NTSB Finds Probable Cause of Crash of Continental (Colgan) Flight 3407: No Surprises
Icing or pilot error?
Last April, the NTSB released the data from Flight 3407's FDR. I blogged about that here. Despite wide spread speculation that icing brought down the aircraft, it looked to me like pilot error -- not weather -- was to blame.
Then, in May, the NTSB released an animation derived from the aircraft's flight data recorder, its cockpit voice recorder, and ATC transcripts. I blogged about that here. The animation, like the raw data from the FDR, made a strong case for pilot error. From the animation, it appeared to me that an inattentive pilot allowed the aircraft to get slower and slower, until it became dangerously close to the speed at which the aircraft would stop flying altogether and simply fall from the sky. Then, when the critical moment came, the pilot pulled back on the control yoke instead of pushing it forward, thereby inducing an aerodynamic stall.
The NTSB made public its official probable cause finding at a hearing yesterday. No surprises to anyone who has studied the data. According to an article in today's Buffalo News, the NTSB summed it up as follows:
The plane got so slow that the "stick shaker" — a device that helps to prevent stalls — activated. But Renslow [the pilot] mistakenly pulled back on the plane's controls at that point, which is exactly the opposite of what he should have done.
In total, Renslow pulled back on the controls three times in response to the stick shaker and "stick pusher," forcing the nose upward. That caused and then exacerbated the stall.
It's almost unimaginable that a professional pilot would make the series of mistakes that the pilot did in this case. Even a new student pilot would know better. But that's what he did.
The NTSB played its animation for those who attended the hearing. The animation shows the pilot's errors mount. The activation of the "stick shaker" is depicted 2 minutes and 8 seconds into the animation. The shaking control yoke was a final warning to the pilot that he must immediately push the yoke forward. But instead of pushing forward, the pilot pulled back. Three times. After the third time, the aircraft stalled and crashed.
There were countless points at which this aircraft could have been saved but, inexplicably, the pilot failed to take appropriate action.
The whole official version of what happened is simply not credible. An experienced pilot does not respond to a stall indicator warning by pulling up on the nose at an angle of attack far in excess of even a takeoff climb. Uh uh, something else happened aboard that airplane.
So I'm watching the PBS Frontline docco (w/ Miles O'Brien) right now, having taken a look at a lot of the info/videos online about tailplane icing a few months back. Seems to me the pilot may have misinterpreted the stick shaker response as if it were a tail stall, opting to pull to unstall the tail. Unfortunately, the FO evidently had a similar mindset, opting to retract flaps (tho' she went all the way to zero instead of prior setting of 5 deg). They had discussed icing earlier in the flight, so it would appear that they were only set up (mentally) for tail stalls, not a wing stall caused by distraction.
Note the timing of the APP radio transmission advising 3407 to switch to tower freq; it's right after the captain asks for gear down, but before the gear is activated by the FO. The airspeed is thus bleeding off fast (gear coming down) while the captain (flying) is evidently focused elsewhere. He now asks for 15 deg flaps, compounding the airspeed problem. The rate and timing of the airspeed decline fits with a distracted pilot tinkering with or monitoring radios and other knobs/dials, and not watching the ASI. Finally, once the captain suddenly feels forces start in the yoke (the shaker), as the 15 deg of flaps is added, both the captain and FO are primed for reaction to - and only to - horizontal stabilizer icing/stall. Hence the pull on the yoke against the stick shaker. (The videos online show that you may need to put in considerable nose up force to overcome an iced tail stall. See, for example, http://www.youtube.com/watch?v=S1daPJJKhEE.) I bet the captain was convinced that his tail was iced and stalled.
If you're still following this thread, Mike, I'd be interested in your take on the FDR evidence as presented in the animation. You'd think that with the relatively high icing probability of these turboprops at the altitudes & routes they fly, that a lot of sim time would be prudent to tease out the correct (and differentiated) responses to wing and tailplane stalls. I know I'd want to have had both stalls thrown at me randomly and frequently in a sim to be able to react to each circumstance correctly.
Anyone know if it's feasible (without adding foam to the tail as NASA did in their tests) to generate an easy tail stall in an aerobatic plane? I'd love to be able to feel the effect of a tail plane stall (at high altitude) in a Great Lakes or a Citabria...
Ben-
Thanks for the thoughtful analysis and the link to the NASA video.
I don't think it was a tailplane stall. A tailplane stall would not have activated the stick-shaker in this aircraft, or so I'm told. Instead, a tailplane stall would have caused the nose to pitch down immediately. But that's not what the FDR shows.
So the next question: Did the pilot misinterpret the activation of the stick-shaker as an indication of a tailplane stall? That's possible. But I think it is unlikely.
Tailplane stalls are very rare. While every pilot has stalled the wings many times, very few pilots have experienced a tailplane stall. Recovery from tailplane stalls is not part of the in-aircraft pilot training, and simulators are not set up to train for them either.
Because tailplane stalls are not experienced in training, pilots are not "primed" to handle them. For a pilot to recognize and react to a tailplane stall, he must almost be expecting it to happen -- like the NASA pilots in your video. This pilot, however, was not "expecting" anything. Rather, he was, figuratively speaking, in a deep slumber. (Harsh, I know, but how else can we explain the fact that he allowed the airspeed to bleed off over a prolonged period of time without taking any corrective action at all?)
It's unlikely that when the stick-shaker finally "woke up" the pilot, the first thing that crossed his mind was the remote possibility of a tailplane stall. Rather, it's more likely that once the stick-shaker activated, he thought "what's going on -- we're crashing" and simply did entirely the wrong thing.