From: kls@ohare.Chicago.COM (Karl Swartz)
Subject: Re: Any comments about the El Al crash?
Organization: Chicago Software Works
Date: Fri, 9 Oct 1992 11:01:56 GMT

In article <1155@galileo.rtn.ca.boeing.com> rfh3273@galileo.rtn.ca.boeing.com (Dick Harrigill) writes:

[explanation of why engine mounts are designed to fail under certain
circumstances deleted]

>Therefore, jet engines are typically mounted in such a way that the mounts will
>fail rather than allow the structure to fail in such an event.  The mounts are
>designed to fail in a controlled way so that the engine will break away without
>causing other structural damage to the aircraft, and allowing it to remain
>airworthy.  This happens infrequently, but it does happen.

Nice explanation!  Several examples of this in practice come to mind,
including an American 727 DFW-SAN (?) which had a leaky john.  A big
chunk of blue ice fell off and was ingested by engine #3 (or #1, can't
recall which).  This took out a number of blades in the engine and the
resulting imbalance produced a severe vibration which, as intended by
the designers, led to the separation of the engine from the aircraft.

A similar even occurred a couple of years ago to a Northwest 727 out
of Florida.  Can't remember what the engine ate but essentially the
same thing happened and a farmer had a JT8D in his field.

As noted, the mounts are designed to fail in a controlled way.  For
example, the DC-10's design received a lot of attention after American
191 crashed at O'Hare in 1979.  The '10 has three bolts which attach
the engine/pylon assembly to the wing.  (Obviously #2 is different!)
Two bolts are at the front, the third at the back.  The intent is to
put the weak spot aft, which will tend to cause a detaching engine to
stay connected longer at the leading edge.  The residual thrust will
then push the engine up, detaching the two foreward bolts, and then
vault the engine safely over the wing.  Of course care is also taken
to ensure that the path will not intersect the tailplane!

The maint flaw in the DC-10 design was that inadequate consideration
was given to just what be torn up in the leading edge and what might
be the effect of tearing up those pieces.  With all three hydraulic
systems right in the line of fire, and inadequate redundancy in
various systems, AA 191 was doomed.

>... it makes me wonder because it appears that two engines were
>lost.  What are the odds of two simultaneous mount pin failures or
>two simultaneous catastrophic engine failures resulting in the engine
>departing the aircraft?  Not too good, I imagine.

Assuming there's nothing akin to the improper maintenance which
contributed to the demise of AA 191 -- and there's no evidence for
it -- simultaneous pin failures are pretty unlikely.  Boeing has
only had reports of 15 cracks in these pins in the world-wide 747
fleet in the past ten years.

However, some external event could easily cause simultaneous failure,
a bird strike, for example.  Or a particularly spectacular failure of
#3 could conceivably spew fragments into #4.

In any case, at least one recent report suggests that only one engine
may have separated after all.  The two pieces seen falling from the
aircraft may either have been separate portions of the same engine or
may have been the engine and the tailcone, which was also found in the
vicinity of the lake where the engine or engines were found.

-- 
Karl Swartz	|INet	kls@ditka.chicago.com
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