Re: 2, 3, 4 engines-- what's actually safer?

From:         Robert Dorsett <rdd@cactus.org>
Date:         17 Mar 93 10:18:34 PST
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>In particular, I'm interested if anyone can think of an incident in which a
>trijet survived (for some value of survived; a semi-controlled crash
>landing, such as the Sioux City crash, would qualify) where a twinjet would
>have (presumably) not done so. 

That is a loaded question.  In some cases, a trijet could be up there by
virtue of having a more flexible MEL, and in more risk, whereas a twin could
be grounded.  In that UAL 727 discussion a few weeks ago, recall that the
airplane was dispatched with the generator control panel inoperative: you
can't dispatch with dual redundancy with one generator out in a twin.

Perhaps a better way of looking at it would be to ask whether there's been
a twinjet crash which a trijet could have survived.   


>(Arguably, the above-mentioned L-1011 might
>qualify, since the early shutdown of one engine allowed them to fly just
>that much longer...)

I think that when we look at airliner certification, we need to consider the
mission profiles involved, and the regulatory environment's requirements,
rather than just the simple binary issues of whether an airplane can stay
in the air following some disaster..

For example, the 727 was originally envisioned to compete against Electras
at out-of-the-way airports.  The CAA rules (at the time) benefitted three-
and four-engined airplanes in marginal weather.  By offering a third engine,
Boeing was able to offer significantly better dispatch reliability.  The
reasoning behind the rules was that a twin-engine airplane, upon suffering
a failure, had to land immediately, at the destination airport.  A trijet
could continue to an alternate.  Thus, is weather was good enough for takeoff,
but not good enough for landing, the trijet could fly, while the twin
would be grounded.

If you suffer an engine failure in a twin, it is an automatic emergency, and
this has nothing to do with how "good" or reliable the engines are.  You
are reduced to one level of redundancy, and this is not an appropriate margin
to be flying passengers with.  With a trijet, if you lose an engine, you
lose an engine.  The flight can continue to destination.

This relates to McDonnell Douglas' wavering between a trijet and a quad in
the MD-12 program: if you experience an engine failure in a twin on an EROPS
flight, and faced with the "land now or else" decision, it could be several
hours before you can reach an appropriate airport.  The need to stay in range
of suitable airports further constrains your actions.  A trijet has lower
dispatch requirements, lower aircrew requirements, and gives the crew (and
the airline's fleet logistics) much more flexibility in determining whether
to keep the flight on schedule.  This is one reason the DC-10 has been pop-
ular with several long-distance operators.

In the short-term, single-instance failure considerations, there's not much
difference between a twin or a trijet or a quad.  But it is the need to 
*maintain* the safety margins, given the possibility of multiple failures,
which recommends three- and four-engined airplanes for longer routes.  Shit
happens, and I'd personally want at least two levels of redundancy *following*
a failure.




---
Robert Dorsett
rdd@cactus.org
...cs.utexas.edu!cactus.org!rdd