Re: Stalls

From:         rdd@netcom.com (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         21 Apr 94 23:18:38 
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In article <airliners.1994.1155@ohare.Chicago.COM> Tobias Henry Lutterodt <luterodt@phoenix.Princeton.EDU> writes:
>
>There have been a number of incidents lately (Singapore 747-400 and others)
>where a jetliner has stalled at high speed at or near cruising altitude.
>
>I don't understand how this happens...could someone explain?

The higher you fly, the higher the stall speed goes.  The higher you fly, the
faster your true airspeed gets, and the closer you get to high-speed buffet.
Buffet is simply defined as a separation of the airflow off the wing: low-
speed buffet is a stall.  High-speed buffet occurs as a result of shock wave
formation.

The higher you fly, the closer low-speed and high-speed buffet get: the point
at which the two curves intersect is called "coffin corner," which claimed
a number of lives in the 50s.

The crtical altitude limitation is defined by the loading you wish to exert.
If you load the airplane, the stall speed will go up; therefore, high-G
maneuvers at high altitude are discouraged, as the stall speed can rapidly
be made to converge on the high-speed buffet speed.

The operational altitude limitation for an airliner is based on a margin to 
allow normal en route maneuvering; this is normally 1.3Gs, or a 30 degree 
bank, and is a function of atmospheric conditions and airplane weight.

The reason we're seeing more high-altitude upsets is because these limits
are being exceeded, either through operator error or autopilot malfunction.
Often a combination of the two: an autopilot malfunction which the operator
does not perceive and correct.

The safety measures for jet transport aircraft come in the form of a stick
shaker (a vibrating motor attached to one or both pilots' control columns,
when approaching stall speed), a VMO pointer on the airspeed indicator, and
deductive reasoning based on projected vs. actual airspeed characteristics.
On glassy airplanes (747-400, A3[2-4]0), there are also indicators to show 
the stall boundary.

The short version here is that the airplanes do not fly very near coffin 
corner; there is actually quite an airspeed margin.  There are major screw-ups
occurring elsewhere, of which the stall is just a side-effect, and often
occurs only part of the time; the incidents I'm familiar with have alternated
between punching VMO and stall, as the pilots attempt to recover.



--              
Robert Dorsett                                                       
rdd@netcom.com