Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: Convair 990 + misc.
Date: 01 Apr 94 13:11:23 PST

In article <airliners.1994.1089@orchard.chicago.com>,
Tobias Henry Lutterodt  <luterodt@phoenix.Princeton.EDU> wrote:
>I'd like to clear up this question:  Is it true that before, Boeing
>designed engine mounts to give when an engine seized and let the
>engines fly while now (post-Amsterdam) they are trying to keep the
>engines attached to the plane if at all possible?  Terry?

I am not clear on exactly how the strut design criteria have changed in the
last couple of months (I could ask my strut stress animal, I guess :-)).
I can say a little bit about past criteria and how we got there.
There are at least two very critical design cases.  One is where the engine
seizes, phi dot = 0, omega dot approaches infinity :-).  Given the enormous
amount of rotational energy stored in the rotating parts of a turbine
engine that would have to be reacted out through the strut, wing, and
fuselage, it is more economical (lighter by tons) to design the
engine/strut to depart the airplane in a controlled manner.  The other
major case that comes to mind is a gear up landing.  The engines hang down
and collect up obscene amounts of concrete during such operations.  If this
were all that happened, it wouldn't be too bad.  It is when the engines hit
an obstruction (like an embankment) and are torn off of the wing that
things become exciting.  We want the engine to depart safely, which means
without rupturing the fuel tanks in the wings.  The single most critical
condition of a safe evacuation is the lack of jet fuel mingling with hot
engine parts at the crash site.

This is all very clear, yet we are finding that the structural solutions to
those problems are vulnerable to corrosion and poor maintenance practices.
There may not be a graceful fix for this exposure (this is not my
specialty) and I have heard noises suggesting we design to keep the engine
on the airplane even if it seizes.  I have not heard anyone address the
crash separation issues - but as I say, this isn't my area and I only know
what I read in AvWeek.  :-)

If this isn't confusing enough, let me know and I'll see if I can murk it
up a bit.  :-)

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."

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: Those swinging engine pods!
Date: 01 Apr 94 13:11:44 PST

In article <airliners.1994.1060@orchard.chicago.com>,
Michael Yip <yip@sybase.com> wrote:
>It seems to me that in recent years more engines are falling/tearing
>off from 747s than before.   Here is a recent article (AW&ST 3/14/94)
>related to another incident in Tokyo:
>
>	For the First time, Japan's Civil Aviation Bureau has
>	asked the FAA to check the safety status of a US carrier
>	after a Northwest Airlines 747-251B bound for New York
>	from Hong Kong lost an engine while making an intermediate
>	stop at Tokyo's Narita Airport on Mar. 1.
>
>	The aircraft's landing was normal, but after several
>	hundred yards its No. 1 engine fell foward and skidded
>	on the ruway for about 1,000 yards.  A fire broke
>	out that damanged the wing, but the situation was not
>	deemed serious enough by the cockpit crew to order
>	emergency evacuation of the 228 passengers and 18 crew.
>	Instead they used mobile stairways.

For what it is worth, the fuse pins were still in place and undamaged from
what I've heard so far.  However, the retaining parts were no where to be
found...

Interesting sidenote, the officials at Narita were quite unhappy with
Northwest back in December and January for leaving chunks of retread tires
all over the runways on what seemed to be a regular basis (shutting the
active runway down for up to two hours).  Now Northwest is leaving engine
parts on the runway.  :-)  You got to feel for these guys!

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."

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: 737 musings
Date: 14 Oct 94 02:23:33

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."

Newsgroups: sci.aeronautics.airliners
From: drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Subject: Re: Engines CAN jettison (Was Rear engined aircraft. (727 DC9 MD80) )
Date: 12 Jan 95 01:56:04

In article <airliners.1994.1795@ohare.Chicago.COM>,
Clemens Emanuel Tillier <ctillier@leland.Stanford.EDU> wrote:
>>>(1) They are designed in a ditching situation to shear off and flip over
>>>    the top of the wing rather than dig into the water & pull the wing off.

Not so.  Engines will shear off in the aft direction in a ditching (or at
least, that is how we analyze it).

>Karl Swartz  wrote:
>
>>This is also for landings on runways without landing gear, but I think
>>the concern is more to keep the aircraft from flipping than it is for
>>tearing off the wings.
>
>How can an engine shear off and go *over* wing?  In situations such as
>described above, I would expect the engine wreckage to go *under* the
>wing.  (Especially for the water landing.)  Also, what is meant by
>"flipping" the aircraft when landing without gear?

An engine can shear off and depart the airplane by going over the wing
if it is still generating thrust and the rear emgine mount failed first,
as it is designed to do.

I think what Karl is referring to is when an airplane skids in and an
engine hit some obstacle, like a berm at the end of the runway or
something, causing the airplane to spin violently.  I think we'd rather
have the engine shear off.  It has the side benefit of absorbing (or
shedding, depending on your point of view) some of the aircraft's
energy.


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