Tailplane stall recovery
Joined: Dec 2002
Posts: 2,775
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From: UK
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.
Joined: Mar 2001
Posts: 4,563
Likes: 35
From: I wouldn't know.
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.
Joined: Sep 2008
Posts: 570
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From: MI
safetypee -
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.
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.
Joined: Feb 2009
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From: Austria
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
Psychophysiological entity
Joined: Jun 2001
Aviation Qualifications: ATPL
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From: Walton on the Naze Essex.
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.
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
Joined: Sep 2002
Posts: 2,188
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From: La Belle Province
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?
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 ....
Joined: Jan 2007
Posts: 200
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From: Vermont
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.
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.
Thread Starter
Joined: Apr 2002
Posts: 45
Likes: 0
From: Hampshire, UK
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
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
