Re: 727 deep stall (Re: tip vortices *do* exist!)

From: (Robert Dorsett)
Organization: Capital Area Central Texas UNIX Society, Austin, Tx
Date:         29 Mar 93 12:15:30 PST
References:   1 2 3
Followups:    1 2 3
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In article <airliners.1993.278@ohare.Chicago.COM> you write:
>Years ago the 727 accident report was published in Aviation Week. As I 
>remember, they hung this one on the pilot. He had set up a high rate of 
>descent inside the Final Approach Fix, and I don't recall that terrain
>was mentioned as a contributing factor. Might have been a failure to 
>stabilize the approach.

There are several such crashes, but none were deep stall incidents.

>Anyway, one of the measures he took to get back down to the glide slope
>was to retard the thrust levers below a recommended minimum setting. 
>This setting exists because turbine engines (particularly turbojets, as
>those installed in that 727) get sleepy at low thrust settings---they 
>become very slow to respond to throttle changes. One reason is that 
>automatic fuel controls are acting to keep from cooking the turbine.
>This in turn is one reason that landing jets have to hang out all sorts 
>of drag-generating laundry to keep approach speeds reasonable while the
>engines are still pushing.

A pilot can normally retard the throttles all the way back to idle
without having to worry about the fuel controller or flaming out the engine.  
The thrust at this setting can range from 300 lbs to 1000 lbs per engine.  

The reason for all the "drag- generating laundry" is the need to slow from 
very *high* speeds to relatively *slow* speeds in short amounts of time, 
combined with the very *clean* design of most airliners--without such devices, 
they would accelerate.

All jet engines have a spool-up time from idle, usually under 8 seconds.
I've never heard of a "safety setting" intended to reduce this spool-up time: 
it would have to be at a relatively high thrust setting, which would help 
defeat the purpose.  Lag times are simply a fact of jet engines that have to 
be contended with.

>If a stall or deep stall was mentioned, it was only as a terminal effect
>of the previous actions.

Do you have an issue date?  Again, allegations of a "deep stall" are some-
what serious, since it suggests the airplane is in an aerodynamic condition
it *can't* escape from.  In the case of the BAC-111, for example, aerodynamic
effectiveness of the elevators was almost nil.  If the 727 is actually capable
of such a stall, I'd like to hear about it! :-)

Robert Dorsett!!rdd