If you’re wondering how the commuter airliner crash near Buffalo occurred, this NASA video on tailplane icing may be helpful. A bit of background… An airplane’s wings pull the airplane up into the sky. The center of gravity is in front of the wing. For stability, the tail surfaces push down (See figure 4-20 within the Pilot’s Handbook of Aeronautical Knowledge (FAA)). If the tail were to stop pushing down, the nose would pitch down uncontrollably. The video explains how extending flaps increases the angle of attack on the tail and, given ice accretion on the tail, that angle may be a stalling angle.
Some highlights… 11:00 they show how they attach simulated styrofoam ice to the tail; 13:00 they tell you not to try this at home; 15:50 unexpected tailplane stall, corrected by retracting flaps.
When I heard flaps were increased shortly before the crash, tail plane stall popped to mind, too. I saw this video a long while back, after reading about another commuter icing accident. My conclusion was that it was ridiculous to expect a pilot to correctly identify a tail plane stall and react correctly while in the middle of managing an instrument approach. If the nose pitches down suddenly, what pilot is going to think “reduce power, pull back, raise flaps”? Caught off guard, your muscle memory will take over, most likely, and you’ll try to break a normal stall, doing almost exactly the opposite.
There are just some things you need to fix with technology, and not NASA pilot education videos…
Do you know if the Dash 8 has deice boots on the horizontal stabilizer, Phil?
http://www.aerospace-technology.com/projects/dash8/ says that the tail is equipped with boots. Some other search results confirm this. No system is perfect, however. Turbojet airliners are less susceptible to ice than a turboprop like the Dash 8 because the higher speeds keep the skin warm and bleed air heats critical leading edges to above the vaporization point of water.
Given the NASA video’s reference to an autopilot’s tendency to hide preliminary indications of tailplane icing, this may be of interest:
http://news.bbc.co.uk/2/hi/americas/7891770.stm
I gather that turboprop engines do produce bleed air, but why is it that
it is less commonly use for de-air in that sort of airplane? Too little
heated air?
Here is a bit more about icing in Turboprops:
http://tinyurl.com/aurbm8
The article starts out with an icing encounter in a Saab 340B that
was flying an approach; the flight crew was able to recover in this
case, however they still had enough altitude to work with compared to
the Continental flight (IIRC, listening to the ATC transcript, 3407 was
at ~2300 ft when they got into trouble). 340B isn’t a high tail config aircraft
like flt 3407.
Perhaps they were too aware of tail plane stall. The NTSB reports that the plane pitched up initially. This would have been the wrong reaction if it were a normal stall.
What kind of control system does the Q400 have on the elevator is an important question. Is the elevator fixed leading edge, and is the pitch control power assisted or is it a aerodynamically balanced pitch control system.
Miles O”Brian (formerly CNN) has some interesting and detailed information on icing.
http://milesobrien.wordpress.com/
Well, it’s now looking like it was a regular stall, according to the WSJ. From the CDR it appears, shockingly enough, that the airplane simply got too slow, the stall shaker went off, and the pilot inexplicably pulled back against the stick pusher, pulling the airplane up into an aggravated stall.
NYT this morning proposes an explanation for the pilot’s pull-back reaction: as Tom speculated, the pilot may have misinterpreted the downward pitch as a tail stall (though it had instead been caused by the stick-pusher in response to low airspeed).
http://www.nytimes.com/2009/02/19/nyregion/19crash.html?hp
It would not surprise me if in the final analysis it is determined that the precipitating circumstances and events including mimicked signs of a tail stall close enough to mistaken for a tail stall. Icing conditions, onset of difficulty upon configuration change (flaps), and action of the stick pusher (simulating dramatic uncommanded nose-down pitch) might well have deceived the pilot into countering the forced yolk forward action of the stick pusher, as pulling back against the yolk is essential to tail stall recovery.
…and now, according to the NYT there is speculation along the lines of Tom’s comment #5 above:
http://www.nytimes.com/2009/02/19/nyregion/19crash.html
Apparently the captain had recently transferred from flying Saab 340s which are notorious for tailplane icing, and he may indeed have used the wrong recovery. But the article also says Dash 8’s are “not susceptible” to tail icing. I find that hard to believe; Dash 8s are great airplanes but that would require something more like magic.
And it still doesn’t explain to me why the stall warning would have happened right when they lowered flaps.
What’s the right way for a pilot to know the cause of his stall? In cases like this, it seems like he has an impossible task – to evaluate the data in time – leaving the correctness of his decision to chance, even if he makes perfectly reasonable decision. ‘Pilot error’, maybe, but if it’s luck, it’s hard to really place that blame when it doesn’t work out. I mentioned some algorithms that the inventors claim can deal with icing automatically here:
http://mcgonigle.us/bill/blog/articles/2009/02/15/how-up-to-date-is-your-autopilot
The problem with the theory that he was correcting for a tail plane stall, is that he didn’t raise the flaps. Also, it strikes me as implausible that a pilot who wasn’t doing well enough to keep the airspeed up would manage to even remember what a tail stall is. I’m not judging him as harshly is it sounds. I’m sure I’d do no better. When I was doing my instrument training, my biggest takeaway from the whole thing was that it was utter bullshit for people’s lives to depend on a pilot doing it correctly when the chips are down. I really think too much is expected of pilots flying hard IFR in any airplane that’s not fully automated. Yes, it’s manageable, but not with the kind of margin you’d like to see when a terrifying death is at stake, and the pilot is flying his first job since instructing in C152s. One’s ability to react correctly is shockingly bad when terrified, and I’m guessing the slow speed situation caught the pilots off guard and scared the shit out of them.
We live in a world where automation has advanced to the point where the space shuttle can deorbit and land itself. We’ve got cars with anti-skid brake systems whose computational power rivals a jet fighter’s. Why the hell are our loved ones flying in airplanes without autothrottles? Why is “drop out of the sky and kill us all” a member of the set of allowable control inputs on a commercial aircraft?!?
Is there a reasonable chance of recovering from an ice-induced tail stall at low altitude in a Dash 8, even if the pilot responds optimally? If not, shouldn’t pilots be trained to perform only a normal stall recovery at low altitude, rather than even trying to identify a tail stall? (Is that in fact the training that’s given?)
Tailplane stall has been an issue on Embraer Banderantis, Vickers Viscounts, Twin Otters and some other aircraft however the cert rules have been beefed up in recent years to address it. The q400 was flight testedthrough extreme pitch maneuvers with simulated tail plane ice and showed no tendency for tailplane stall. Tailplane stall occurs usually at full flap and the Colgan aircraft did not get to full flap if I understand correctly. Also, the nose-down is sometimes caused by aerodynamic force reversal on the elevator which for a reversable flight control system can push the stick forward violently, causing the nose-down. The Q400 has a powered system – the pilot does not feel the actual aerodynamic forces on the elevator as the force comes from an artificial feel unit. The aircraft behavior, as described by the NTSB, is not consistent with a tailplane stall.