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Date: Tue, 15 Dec 1998 07:39:31 -0600
From: Snowbird <snbird@ibm.net>
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Subject: Re: emergency decents
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JStricker wrote:
 
> It depends on the purpose of the emergency descent.  Actually, the 
> real need for a true emergency descent in an unpressurized aircraft is > rare.

On what data do you base this statement, John?  And what do you mean
by it (rarer than other emergencies such as engine failure?)?

I've already had one smoke in the cockpit incident which fortunately
didn't end in the need for an emergency landing, but I wouldn't want
to bet that they're so uncommon.  

> The quickest way down in most a/c is power back, prop full ahead, 30 
> degree bank toward the pilot (maybe away from in the case of a Cessna, > but in low wings it's toward the pilot for visibility) and speed to 
> Vno.  

I agree, this is a quick descent, but I'm not sure it's the quickest
way down nor the best.  For example, one reason for an emergency
descent would be fire (and/or smoke in the cockpit).  One doesn't
just want to descend, one wants to have the best chance of keeping
the plane under control and of LANDING safely in minimal distance
once descended.

Speed at the top of the white arc, full flaps (or 20 degrees in a
C172), full control deflection slip will IME produce a very similar
rate of descent to Vno, while allowing much more margin for error
(no concern for overspeeding or overbanking).  And it won't get you
to the ground with a large quantity of excess energy which must be
expended before the plane will consent to settle down and land.

> In the Twin Comanche, I was taught two "emergency" descents.  One my 
> MEI called a "rapid descent", which was slow to Vle and drop the gear, > slow to Vfe and drop the flaps, and speed nailed on Vfe.  This 
> resulted in about 2,200 fpm or so.

I think you'll find if you toss a slip in there, the descent rate
will increase substantially.

> The other was a true emergency descent that I described above

See above comments on whether this is a good idea, depending upon
the nature of the emergency.

> and very nearly pegged the VSI at the 4,000 fpm mark.  In fact, 
> unless you're above 10,000' or so, you'll have trouble sustaining
> Vno for more than a few seconds 

If you're at 5000 feet over 1000 ft terrain, a 4000 fpm descent
takes 1 minute to get down.  That's a lot longer than "a few seconds".
And will doubtless seem even longer with smoke filling the air.

Snowbird


