Dear Idiots: Cirrus Stall Revisited
This week's video on the Cirrus stall accident drew an immediate hey-wait-a-minute trickle of e-mail calling the report's premise into question.
"The real problem and the story not being addressed here," wrote one reader "is that the stall is not the cause of the accident--and the NTSB is very clear on who caused the accident. The headline needed to be: 'Stupid flight instructors can kill you.'"
He's referring to the fact that the NTSB cited the cause of the accident as the instructor's failure to maintain control of the airplane. The stall was the result, the cause was the instructor's bad judgment. This frames the paradox we face when reporting about accidents. Do you blame the nut behind the wheel or try to explain what he did that produced the tragic results?
Taking this to the illogical extreme, all accidents that aren't mechanicals are due to bad judgment. Do we emphasize the judgment or the action that followed? Ideally, both, since understanding human factors is part of the safety equation, too. In the video, I decided to frame the stall issue because that's the research I was working on at the time. Were I doing it again, maybe I'd hammer the instructor's bad judgment instead, although I think it's self-evident while the actual chain of events isn't.
Another reader wrote this comment: "While the video does a great job at explaining the well-known 'over-the-top' spin entry issue, it also states that the descent when making the initial low-level turn to the right lessened the effect of the steep turn. Here again, this indicates that the producers of this video do not understand the basic physics of turning while descending."
We get this kind of thing a lot and it's yet another example of emphasis and focus. For the record, even though I'm not the sharpest knife in the drawer, I get turning dynamics. What the reader was torqued about is the statement that in a steep bank—say 45 degrees—lowering the pitch angle and allowing a descent unloads the wing and thus increases the stall margin. This is demonstrably correct, because lowering the pitch effectively decreases the load the wings must carry, meaning less lift is required and thus a lower angle of attack is needed in the steady state glide. That's not to say you can't stall an airplane in a descending turn if you continue to increase the pitch and angle of attack, but that's the point we're trying to illuminate. The descending 45-degree turn is the classic graveyard spiral. There's no danger of a stall; collision with the ground is the problem.
But the point we missed, said our reader, is the deceleration. Nope, we didn't miss it. As noted, the angle of attack increased from 6.5 to 10 degrees in nine seconds, a moderate pulling of pitch indicating deceleration. But just because the flight isn't steady state, doesn't mean a stall is in your future. In this example, 10 degrees is still well below the stall angle of attack and sufficient margin for a pilot whose butt is sensitive to which way the trend is going. And I submit that despite the Cirrus's impressive EFIS, this situation calls for seat-of-the-pants feel for load factor. Even a dim bulb can tell when the trend is rising from 1.2 to 2.0. To keep from overloading the wings, null the rate or reverse it by…relaxing pitch or rolling out some bank or both.
Speaking of overloading, another reader said that in relaxing pitch, you aren't really "unloading" the wing. He's right. To really unload the wing, you'd have to do a zero-G pushover. But when using the term "unload" I think most of us really mean "reduce the load" not null it. I'll concede in the context used in the video, the term lacked precision, but that's TV for you.
The larger point we were trying to make in the video is that pilots lack an understanding of how bank angle and load factor work together. It's not the bank angle that kills you. You can fly an overhead approach with 45-degree descending turns all day long and never provoke a stall. It's when you get ground fear and try to arrest it with sharp pitch and bank that you get into trouble.
And that's what got the Cirrus pilot and his instructor or, if you prefer, the instructor and the pilot.