Over Stressed?

[SEP Flyer]

Hi all, something that's been on my mind for a while, wondering if you have any thoughts on it.

Almost all of my flying is done in C152 or PA28 trainers, all of a certain vintage (most seem to be around 30 years old). Some are in reasonable 'visual' condition, while others are pretty tatty. I trained on them, did my share of 'firm' landings in them and pulled a little G in steep turns and spiral dive recoveries.

But, how would I know if the plane I am about to fly in has exceeded the G force/structural limit with other flyers? - when I do a walk around, the bolts holding the wings and under carriage on are covered, so I can't see them (and to be honest, I'm not sure if I could tell if they were over stressed anyway!). I'm assuming that these items are visually checked at the 50 hour check and at the annual, but is it more than just a casual visual inspection? Do planes have some sort of G force device/marker that indicates if the structure has exceeded the safe limits?

Structural integrity has crossed my mind occasionally on particularly turbulent flights - I 'd like to think I'd be okay if the engine stopped at 2000 ft, but not sure how I would cope if a wing fell off

Do these well used planes have a finite shelf life?

Thanks for reading!

[Jetblu]

I have trained for virtually every eventuality. If the wing ever falls off at 2000ft
Close the throttles, mixtures back to ICO, mags off, master off, place your head between your legs and kiss your a** goodbye

On a serious note, you will not be able to visually see if bolts etc are stressed unless they are physically apparent by the naked eye. I have seen stressed skins which looks like ripples.

[Pilot DAR]

how would I know if the plane I am about to fly in has exceeded the G force/structural limit with other flyers?

Aside from perhaps noticing a wrinkle in the skin which should not be there, sheared rivet heads, or something else apparently broken, you're not going to know.

Do planes have some sort of G force device/marker that indicates if the structure has exceeded the safe limits?

No, unless it has an accelerometer, and someone takes notes, before it gets reset.

That said, the capacity of those (and all certified aircraft) to carry high G loads is considerably more than most people expect. It will be (flaps up) a minimum of 5.7G before the aircraft becomes not flyable. I won't speak too much to the Piper, but for the 152, remind yourself that it has been said that there has never been a failure for the structure due to flight loading, of a strut braced Cessna. 5.7 G would be really hard for most pilots to ever achieve. Having done hundreds of loops and rolls in 150 Aerobats, I have never exceeded 3.5G. I did approach 5G in the Aerobat, but I was really trying, and it's approved for it.

When doing the walk around of the 152, it is not so much the condition of the wings which should concern you in the over stressing sense, but the H stab. When you're pulling G, you're doing it by making the H stab push the tail down. In that context, in begins to make more sense that Cessna designed it so it's sitting on to of the tail, rather than hanging from, or being bolted to it.

The H stabs are subjected to G forces, careless ground handlng forces, and buffetting forces. The result is the occasional cracking of the forward ribs of the H stab, which mate to the fuselage. Losening of the rivets in the H stab is also known. Both of these are detectable by the pilot, and in their early stages do not make the plane immeidately unsafe to fly.

In general, any of these planes with no unexplained wrinkles, no sheared rivets, and nothing falling off is very likely structurally safe to fly. As for the landing gear, if the nose wheel is not broken off, or a main tire not burst, it is very unlikely that the main landing gear has been damaged by a hard landing.

During the required inspections, the maintainer knows what to look for. If they don't like what they see, you won't be flying the plane. A few "more recent" light aircraft do have an airframe hour limit (Tomahawk, I think). The much older ones were built before there was an intent to limit light aircraft airframes this way, and are just "on condition". The safety record supports this being a good way of doing things. Aircraft with pressurized fuselages are something completely different!

[B2N2]

You would never be able to see if it is overstressed.
In extreme cases you can tell like very hard landings but other then those....no.
You put your life in the hands of the mechanics/engineers.
But even then, if there is no reason for extra vigilance during an inspection, such as a report of exceeding limitations they may not even find anything.
Have you ever seen an idiot spinning in the parking lot with a rental car?
Those morons rent airplanes too.
I'll bet you that every C172 and Pa28 out there has done a loop or a roll at least once in it's life.
Every turkey thinks of itself to be an eagle...........

[bravobravo74]

I must admit that I've wondered about the structural integrity of ageing light aircraft for a while. How much of an airframe's original strength remains after forty years of use and exposure?

The thought has occurred to me that we'll only retire the current fleet of 60s/70s GA aircraft after a spate of horrific in-flight break ups. Metal eventually breaks after repeated loading and I can't imagine that owners will withdraw their machines from service 'prematurely'.

What bothers me slightly is that a fatal weakness would almost certainly be undetectable by a pre-flight visual inspection and possibly an annual check.

[B2N2]

fatal weakness would almost certainly be undetectable by a pre-flight visual inspection and possibly an annual check.[/

Absolutely correct and one day the wing will fold on an unsuspecting innocent third party.

[Contacttower]

The thought has occurred to me that we'll only retire the current fleet of 60s/70s GA aircraft after a spate of horrific in-flight break ups. Metal eventually breaks after repeated loading and I can't imagine that owners will withdraw their machines from service 'prematurely'.

Well there are plenty of metal aircraft even older than that that are still flying...I don't see any particular evidence to support the belief that once aircraft get to a certain age they are liable to start falling to bits, provided they have been well maintained and not over stressed.

[AdamFrisch]

It's probably every pilots greatest fear - catastrophic structural failure. And I sometimes think to myself flying a 57 year old aircraft "hang on a minute, how do I know my main spar doesn't have a crack in it?". I don't, of course.

But I do know that aircraft get inspected every year very thoroughly. I also know that long before metal cracks, it has to reach its so called plastic state, which means deformation, which is often detectable to the eye. This doesn't mean there can't be cracks that are not detectable, of course. But a crack doesn't always mean a structural failure is imminent either. I also take courage in knowing that a member of our owner's club, who operates 30 of these in his night freight operation, regularly puts 15.000hrs on them or more. So I have some 10K hours still to go, I suppose.

We've all seen the horrific Hercules crash and the failure of the spar box on that:

C130 Crash - YouTube

In the Hercules case the plane was also at max gross and the pull-up after the water release probably took it over Va. But nevertheless, an undetected crack hidden behind a doubler was the main culprit.

My aircraft has a cold formed one piece main spar that runs the length of the wings. It also has a smaller spar behind it, but I doubt it could take the entire load should the main one fail. Still, it adds some extra security. Many designs use more than one spar and if you're extra nervous, seek out aircraft that employs this design.

As a complete side, the new composite spar structures are in a way much more insidious as they're virtually impossible to detect cracks or internal debonding in. At least with metal you can spot trouble as long as you inspect properly. So in a way, I feel a lot safer in a 57 year old metal plane than I ever would do in a 57 year old glass fibre one.

[Pilot DAR]

You would never be able to see if it is overstressed.
In extreme cases you can tell like very hard landings but other then those....no.

Hmmm... Disagree.

If you've ever seen bent metal, you've seen stressed, or overstressed metal.

I have seen aircraft with bent metal, and sheared rivets. They had been overstressed, though usually a result of contact with something harder than just air.

When I fly a Lake Amphibian, I can pull 2G, and see a whole lot of compression wrinkles form on top of the wing. It was stressed - but not ovestressed - they went back when the [within limits] stress was removed. If they did not go back, I would realize that I had pulled a lot more that 2 G!

For an aircraft structure to fail in flight, either a manufactured joint has to open, or something has to crack open. Manufactured joint opening has been known, particularly fuselage tops peeling off B 737's, but they are a very different case than a GA aircraft. Cracks do happen in GA aircraft, but rarely from initiation to release in one or a few flights. One of the many premise of light aircraft design is that cracks will be seen long before they create a safety concern. It does not always work, but it does work much more reliably than pilots keeping the tanks full!

they may not even find anything

Then likely there's nothing to be found - it's safe. Aircraft and inspections are carefully designed to assure that any meaninful defect will be detected before becoming unsafe.

Of course, take care with pre flight inspections, but more for hangar/ramp rash risk, that the remote chance you'll pick up an undetected structural fault.

[Pull what]

Not quite true that you may never see it. In the 70s a PA28-180 was put into a spin with four people on board by the front seat passenger. The pilot was unable to recover until a rear seated instructor intervened, they were however so close to the ground that in the ensuing massive attempt to then stop it hitting the ground the wings were overstressed VISIBLY(wrinkling). The front seat passenger was later prosecuted in the local magistrates court and the aircraft received two new wings and is still flying today!

[vjmehra]

That last post is quite frightening to read:

A student pilot got prosecuted for making a mistake and having an instructor correct it in effect...

Surely that could happen to anyone whilst learning?

[mcgoo]

He never said a 'student pilot' got prosecuted, he said a 'front seat passenger' got prosecuted.

[gasax]

How will you know if the airframe has been over-stressed? Almost certainly it will have been if is 30 years old. Gusts and hamfisedtness will pretty much guarantee it!

With a monocoupe structure you might see some deflection - the pre-flight 'walk around' is not supposed to be window dressing. With a pin jointed type structure - Pa18,20,22 and many others it is possible to damage the bolts and so the walk around needs to look at these joints - particularly looking for any witness marks.

But realistically unless the structure isvisibly deflected most pilots will not notice. But light aircraft do not fall out of the sky very frequently -and that tells you alot about the design code reserves of strength and maintenance.

[foxmoth]

This is the reason you physically move the wing up and down on walkround, it may not show up all problems but if something is already broken you will hopefully pick it up.

it has been said that there has never been a failure for the structure due to flight loading, of a strut braced Cessna.

when I learnt (1973 at BOH) there had just been someone killed in a (non AERO) C150 - my understanding at the time was that this guy had aeroed it, overstressed it and it had failed structurally, ASN Aircraft accident 24-JUN-1973 Cessna 150C G-ASHF

[Monocock]

Steady on.

How may GA fleet wings have recently just "fallen off"?

[abgd]

Aside from fatigue, is corrosion a big issue in older GA aircraft? I've heard it said that old spitfire spars normally need to be condemned.

[foxmoth]

I've heard it said that old spitfire spars normally need to be condemned.

It may be the case - but have you [I]seen[I] a Spitfire spar? A bit like an onion with square tube inside square tube, must be incredibly strong!

[A and C]

A bit of a pointless thread this ? I see no in-flight breakups excepting due to gross abuse of the aircraft.

The thing that is most likely to result in the degradation of the structure is corrosion, this is a real problem and it can be addressed by good maintenance practices.

I think that corrosion and the economic situation that had depressed aircraft prices is likely result in aircraft being withdrawn from service as the price of repair exceeds the value the aircraft.

[Pilot DAR]

my understanding at the time was that this guy had aeroed it, overstressed it and it had failed structurally

I had not heard about that one. I would suggest that it is somewhat beyond the scope of a simple over-stressing structural failure though. Tail not airworthy to begin with, then comes off, then resulting loss of control and overstressing failed the outer wing in negative G. That is not a maneuver that a pilot could accidentally do to a whole plane!

The tails of the earlier C 150's were a little less robust than the later ones. When the Aerobat was designed, a doubler was added in the tail attachment, which later became standard on 150M's and all 152's. Inspection for this defect is very easy, and it takes a long time to go from perfect contition to "falling off", so there are many preflight and full maintenance inspection opportunities to discover and repair it before it becomes unsafe.

In any case, the preflight inspection of a C150/152 H stab should include a good look at the area where the leading edge of the H stab attaches the the fuselage. Any black stains (fretting), popped rivets, evidence of anything moving, or (obviously) cracks is cause to request a maintenance inspection before you fly it. This damage is not usually caused by flight loads, but pilots carelessly pushing down on the tail to ground handle the aircraft.

The H stab of a 150/152 (or any aircraft) is designed to transmit enough force to lift the nose off the ground - when the force is applied symmetrically along both sides of the entire H stab, not concentrated at one point half way out! If you must push the tail down on a 150/152, there is only one way: One palm on the line of leading edge H stab rivets, as close to the fuselage as you can manage, and the other palm on the fuselage just at the front of the fairing which covers the joint between the V and H stabs. Apply pressing force there equally. If you have decided to apply a force to the tip of the H stab to satisfy yourself that it is still attached to the fuselage, apply no more than required to very gently "oil can" the top H stab skin. This force should be applied as close as possible to the elevator hinge. You may find that the owner/maintainer of the aircraft is not happy having "just anyone" doing this, so handle the aircraft in the manner they specify to you.

As has been said, the thickness of metal used in the structure of light aircraft is often that required for the minimum stiffness needed for that part of the aircraft, not the required strength. So sometimes they are a lot stronger than required, just to be stiff enough. That introduces ironically a whole other problem for the designer, of structure too stiff, causing adjecent strucutre to endure undesired loads, but that's a whole different story!

[foxmoth]

I would suggest that it is somewhat beyond the scope of a simple over-stressing structural failure though.

I would agree here - merely pointing out that there had been at least one accident that went against your statement

there has never been a failure for the structure due to flight loading,

There was no suggestion though that the tail was not airworthy to start with, Aerobating a non Aerobatic aircraft though is asking the aircraft to put up with loads it was not designed for.