AF 447: High Altitude Stall or Swept Wing Stall? Did the Mishap Investigation Boards Make a Fundamental Aerodynamic Error?

The mishap investigation boards have given a less than
aerodynamically correct presentation of “high altitude stalls” in the 2009 Loss of Control LOC mishap investigation of AF 447 and the  2005 Loss of Control LOC mishap in Venezuela of a West Caribbean Colombian MD82.
The result is that these mishap investigation reports are not putting out satisfactory recommended corrective actions.
Here’s the problem: Swept winged aerodynamics differs from straight winged aerodynamics. Swept wings stall at the tips first. Straight wings stall at the root first. Swept wings pitch up when they stall. Straight wings do not. Swept wings tend to go deeper into the stall. Straight wings do not.

Pilots of swept winged transport category acft need to know this because many of these pilots received their basic and primary instruction in straight winged trainer
acft, and as such have learned straight winged stall recovery procedures.
Pilots who are now operating swept winged acft, who have not had
specific swept winged stall recovery procedures training per se, may not be adequately trained to handle a swept wing stall.
The various LOC mishap reports of the Colombian MD82 and AF 447 make reference to a problem the boards call “high altitude stall.”  This is really just an explanation that at high altitudes, the spread between true airspeed and indicated airspeed causes longitudinal pitch changes to feel exaggerated. But that extra pitch feel exaggeration is not the problem that keeps the acft stalled from loss of control at FL 370 to impact. What keeps the acft stalled is high angle of attack. The stall is the result of longitudinal controls being in the what is known as the region of reverse control, where the acft is on the back side of the power curve. This means that drag produced by the production of lift, induced drag, is so large, that it is greater in magnitude than the thrust available from the engines, meaning that even at full throttle, the pilots must push the nose down, in order for gravity to add force to thrust, to accelerate the acft forward to a speed great enough, and an angle of attack low enough, to bring the aerodynamic force vector vertical enough so that induced drag is lowered enough so that thrust can now accelerate the acft back into the normal one g flight envelop.

I apologize to all for the very long statement above, especially if it is confusing. In layman’s terms, the pilot has to push forward to get the plane going fast enough to begin flying again.

The boards should have cited swept winged stall and failure to employ swept wing stall recovery procedures as the mishap cause. Recovery procedures for swept wing stalls are different from procedures from recovery procedures for straight wing stalls.
Notice that when both AF447 and the Colombian MD82 descended through lower altitudes, they remained stalled and did not recover. The stall was not a result of high altitude, but high Angle of Attack. In both cases, if the correct swept winged stall recovery procedures had been used, the pilots could have recovered the acft, would have recovered the acft at much higher altitudes than the terrain CFIT and would have recovered the acft immediately, in my opinion. Stall recovery comes by lowering the angle of attack, not by lowering the acft altitude.

This is very important information and needs to be put out.

When a swept wing stalls, the stall emanates at the trailing edge of the wingtip due to span wise flow thickening the boundary layer. The aileron is actually one of the first wing components affected by a swept wing stall. As the stall progresses back up the wing, the aerodynamic center (AC) shifts forward, raising the nose and angle of attack (AoA). This causes the angle of the aerodynamic force (AF) to shift aft, resulting in a rapid and high rise in induced drag (Di), the horizontal vector component of AF. This induced drag opposes thrust, slowing the acft further and raising the AoA, deepening the stall.  This is known as the Region of Reverse Command or the back side of the power curve. Because induced drag rises and rises quickly, there may not be enough power alone to thrust the acft to a higher speed.

The stall is AoA dependent, not altitude dependent. The thrust available is limited by altitude, therefore the thrust deficit above induced drag is altitude dependent. Therefore the only recovery possible is to dump the nose down, reduce the AoA, reduce Di low enough, to where available thrust, as it is added, is sufficient to overcome Di and parasite drag Dp and accelerate the acft Indicated Air Speed (IAS) fast enough to regain lift and therefore one g level flight.

This is the only recovery possible. Swept wing aero is so important to know, that the US Navy has an entirely separate course on it, and it is taught after a flight student has learned to fly straight winged aerodynamic acft. In straight wing aero, the stall begins at the wing root instead of the tip. The AC as a per centage of the mean aerodynamic chord does not shift much, thus AF doesn’t shift aft and doesn’t result in a rapid rise in Di, slowing the acft further. Recovery is quicker with lowering the AoA, adding power and quickly regaining IAS.
Is it possible that the crews of neither of these LOC mishap acft received training in swept wing aerodynamics and the stalls that occur to swept wings?

There is a lot to know in swept wing aerodynamics that is different from straight wing aero, quite a bit to learn (I’ve only touched on it here). This knowledge is critical to understanding swept winged stall recovery procedures and successfully implementing them.

See “Aerodynamics for Naval Aviators,” by H. H Hurt Jr, NAVAIR 00-80T-80, Jan 1965, Naval Air Systems Command,  page 353-354, concerning the “Region of Reversed Command.”

Pilots have got to know this swept-winged aerodynamics if they are going to fly swept winged aircraft safely in all situations, I believe.

In my opinion, this is especially true, if they chose to become test pilots by conducting uncertified operations into FL 600 thunderstorms or operating acft over the certified gross weights indicated for altitude.

Let me know what your thoughts are. Thanx, Paul

Published by Capt. Paul Miller

Aviation safety expert with 43 years in the sky

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