Re: Are Two Engine 757 & 767 Jets Dangerous?

Date:         12 Dec 96 03:49:25 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>>Try, then,
>>the more obvious need for more rudder authority on a twin to
>>compensate for an engine out than on a four-engined airliner.

>That's a good point - but is the adverse yaw from a failed outboard
>engine on a 4-engined a/c much less than on a twin (assuming placement
>equivalent to the inboard engines of the quad)?  I guess in the
>former case you have the option of reducing thrust on the other
>outboard engine, and still have enough thrust left to stay in the
>air.

Not only can you reduce the thrust on the remaining outboard engine,
you can also increase thrust on the inboard engine on the opposite
side (the side with the failed outboard engine).

The big question is what happens when both engines on one wing fail.
The certification requirements require that you be able to take off
and climb if *one* engine fails after reaching V1, but what about
a double failure?  I assume you have to handle this in cruise since
the pre-ETOPS ICAO rules for long overwater flights mandated the
ability to limp to safety after the loss of two engines, but losing
two at cruise, with relatively low thrust levels is a very different
matter from losing two during takeoff or climb, at high thrust levels.

The El Al crash in Amsterdam, as well as the 1991 China Air crash near
Taipei (both 747-200Fs) suggest that losing two during climb may not
be recoverable -- I'm not sure if any of the wing damage in El Al was
significant with regard to control of the plane, and I know even less
of the details of the China Air crash.

On the other hand, United 811, the 747-100 which lost the cargo door,
shows that a 747 *can* recover and return safely after losing two
engines at higher altitudes (though in that case still during climb).

--
Karl Swartz	|Home	kls@chicago.com
		|Work	kls@netapp.com
		|WWW	http://www.chicago.com/~kls/
Moderator of sci.aeronautics.airliners -- Unix/network work pays the bills