Negative G's breaking wings?????
 

For a few decades now I have studied anything I can find about air safety. There are sometimes obvious lessons from this data, and sometimes some "not so obvious" lessons. Sometimes just mystery.
Some Aero Commander Shrike accidents have had the ultimate scary "in flight breakup" that has not been explained, and there are remarkable similarities.
Some common factors are,
probable moderate to severe turbulance
probable simultaneous wing failure at the same point on each wing.
Probable high speed maybe at or exceeding VNE
Severe downward forces on outboard wing sections causing breakup.
This aeroplane has a significant "washout" on the wings, and engine nacelles extending forward of the wing leading edge, with significant weight ahead of the wing spar.
I have to wonder if severe turbulence put high positive "g"'s on the aeroplane causing the weight of the engines to twist the wings so that the outboard sections were at a negative angle of attack, generating the high speed and forces to break the wings in a downward direction.
Crazy??
Maybe. but we are getting repetition of some scary things.

In '81 a Fokker F28 flying over Moerdijk in the Netherlands experienced severe turbulence after entering a thunderstorm, causing one of the wings to separate from the aircraft after it experienced loads of up to +6.8 G and -3,2 G. The F28 is a plane with the engines at the tail. (And quite a tough one, too)

No offence, but I think you're clutching at straws here. Aircraft are designed and built according to certain operational requirements, and those include the limit and ultimate loads ("g") on the airframe. Exceed those and anything can happen.

Spoke to a guy working for GAM a few years ago about the wings on the shrike and his simple response was that the factory came back saying that they pushed them out to fast off the production line and the spars were not properly heat treated.

Yeah right...someone at an aircraft factory would admit that.:ugh:

Factory, specialist, assesor, whatever. My story teller may not have gotten his story correct about who it came from, but he did work for GAM (Australias largest operator of AC500S and distributor).If it is not entirely correct, its a good theory.

Bushy
 

The structure around the engine is a fairly deep (chord wise) box section into the wing, within a short distance span wise of the fuselage attach and carry through point. I would be surprised that substantial untwisting of the wings could occur within the rated ultimate load of 6.6 G's.

If significant twisting did occur it would then tend to unload the lifting ability of the wing and hence reduce the G load. But it would need to create massive distortion and skin buckling. Momentary heavy negative loads associated the inertia of heavy landings would, I suspect have shown this as weakness very early in it's history. Additionally with the torque effect created in the same area from landing and braking loads on the relatively long U/carriage legs attached to the same structural area. This is probably an area of high surplus structural strength.

I would also assume (?) that the wing was statically loaded to verify ultimate loading and that at least sound mathematical modeling was done to determine the twisting moment of the forward of the CP engine mass. I don't know, but I figure that if the factory went to the trouble of static testing the main plane then it would not be an overly onerous task to load the engine mountings to verify the bending moment at the same time.

As one who makes a dollar flying, among other types, the lovely old Shrikes in low level ops, I look and listen carefully to matters of structural integrity for this type, whilst always keeping the fingers on one hand firmly crossed.

M

I have a question, for the folks who might know -

With auto pilot engaged - altitude hold - what occurs in the event of an updraft?

Does the auto pilot try to hold altitude through VNE?

Ad/ac/86..

Folks,
At the risk of oversimplification:

1-Almost all aircraft have washout towards the tips.

2-At a high enough IAS, the AoA towards the tips will become negative, inducing a downwards bending load.

3-Wings are generally designed for a greater positive load than negative, therefore it takes less downward bending (-)for a wing to fail than upwards (+)

4-Therefore you can calculate the theoretical speed in a dive, at which the wings of an aircraft in trim will fail in downwards bending.
5-Add all the complications of turbulence etc., and that speed may not be all that fast.

6-I have a vague memory of several strutted Cessna singles breaking up this way, with the wings failing just outboard of the struts, and failing more or less symmetrically, as we have had with two AC500S.

Know your AFM Limitations, and stick to them.

Tootle pip!!

PS: have a look at:

<http://www.casa.gov.au/airworth/papers/AeroCommander.pdf>

Indeed, the factory did say that there were spars built of lousy material, once the problem was discovered.

:hmm: The AC 500S is the only twin engined aircraft Bob Hoover would perform in because of its structual strength. Having said that it does appear that inflight breakups could be a problem.

Currawong

Autopilot SHOULD disengage.

This might make them break.....
 

Have the failed AC/Shrikes been used for aerobatics? :(

Read the PDF file linked above. I think only 2 out of 24 wing failures involved overstress. Anyway, overstressing can occur in any flight regime (unusual attitude recovery during an instrument rating test for example,) doesn't have to be aerobatics.

I believe that the low level survey Shrikes had an extra (or strengthened spar).
Bobby H probably used the same during his aeros display.

I don't think it's an extra spar but rather a stainless steel spar strap that strengthens the point at most risk of fatigue.

Folks,

You can read all about the saga of the Aero Commander spar caps, spar straps,lousy alloys delivered, bi-metal corrosion, cold bent extrusions etc, inspections and all the other ADs in Steve Swift's paper, the link is in my post above. Even the Gulfstream 1000 (aka AC695) had problems.

The old Department of Aviation AC695's in WA seemed to attract (now) FOI's with a memory problem about gear levers and checklists, maybe that is why DCA on had so many, every gear up (and there were a few) was little more than a gravel rash belly, unlike the NSW DCA Region C-310 or the DH-125.

The 500A and on were initially certified in the utility category, all the Hoover aeros fitted those permitted in utility, he did have some mods to his display aircraft, but not structural.

That Hoover worked for North American Aviation for years before they bought the Aero Commander line might just have something to do with him choosing to fly the Aero Commander line in airshows --- along with his P-51D and occasionally a T-28B.

Tootle pip!!

Are these AC50s ex Coastwatch?

I am aware, from an ex observer, that these were subjected to negative G as a demo of pilot prowess to the observer/s. Heard of one case severe enough to produce a loss of power on one engine.

Having said that, I would expect that these problems would have been identified by now with the required spar inspections needed in the course of their maintenance schedules.

The positive load factor in a Ac500 is around 4.4g, the negative 2.7g.

If your buddies in CW ever got anywhere near -2.7g, I would be very surprised.

What you may have experienced is hard nose-down at the top of a climb. In the order of zero to -1g.

Don't know what could have caused loss of power on an engine, perhaps fuel starvation.
Normally aspirated engines lose power very easily due to the carb float moving to the cutoff position.

The accident in Tassie was likely to be possibly caused by autopilot down-trim runaway, taking the hapless pilot through VNE very quickly.

You don't need severe neg. G to starve a Shrike of fuel, anything less than 0 G can do it, and the leaner the mixture the apparent quicker the onset of fuel induced hesitation, returns with positive G.
The culprit is I believe the fuel vapour separator located on the firewall of each engine which has a ball float of some description inside. It is not unheard of (not common either) for a sudden gust to stick this ball float at the top of the vapour separator and stay stuck until removed or tapped with a hammer.
A good reason to stay within the positive envelope of the Shrike at least with the 540 E1 B5's which are incidentally normally aspirated and fuel injected.

D.O.G,

The Tasmanian accident report is a matter of disagreement, those of us very familiar with the it do not believe an autopilot runaway nose down was the problem. We do believe severe negative G was a factor.

As to the most recent, it was a very low time aircraft, delivered in the last several years, and in my experience all GAM aeroplanes were maintained to a very good standard.

Coastwatch has nothing to do with it, the AC structural problems have a history almost as long as the type, go have a look at Steve Swift's paper, the link is several posts back. At the time a bunch of Coastwatch aircraft were sold off, all the ones I surveyed had low times on rebuilt wings, with new sparcaps, straps etc.

Tootle pip !!