Re: 737 musings

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         14 Oct 94 02:23:33 
References:   1 2 3
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In article <airliners.1994.1607@ohare.chicago.com>,
Dr. Martin Erdelen <HRZ090@AIXRS1.HRZ.UNI-ESSEN.DE> wrote:
>On 19 Sep 94 01:28:39 Karl Swartz <kls@ohare.Chicago.COM> said:
>>
>>The usual design for a wing-mounted engine intentionally puts the weak
>>point in the mount at the rear of the engine.  This way, if something
>>happens that causes the mount to break, it'll break at the rear.  The
>>engine then rotates up around the front mount, breaking it too, and
>>the residual thrust carries the engine up, over the wing, and out of
>>harm's way.  (The trajectory is also designed to avoid the horizontal
>>stabilizers.)
>
>Amazing... there seems to be no end to possible design goals.
>But, speaking naively (euphem. for ignorantly): isn't this a somewhat
>roundabout approach? Why not making the *front* mount give in so that
>engine rotates downwards around rear mount and leaves earthwards with
>nothing else in the way (instead of trying to sneak it between wing
>and stabilizers)?
>Just curious.

Because if the front mount breaks first, the thrust vector will create a
nose-up moment around the rear mount (the thrust line is below the mount)
causing the engine to push up against the strut or wing (depending on the
level of failure designated) and probably not depart the airplane.  In
such a position, a thrust-producing engine banging away at the wing could
cause considerable damage whether it eventually departed the airplane or
not.  Given the relative importance of the wing to continuing flight, we
airframers tend to prefer a clean breakaway of the engine, and the simplest
manner of ensuring this is to design the rear mount to fail first.

Let us postulate two possible failure modes.  First, the front mount fails
first, and the engine continues to run normally.  The engine would most
likely hammer against the remaining strut until parts of it fail or against
the bottom of the front spar if the strut fails totally.  This is
considered undesireable for those who were wondering.  A more innocuous
version of that failure would be for the engine to find another
equilibrium position using the failed structure above it to transmit those
upward loads (which are alleviated to some extent by the weight of the
engine).

The second scenario is for the front mount to fail, and the fuel lines to
crimp and break.  The engine will run normally for several seconds as the
fuel in the line below the break continues to feed in.  The crimped and
broken fuel lines will blow exciting amounts of raw fuel into the strut and
out into the airflow, causing a truly inspiring white cloud of vaporized
fuel to trail the airplane - right above the engine.  This would be a severe
fire hazard.

Both of those scenarios can be avoided by designing the rear mount to fail
first, and the engine to break away cleanly afterwards.

There are a couple of arugments against designing for this sort of thing.
One is that it could be considered poor form to go about deliberately
littering the environment with relatively low-time engines which could
probably have been economically repaired had they been returned with the
airplane itself to the proper repair facility.  Some people are simply not
comfortable with the concept of sequenced failure.

The other argument is that with a deliberately "weakened" mount, you are
more likely to encounter inservice problems, the fuse-pin argument, if you
will.  Understand that the concepts of fuse-pins and sequenced failure of
the engine mounts were conceived during the early years of jet transportation
when engine failures were more frequent and rather more spectacular than
they are today.  Perhaps a future airplane design will assume that the
engines are so reliable that they "never" (10^-9 probability of failure)
fail in such a manner as to endanger the aircraft.  We aren't there yet. 

Terry

-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."