Tailplane stall recovery

[Windrusher]

Apologies for being particularly dense, but I'm puzzled by the principles behind the 'stick back' tail stall recovery - how is it that increasing the (conventionally negative) angle of attack increases the (negative) lift of the stalled tailplane? There's been a previous Pprune thread (http://www.pprune.org/tech-log/153740-tail-stall-recovery.html) on this but it doesn't seem to have answered the question; I've found an FAA guidance note (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/451296dbdf212c81862569e70077c8f9/$FILE/AC91-51A.pdf)on the topic, and the NASA folk who produced the excellent video (http://video.google.com/videoplay?docid=2238323060735779946) (referenced in the thread on the Buffalo crash (http://www.pprune.org/rumours-news/362055-continental-turboprop-crash-inbound-buffalo.html)) have also produced a written report (http://gltrs.grc.nasa.gov/reports/1999/TM-1999-208901.pdf), from which a couple of pertinent paragraphs are:

Reducing thrust was the first part of the procedure because it was increasing thrust that led to the stall event that was encountered. Pulling back on the yoke increased the camber of the tailplane, which provided enough tail download to counteract the nose-down pitching moment and increase the atail. Raising the flaps was initiated by the copilot immediately, but the flaps are hydraulically actuated and movement is rather slow (~ 1º/sec). The major lesson learned to recover from a tail stall was to undo what was just done to cause the event.

It was noted that this tail stall recovery procedure is opposite of the recovery from a wing stall. The reason for the difference is the location of the flow separation. In a wing stall, the flow separates from the upper surface of the wing, therefore reattachment is made by decreasing the wing a. In a tail stall event, the flow separates from the lower surface of the tail and requires a positive increase in tail a to reattach the flow. Because of these differences in the stalling mechanisms and recovery procedures, it was determined that pilots should be made aware of the cues that may occur prior to a tailplane stall. Efforts to increase pilot awareness on this topic are described in the following section.

It's reasonable that to unstall the elevator one will wish to make its angle of attack more shallow (less negative), and that this will correspond to a more nose-up aircraft attitude, hence lower airspeed, reduced flap setting etc. But recovery seems to assume that elevator control remains to some extent effective: pulling back the column, raising the elevator, will increase the angle between the tailplane chord and airflow - ie tailplane alpha will become a larger (but conventionally negative) value. Naively, one would not expect a stalled aerofoil to respond with an increase in (downwards) lift.

A few thoughts...

The NASA report refers to increasing the tailplane camber, suggesting that the effect is a profile-related increase in the lift of the stalled aerofoil, which is sufficient to change the aircraft attitude and subsequently unstall the tailplane.
Conventional (wing) stall recovery relies upon a separate aerofoil (the elevator), which we don't have in this case. Pulling back is therefore equivalent to unstalling a wing by deploying flaps.
There will presumably be an increase in the tailplane drag which, depending upon the aircraft, could help or hinder recovery.


Apologies for the curiosity: I appreciate that what works is more important than why it works, and the NASA video is compelling, so this is entirely academic - but then, so am I!

Many thanks,

Windrusher

[Checkboard]

The tail is conventionally producing negative lift. Think of it as an "upside down" wing. If you were just into a wing stall, and lowered flap, the increased camber effects the airflow of the air ahead of the wing, and the stall is recovered. Same for the tail, just upside down.

[Windrusher]

Okay, happy with that - but I guess it prompts me to clarify what's meant by the stall in this case (I'll assume that we're looking at the tailplane upside-down, so that lift and angle of attack are positive):

the tailplane is stalled if the elevator deflection is fixed - ie lift decreases rather than increases if the angle between airflow and chord is increased (which, from this point of view, makes the stick-fixed aircraft unstable in attitude, as observed); but nonetheless
the lift increases as usual with elevator deflection. The increase in camber therefore wins over the increased effective angle of attack.

I guess I'm comparing this with the usual tale about ailerons being ineffective at the stall...

Windrusher

[Chesty Morgan]

Checkboard I'm not necessarily sure that's the case.

Imagine using ailerons in a stall situation (a BIG no no!) you merely deepen the stall on the wing with the downward moving aileron by increasing the AoA, this being one of the reasons that you may get a wing drop. I would suggest that using flaps in the case of a wing stall you would deepen the stall on both wings.

The standard stall recovery is to reduce the angle of attack of the wing, to unstall it. To use flaps or ailerons has the opposite and completely unwanted effect.

As I understand it a tailplane stall causes stick force reversal and leads to the elevator being forced into the nose down position, or vice versa, and the reason a hefty pull is required is to overcome this force (which can be huge).

[hightimecfi]

Part of the issue with the tailplane stall is 'snatch' of the control surface, more accuaratly called a 'reversal of the hinge moment.' The ice contamination at or near the leading edge results in a 'bubble' of separated flow on the underside (vacuum or lifting side) of the tailplane. As the AOA (of the tail) increases, the bubble grows in length, with the re-attached flow point moving further aft on the tailplane chord, until it encompasses the control surface. Then the control surface is 'sucked' into the bubble and the flow fully separates. This 'snatch' of the elevator will typically pull the control column right out the pilot's hands and slam it into the full nose down position.

Part of the reason for the 'apply up elevator' step in the tailplane stall recovery procedure is to move the elevator back out of the 'snatched' full down position, which helps get the airflow to re-attach.

The same condition can happen on the ailerons when operating behind a contaminated leading edge. This aileron snatch is theorized to be the dynamic that caused the loss of roll control in the Roselawn accident with the ATR.

The aileron snatch phenomenon is more likely as the main wing approaches the stalling AOA (which is much less than normal due to the icing contamination of the leading edge.)

[FE Hoppy]

I assume this "snatch" only applies to non powered flight controls.

Just watched the movie from the first post. It answered my question for me.

[hightimecfi]

You are correct; control surface hinge moment reversal is cited in the literature as not an issue in aircraft with powered controls.

Of course, that does not offer immunity from ICTS.

[oligoe]

Quite an interesting video on that subjet :

Tailplane Icing (http://video.google.com/videoplay?docid=2238323060735779946)

Regards,

og

[PEI_3721]

See the explanation (with diagrams) of tailpane stall (http://uk.geocities.com/[email protected]/alf5071h.htm) (listed under Tail Ice) - from the link in #1.

[DC-ATE]

Chesty Morgan (http://www.pprune.org/members/48495-chesty-morgan) -

The standard stall recovery is to reduce the angle of attack of the wing, to unstall it. To use flaps or ailerons has the opposite and completely unwanted effect.

Don't know what airplane you're referring to, but every transport catagory airplane I've ever flown requires flap extension as part of a clean stall recovery.

[Condor]

control surface hinge moment reversal is cited in the literature as not an issue in aircraft with powered controls.

However there must come a point where the aerodynamic forces at the elevator will be stronger than the hydraulic powered controls (as per the well known servo transparency / jack stall in the rotary world) - has anyone any idea what sort of forces we are talking about in this fixed wing scenario?

[Chesty Morgan]

DC-ATE, Q400, BAe 146 and E195. None of these require flap extension to recover from stall situations. Even after pusher activation.

In fact the E195 doesn't have a pusher only a shaker. The recovery from that is to apply full power and maintain the pitch attitude.

The only way to unstall a wing is to reduce the AoA of the wing not increase it by using flaps and or ailerons as this only causes the wing to enter deeper into the stall.:=

[DC-ATE]

Chesty Morgan (http://www.pprune.org/members/48495-chesty-morgan) -
The only way to unstall a wing is to reduce the AoA of the wing not increase it by using flaps and or ailerons as this only causes the wing to enter deeper into the stall.

Well, that might be true in those "little" machines you're talking about, but not the stuff I've flown.

[Chesty Morgan]

Well I'd have thought the only way to unstall any wing is to reduce the AoA.

I'd be interested if you could expand a bit on your comments and which types you're referring to.

[DC-ATE]

Chesty Morgan -
I'd be interested if you could expand a bit on your comments and which types you're referring to.

Lockheed Constellation, Douglas DC-6/7, Boeing 737 (-200), and Douglas DC-8 (-50, -61, -62, -71). Flew the Lockheed Lodestar as well, but have no recollection of the stall recovery procedure any more!

[Chesty Morgan]

Well obviously those old fashioned things you flew have different characteristics to the "Little" modern things I fly!;):}

[DC-ATE]

Chesty Morgan -
Well obviously those old fashioned things you flew have different characteristics to the "Little" modern things I fly!

Well, those "old fashioned" things took pretty good care of me for thirty years and I never bent a piece of metal, so something must've been right compared to all the problems I see and read about now-a-days.

Good luck to you.

[Denti]

737s do not require any flap selection for stall recovery anymore, at least not 300 to 900s.

[DC-ATE]

737s do not require any flap selection for stall recovery anymore, at least not 300 to 900s.

Well, I flew the 300 (but it was in the last Century!!) and from the Clean Stall we selected flaps. The incriment changed through the years, however. Time has a way of changing things I guess.

[Mad (Flt) Scientist]

Can I just comment that you are not pulling back on the elevator in a tail stall to try to unstall the tailplane - you're puling back on the elevator to try to get some nose-up aircraft motion, because with the tail stalled the aircraft will be nosing over like mad if you don't.

To unstall the tail you need to reduce the AoA at the tail - which is achieved primarily by selecting wing flaps up and reducing the downwash over the tail.

The normal effect of flaps on an airfoil is to reduce the AoA at which it will stall (you're making the airfoil work harder, so you'd expect to have it "give up" sooner) - although the maximum lift for the flapped airfoil does increase. I would therefore be shocked if any of the cases cited used flaps ALONE to recover; lowering the AoA has to be part of stall recovery.

What lowering the flaps will do, in combination with a lowered AoA, is enable you to generate more lift once you're unstalled, so you'd need to lose less altitude to recover back to a safe flying speed. I don't know, but I'd think that's the reason for flaps selection on some types - to make the recovery better.

[safetypee]

DC-ATE the aircraft types that you quote are relatively old. If your experiences are similarly dated, then since that time the FAA and US industry view of stall recovery has change significantly. The old focus on minimum height loss (applicable on final when more flap might be unavailable), has changed to AOA reduction and specific techniques related to the characteristics of modern aircraft, i.e. follow the manufacturer’s instructions.

[Denti]

Yup, that makes sense. I guess Boeing ditched the flap selection on clean stall recovery because one case is the high altitude stall recovery and flap selection over 20.000ft is not allowed, so using that procedure would put the plane immediately into a non approved (and not tested) configuration. And to make it simpler for the flight crew they just dont use it anymore.

[DC-ATE]

safetypee -

DC-ATE the aircraft types that you quote are relatively old. If your experiences are similarly dated, then since that time the FAA and US industry view of stall recovery has change significantly. The old focus on minimum height loss (applicable on final when more flap might be unavailable), has changed to AOA reduction and specific techniques related to the characteristics of modern aircraft, i.e. follow the manufacturer’s instructions.

Oh, I have no problem with your comment about the "age" of the aircraft I flew. I'm even older than they are!! However, although I don't have any current manuals, I would imagine the stall recovery procedures for those aircraft I listed haven't changed. But, naturally, I could be wrong. Nevertheless, the procedures worked back then! Maybe we'd better go back to those types instead of all this "new" stuff that the procedures have to be changed all the time.

[Tu.114]

It would seem that the difference between a/c requiring flap extension in case of stall and those that do not is that those that seem to require flaps come with slats installed. To me this would make sense, as slat extension provides the upper boundary layer with more energy and might help reattach the flow. The types I know (DH8 and F70) are not equipped with slats and do not require flap extension for stall recovery. Tu.114

[Loose rivets]

What lowering the flaps will do, in combination with a lowered AoA, is enable you to generate more lift once you're unstalled, so you'd need to lose less altitude to recover back to a safe flying speed. I don't know, but I'd think that's the reason for flaps selection on some types - to make the recovery better.

Quite so, I was going to say that the lowering of flaps would contain the speed a tad...in some ways it's saying the same thing. However, a lot of candidates - often ones that should have known better - let the speed build up far too much, so I guess training like that would have seen a lot of limiting speeds busted.

We used to train to the push in the real T-tailed airplane (1-11) - there was no choice, we had no sim - and the 20,000 feet Denti mentioned, was 15,000 for us, (until more experience had been gained by BAe) so no experimenting with flap if the near-stall was from clean.

We went to the shake and a tad of real buffet with flap down, to cover that aspect of training, but did not ever go to the push.

BTW, the push was quite docile - once you got used to the klaxon - and I'm told that we could easily have restrained it...memory fails me, but I think it was about 80lbs

[Basil]

Perhaps the canard stalled.

The NASA tail icing video posted by oligoe is very much worth watching.

Re flaps for stall recovery; I guess that technique had gone when I learnt to fly.

[Mad (Flt) Scientist]

However there must come a point where the aerodynamic forces at the elevator will be stronger than the hydraulic powered controls (as per the well known servo transparency / jack stall in the rotary world) - has anyone any idea what sort of forces we are talking about in this fixed wing scenario?

Sorry, didn't see this before. The answer is, absolutely massive forces required to backdrive the PCUs in any but severe system failure cases.

Consider that because the controls are powered, there's no real design need to aerodynamically balance the controls, so the PCUs can overcome the large hinge moments associated with large control deflections at high speeds, and also can maintain control positional accuracy against the risk of fluttter all the way to VD and beyond. Then there's the fact that to backdrive a hydraulic actuator you've usually got to overpressure the cylinders enough to blow off a protective valve or some such (or just blow the seals) and you're probably talking forces an order of magnitude greater than those which can snatch the controls from a human hand.

Perhaps in the event of mutliple hydraulics failures you might get the system power down enough. But that's one Hell of a bad day you're having ....

[Mansfield]

Hightimecfi gave a perfect explanation of the snatch issue aerodynamics.

I think it is also important to emphasize the power of the control force in a fully developed situation. In the elevator case, Rich Ranaudo of NASA measured around 175 pounds of pull force in the Twin Otter during the one test in which they inadvertently let the situation get a little out of hand. That is more than he weighs, which says something about one's motivation in such circumstances.

Gilbert Defer of ATR discussed the aileron behavior during the ATR tanker testing following Roselawn. In this test, the ailerons rapidly oscillated back and forth...so it may not always be a straight forward snatch.

More importantly, with regard to the elevator case, is the change in stability, and then stick force gradient, prior to the snatch. As the stabilizer flow changes due to ice accretion, a nose down elevator input will lead to more flow separation and thus a marked reduction in nose up lift provided by the tail. The nose up elevator input "recambers" the tail and re-attaches flow, returning the tail to a full lifting capability.

So what typically happens in these events is that a slight nose-down input is called for, for example if one is a bit high on the glide slope. The nose-down input results in a radically larger nose down translation. The pilot responds with a larger pull force, generating a nose up input. The tail is re-cambered, flow re-attaches and the fellow gets the correct response to his mighty pull...a large nose up translation. Naturally, he walks right into the trap. He then applies a larger than normal nose down input, drastically de-cambers the tail, and gets a more substantial flow separation. The nose plunges, and now the required pull force can be enormous.

Keep in mind that throughout this event, the elevators still work; they change the lift produced by the tail.

In extreme cases, the stick force gradient may reverse. When this happens, increasing push results in less aerodynamic resistance, rather than more resistance as normal. Thus, a stronger push leads to far larger changes in elevator position than the same push does in a normally balanced situation. Now we have a really bad situation with the elevator moving far into the de-cambered profile.

Many of these type of accidents will have a small pitch down, followed by a strong pitch up, followed by a pitch down to the lawn dart position into the ground. Note that this is best experienced at speeds far above the main wing stall, so the two really don't co-habitate. One question which remains, and which I personally believe was significant in the Columbus J41 accident, is whether one can mistake a stick pusher for an elevator snatch. Based on my experience with the Fairchild Metro, I believe you could make this error if you were overly vigilant towards a tail stall.

The DC-9/MD80 series is capable of this, and it has happened. The difference is that full flaps are usually selected around the outer marker (or equivalent), allowing a lot of altitude for recovery. Turboprops, such as the Viscount and Jetstream, call for landing flaps much closer in. No altitude leads to no recovery. Even then, a successful recovery was achieved in a couple of the Viscount accidents initially...followed by an unrecoverable nose-over farther down the runway as the fellow tried to manage the stick force gradient problem. As we now know, retracting the flaps just one notch solves the problem.

I had the opportunity to fly the Twin Otter at NASA with Rich Ranaudo while ice shapes were on the tail. I have to say that you don't really appreciate things like longitudinal stability and stick force gradient until you try flying with them in a very altered state. You really have to understand how this beast will manifest itself in order to resist walking right into the trap.

[Windrusher]

Many thanks everyone, and particularly to Mansfield for what seems the definitive and authoritative answer!

This has certainly prompted me to ponder rather more carefully the different characteristics of the 'stall', though I confess that it still isn't clear to me whether the tailplane is stalled in the simple sense - that an increase in angle of attack no longer results in an increase in lift. Certainly, for elevator authority to be retained, there must still be an appropriate, if reduced, response to a change of camber - but the massive flow separation clearly limits the authority of the pilot, if not of the elevator itself, and the asymmetry in effectiveness makes the 'correct' recovery crucial. It sounds as though with increased icing the elevator doesn't so much reverse its effect as lose it altogether, and the pilot's job is to retain and exploit what little authority remains.

That all refers to roughly level flight, though, and I can well imagine that in an uncorrected pitch down and acceleration the tailplane will indeed stall - in all senses.

Thanks again!

Windrusher