter, written by Earl F. Weener, PhD:
While the affected airplanes all have a similar main spar design in the wing attach-point area where the fatigue cracking was found, engineering data presented by Piper separately to the FAA and the NTSB showed that the localized stress level can vary significantly depending on airplane gross weight, cruising speed, and amount and location of fuel in the wings. The data also showed that the airplanes at greatest risk for fatigue cracking are the PA-28-235 model airplanes, all PA-28R series airplanes, and the PA-32-260 and PA-32-300 model airplanes. The NTSB supports the inspection requirements of the proposed AD for these airplanes.“

always learning
LO__, Austria

The main difference between the TB20 wingspar to the Piper Wingspars is that the TB20 wingspar is one piece throughout the whole wing. The same goes for Mooney and some others.

The Piper Wings are separate and mated to the fusellage with bolts.

Clearly a througout wingspar reaching through both wings is a lot stronger than 2 wings attached separately. I don’t know about the TB20 but I never heard they had an airplane break apart in the air (I may be wrong with that). The only Mooney which ever came apart in the air did so in a TS many years ago. One out of the whole production.

Pipers on the other hand have quite frequently come apart in the air, still a minuscule number of times in relation to the numbers flying but still significantly more than other makes. So while I’d happily fly (and have flown) Pipers, I do not expect the tank like design other makes have with steel cages and single spars

Last Edited by Mooney_Driver at 13 May 21:08
LSZH, Switzerland

The inner portion of the Piper wing is an aluminum “I” beam, further out it transitions to riveted aluminum structure.

Piper wings have a rear spar, just like a Mooney.

Piper wings have a front attachment fitting, just like a Mooney.

Piper wings are subject to fatigue and corrosion just like a Mooney.

Pray that your spars aren’t corroded inside your fuel tanks like we’ve seen here, they could fail just as easily.

Not all of its flights resulted in a hard landing, just like your landings, not all are hard.

From the picture above you can see that the Mooney spar is not one piece from tip to tip as many here believe. It is assembled from many different pieces all fastened together with hundreds upon hundreds of fasteners. The butt end of the main spars are joined with several splice plates which are under the aft seat, take the plates off and you have 2 separate spars.

- Mooneyspace.com

Last Edited by Snoopy at 13 May 21:17
always learning
LO__, Austria

I think one is more likely to break their necks first before wing breakup…many aircraft are solidly built M20/TB20/C182 but they would break in loss of control when they have gone past their VNE or past 10G

For TB20, there is one documented in-flight break up after an aircraft that went in super-cell over Belgium

For M20, there is one documented case of loss of control in IMC

I think there was a case of Mooney going 70kts above it’s VNE after dual loss of Garmins in the cockpit and the wings are still attached, I think there is a report somewhere (I would need 2 cup of tea after such flight), there was another Ovation that was flown with EFIS recording past 10G in another loss of control event…

https://www.euroga.org/forums/flying/13153-loss-of-control-and-high-speed-after-losing-attitude-indicator-gi275-dual-failure#post_290227

For business as usual flying, the possibility of wing detaching is very remote

Last Edited by Ibra at 14 May 09:58
Paris/Essex, France, , United Kingdom

As far as I remember the design is such that only one of several bolts carries the load, or most of the load. The bolt itself is OK, but not the structure around it because that structure is designed with friction in mind. With a perfect fit, the right amount of torque, no corrosion, it probably will be OK and will last for ever. But too much torque will insert too much stress on the structure around the bolt. Too little torque will weaken the friction between the members so too much load is applied to the bolt and the structure around it. A less than perfect fit will weaken it considerably, which could be problematic when changing a wing for instance, unless this is done 100% correct. Also, the condition and fit of the aft spar matters.

It really is an odd design that demands perfection in production and fit. What holds it together is not the bolts, but friction due to the torque of the bolts (which is a normal way of bolting stuff together, works for propellers for instance, but not the way it is done here, due to asymmetric/uneven load). The typical way to design this is to assure all the forces go through one single bolt, one at the upper and one at the lower, or a series of bolts all taking equal load, either pure shear or pure tension.

Another way to look at it, is it would be OK if the bolts worked as pure shear bolts (un-torqued), and the surrounding material allowed for this kind of load. Then it would be obvious that only one bolt (two) was needed.

ENVA ENOP ENMO, Norway

TB20 spar is not one piece from wingtip to wingtip.

2 or 3 have broken up in TS.

Administrator
Shoreham EGKA, United Kingdom


I wonder if the outermost bolt hole is the weak point because the hole obviously removes material. Why not move the spar (C) futher inboard and fasten it there, leaving more material at the area of greatest bending moment?

always learning
LO__, Austria
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