Saturday, March 19, 2011

Stalls

Stalls? Yes, stalls. An article in the current issue of Aviation Week and Space Technology describes how loss of control has replaces controlled flight into terrain as the number one cause of accidents, with details on several accidents in which stalls were a factor: the Colgan Air Q400 at Buffalo, NY, in 2009, a West Caribbean Airlines MD-82 in Venezuala, and a Turkish Airlines Boeing 737-800 in Amsterdam.

How could a professional pilot stall an airplane into the ground? There are many contributing factors, but I think one of the must be the way that our current training materials make the meaning of "stall" obscure. To quote the Airplane Flying Handbook [FAA-H-8083-3a], "A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly."

A young instructor recited a similar definition to me the other day, and I compared it to hay after the horse has eaten it. The reason is that while this definition describes the aerodynamics effectively, it doesn't tell you how to recover from a stall. How does a pilot smooth the airflow? By polishing it clean? How can you reattach the boundary layer? Is that what duct tape is for?

In the old days (and I'm not that old) we used to teach that a stall meant too high an angle of attack. That definition tells you how to recover! "Stalled," I say, pulling an invisible yoke into my gut. "Unstalled," I say, pushing an invisible stick to the invisible panel. "Stalled." "Unstalled." "Stalled." "Unstalled." Until the point is made.

The FAA also says "the application of more power, if available, is an integral part of the stall recovery." That's wrong; ask any of my glider students! You add power to climb away, not to recover.

So I have taken pen to paper again (metaphorically) and written another cranky letter to the editor of Aviation Week. (AOPA Pilot didn't publish this one, by the way, although I got a nice note from Bruce Landsberg about it). Here's what I wrote, working to fit their 200 word limit:



The increase in loss-of-control accidents is partly explained by the confusing way we teach stalls. Today’s instructors talk about stalling as a boundary layer effect, which does not offer any insight into recovery. We used to teach that a stall was excess angle of attack, which also describes the recovery procedure: Reduce the angle of attack. A pilot who understands stalls this way is unlikely to descend to the ground with the
yoke fully aft.

Pilots are also taught that power is part of stall recovery, although my glider students do not have that option. The purpose of power is to climb away. The net altitude loss from starting recovery before adding power is minimal, and the risk of loss of control is reduced. This recovery sequence is most important in an airplane like the Bombardier Q400, which risks both spin entry (due to the large left-turning tendency from two propellers) and Vmc loss of control (in the event of an engine failure). (Granted, if engines have long spool-up times it is prudent to start that process as soon as possible.)

Advanced training as offered by the major simulator centers offers few opportunities to correct this basic misconception, leading to the tragic results you described.

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3 Comments:

At March 19, 2011 at 5:10 PM , Anonymous Dave Starr said...

Great article indeed, Jim. As a mere low-time pilot, but a diligent student of safety and accident investigation, I've been troubled by the same accidents you cite ... there are so many things that can 'get' a pilot, but for a pilot of the aeronautical skill level and experience of these folks to make such early primary student errors is really mystifying.

We obviously can do better at teaching stall recognition and recovery.

 
At March 19, 2011 at 5:57 PM , Blogger John Ewing said...

Here, here!

Another common problem is pilots who overdo the stall recovery by diving toward the ground. When asked why, the usual response is "to gain airspeed." Whether they were taught this or their instructor simply condoned their stall recovery technique, it shows a lack of understanding.

My theory is that a contributing factor may be the stall warning indicator itself. If a pilot is unconsciously conditioned to become anxious when they hear or see the stall warning indication, it's natural that they may unconsciously want to silence it during the stall recovery phase. A simple demonstration that the warning goes away when the stall occurs may help the pilot to understand that it is more like an "I'm not stalled yet" indicator and that hearing or seeing the stall indicator during the initial part of the recovery is actually desirable

 
At March 19, 2011 at 6:44 PM , Anonymous Ron Gleason said...

Thanks for the article Jim and another thought provoking subject. While a wing stalls based on angle of attack a stall can occur at any angle of attack as the definition of 'high angle of attack' is relative. As a glider pilot with no power experience the only option we have is to get the nose or change the CG. Since we have little way of changing CG, other than dumping water if relevant, the only option is getting the nose done regardless of the percieved angle of attack.

 

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