Let me start with the same caution as in yesterday's item
about this sad incident: it can take months or years to get the full explanation of an airplane crash, sometimes the real answer is never known, and any hypotheses now are tentative.
So my purpose yesterday was not to say definitively what had happened in the crash but instead simply to explain what a "stall" means in aviation, since the implications are so different from the normal sense of that term. And my purpose now is to explain the possibly complicating factor of a "tailplane stall," which is emerging in recent stories about the incident.
The "horizontal stabilizer," or tailplane, is the flat part of an airplane's empennage, or tail. (If this is not clear, check the NASA diagram here
.) Like the wings of an airplane, the horizontal stabilizer is an aerodynamic surface, which provides lift. In essence, it is a wing mounted upside down
. The curved, airfoil surface is on the bottom of the horizontal stabilizer, not the top as with a wing. The "lift" it provides is downward --
the purpose of which is to raise the nose of the plane. You can think of this like a see-saw: downward pressure at the back of the plane pushes the nose upward. This is necessary for reasons I won't get into, having to do with the center-of-gravity and center-of-lift of most airplanes.
When an airplane stalls, it is usually because the wings, which lift the aircraft as a whole, can no longer do so (as explained yesterday). This is a "wing stall," and when it happens the airplane stops flying and starts falling to the ground.
In an "tailplane stall," the upside-down wing at the back of the airplane can no longer do its job of "lifting" the tail down and thereby pulling the nose up. This usually happens because the tail becomes covered with ice. When it does, the airplane's nose suddenly pitches down. The airplane is still flying (since the wings still work) but is heading for the ground. This 23-minute video produced by NASA
does a superb job of explaining the theory and practicalities of the problem. Also, it's a nice sample of the tone and approach of a lot of aviation-training material. (Other discussion of the video here
Here's why this matters. The WSJ report
mentioned yesterday says that in the Buffalo flight's final seconds, the air crew pulled the plane's nose up as hard as they could. In "normal" stalling situations, this is exactly and catastrophically the wrong thing to do -- as every pilot knows through repetitive training. But in a tailplane stall, as the NASA video shows, pulling up is the right first thing to do. So if the pilots thought they were facing a tailplane stall, they could have -- mistakenly -- reacted in a way that made a normal, wing stall worse.
Other reports (including yesterday's
in the NYT) suggest that tailplane icing is not normally a problem in the plane involved in this crash, a Dash-8, but that it is more common in the model in which the pilot had previously flown, a Saab 340. If this is true, it might suggest why the crew (may have) reacted in the wrong way for these circumstances. But here we enter the realm of speculation, subject to the caveats with which I began. It is a tragedy, which stalls in some form will probably help explain.
Read the complete post at http://jamesfallows.theatlantic.com/archives/2009/02/one_more_possibility_in_the_bu.php
20 Feb 2009 3:12