Stalling in a loop

A stall at the top of a barrel roll can certainly have tragic results. Of course, if there is sufficient altitude for recovery then you will be ok.

Another way that enormous strees can be put on an airframe is by using large deflections of the controls. The speed at whch significant damage can be done is well within the normal speed range for most non-aerobatic light aircraft. Remember the Airbus that crashed in New York because the fin detached after extreme rudder deflections caused by wake turbulence.

This link is to the accident bulletin of a PA28 Arrow, which suffered a catastrophic structural failure because of this.

http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_025533.hcsp

The scenario that the AAIB believe was the most likely is that the front seat passenger inadvertantly moved the controls and the pilot then reversed them.

I fly an Arrow, and was surprised by how much below the normal cruise speed of 130 knots one had to be to avoid structural damage – 118 knots, in fact. Should have known, of course.

There was another case of structural failure of a Commander 114, which flew into a snow storm and ended up diving into the ground after the wings came off as the pilot tried to recover from the dive. The manufacturers estimated the g load at the time of failure to be 8.8g. The link is at:

http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_500069.hcsp

I also fly a Bulldog, which has 8 g meters for monitoring the Fatigue Index, as well as a resettable one on the instrument panel. These have to be logged after each flight, along with the number of crew, number of take offs and landings and the total time. These are analysed annually and the remaining FI is calculated. Once that is used up, the aircraft stops flying and will have to be resparred.

At least with the meters, you know if you’ve pulled too much. And you cannot hide the fact from other people, because the FI meters cannot be reset. This has the advantage over the average club plane, where you can never be too sure what the last person to fly it had done.

YR

This is a tough one aj 😀 I will try to make some sense out of all of this.

All light aircraft have to be able to withstand a certain amount of g to get certification. In which way that g is applied does not matter at all. It can be in a pull out from dive, or performing aerobatics the aircraft is not stressed for.

Let’s look at your Cessna C172 example. A fully loaded C172, at 2300 Ibs, has a limit load factor of 3.8g positive. What that means, is that it can go up to 3.8 g without any damage to it’s airframe, through out it’s service live. Then there is another definition, the so called ultimate load factor. Ultimate load factor has to be at least 150% (FAR 23 regulations) of limit load factor, which means it is 5.7 g for the C172 (when new!). Once an aircraft reaches it’s ultimate load factor, damage is caused to it’s structure. It is not neccesarily a catastrophic failure (though it might be), it might be ‘just’ wrinkles in the skin or popped rivets etc. But there is a grey area, between the limit- and ulitmate load factors, that is dangerous as well. If an aircraft exeeds it’s limit load factor, but does not go all the way to the ultimate load, no damage will be apparent from the outside, no matter how well you examine. However, fatique will set in, and lower the g threshold at which structural failure will occure. On an old aircraft that has many cycles of exeeding the limit load, a structural failure might happen at a g load well below the original ultimate load factor.

Now, how are these things monitored you might ask. Well they are not. In the standard light aircraft today, there is no, non-resetable g-meter to monitor loads on the airframe. And they are not needed in my opinion. History tells us that the pilots flying these aircraft are careful, and I can not remember any accident where a light aircraft just broke up in flight with no apparent reason. Most of the in-flight brake ups happen after loss of control in IMC, or, after performing unauthorized aerobatics. Both scenarios place heavy loads on the airframe.

As to the checks that would have to be made after a high g pull out from a dive, I do not know of any such checks, and it is difficault to know exactly how much g was pulled as g meter is very seldom installed. Sure, if you have obvious structural damage, like wrinkles in the skin, or popped rivets, the relevant part (most likely the wing) would have to be replaced.

I hope this answers your question.

That would figure 🙂 (C of G)

I’ll stop after two 😮 I promise.

Moggy

Hi Moggy,
No, I don’t think there is anything you need to know about how to fly your Colt, at least not from me 😉 😀

This reference to the PA 22 was a bit of a mistake on my part, as I did not remember the Colt 😮 The PA 22 I was referring to was the PA 22-150/160 Tri Pacer, on which I have some time. In the AFM for that model it is clearly stated that intentional spinning is Stricktly Prohibited, and it is placarded on the instrument panel as well. I remember reading a couple of reports on Tri Pacers lost in unrecoverable spin to the ground. I suspect that aft C/G played a big part in some or all of them, so it may not be entierly the aircraft’s fault.

So, happy spinning in your Colt 😀 Just remember not to let it turn more than twice in the spin!

some light aircraft are not save to spin upright (the Piper PA 22 for example)

Eh?

Have you a source for that information?

Our PA22 (2-seat 108 bhp Colt) is approved for spinning as long as it is operated light. Is there something I should know?

Moggy

Well first of all, stall, as such, is very seldom dangerous in it self. 😎

If we assume that you were doing the barrel roll more or less correct (that is, had positive g) when you stalled, it does not matter which side of the aircraft is pointing upwards. If you have positive g, it is just a normal stall, of the variety seen in straight and level flight. To affect a recovery, ease the stick forward to decrease angle of attack and increase engine power to increase speed.

In case the barrel roll was less than perfect :rolleyes: and you had negative g when the aircraft stalled, you would have had………a negative g stall, or the kind of stall you would expect from the aircraft when flying up side down. To affect a revovery, pull the stick torwards you to decrease angle of attack and increase engine power to increase speed.

Most important thing to remember about stalls, is the fact that it does not matter which side of the aircraft is up or down. If you have a positive g, it is a normal upright stall (even though the aircraft might be inverted) and if you have a negative g it is a normal inverted stall (even though the aircraft might be upright). Sound complicated, but once you start flying aerobatics it all falls into place 😉

Now to your question. To say that only two aircraft (types) are able to recover from a stall in the situation you descripe (wether it be positive or negative stall) is absolutly absurd! All aircraft can recover from a positive stall and all aircraft can recover from a negative stall. The only tricky part is, some aircraft (i.e. transport category) would not neccesarily be able to recover from inverted flight (if the stall happened in such conditions). But if you are talking light aircraft, ALL of them would be able to recover from either stall.

If you are taking the stall to it’s next level, the spin, the waters are muddied somewhat, as some light aircraft are not save to spin upright (the Piper PA 22 for example) but most of them are ok. None of the standard light aircraft have been spin tested inverted, so recovery from that situation is unknown. However as you were talking about a Chippie, I think it would be fairly save to say that it can be recovered from both upright and inverted spins as it is fully aerobatic. Most aerobatic aircraft don’t care which side is up when they are spinning.