From: (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         12 Oct 96 21:34:31 
References:   1 2 3
Followups:    1 2 3
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In article <airliners.1996.2014@ohare.Chicago.COM>, (Robert
Dorsett) wrote:

> In article <airliners.1996.1975@ohare.Chicago.COM>
(C. Marin Faure) writes:
> >
> >There aren't any risks, any more than there are on any flight in a modern
> >commercial jetliner.
> There are more risks, hence the operating and regulatory requirement imposing
> extraordinary crew, airframe, maintenance, and dispatch procedures for ETOPS
> operations.

The original question was is an ETOPS flight riskier than a flight on a
three or four engine airplane?  The answer is still no, because whatever
additional risk there may have been because of the fact there are only two
engines has been eliminated by the reliability of today's equipment, the
extra redundancy of the ETOPS systems, and the stricter maintnenance
standards for an ETOPS airline.

> I'd still like to see whether these fabled "safety analyses" "proving" that
> twin operations are as good as others take into account the recent history
> involving volcanic dust ingestion.  In each case, *multiple* engines were
> shut down, and *multiple* engines and systems suffered severe damage.  If
> it's a question of eeking out every last bit of thrust, I'd much rather
> be in a 747 (or even an A340) for transpacific travel.

First, the dust incidents.   You've proved my point.  ALL the engines on
the four engine planes shut down, and ALL of them were damaged to some
degree, so at that point it didn't make much difference how many there

Three and four engine airplanes don't have three or four engines simply as
a margin of safety, they have three or four engines so they can get off
the ground and STILL MEET the required margins of safety.  The SAME
margins of safety that apply to twin engine airplanes, by the way.  If a
twin loses one engine on takeoff, its remaining engine has to provide
sufficient power to safely continue the takeoff.  That's the rule.  Ride
in a 757 sometime and you'll experience the result of having to have
engines that are capable of continuing a takeoff after an engine failure.
The performance is pretty impressive.  If a three or four engine airplane
loses one engine during takeoff, it has to be able to safely continue the
takeoff.  Same rule.  But if a four engine airplane loses TWO engines on
takeoff, it's in DEEP trouble.  And if the three engine airplane loses two
engine during takeoff, it's all over.  In cruise, you can get away with
losing two on a four and keep going, albeit at a lower altitude.  But in
takeoff, where it's really critical, lose two engines on that four engine
plane, or worse yet, on the three engine plane, and you're going to hike
home if you're lucky.
> >And since twin-engine ETOPS planes use the same types of systems used in
> >three and four-engine airplanes, the chances of an inflight problem are
> >the same for all of them.
> The problem is, you lose one engine, you've lost half your redundancy.
> You lose two in a 747, you've lost half of your redundancy.  You lose
> two in a 777, you're going swimming.

The problem is that you're doing what so many people continue to do-
focusing on the engines.  The engines these days are so reliable (I've
seen the current data on the numbers of in-flight shutdowns of today's
high-bypass fanjets on all airplanes regardless of the number of engines,
and believe me, it's a tiny number), that they're almost not a factor in
airplane reliability anymore.  That wasn't true ten or twenty years ago,
but it is today.  Of the very few problems there are on today's airplanes,
most of them are caused by a problem in the airplane's sytems, NOT in the
engines.  An electrical failure is going to have the same effects on a
four engine airplane as it does on a twin.  Your argument will be that a
four engine airplane has four sources of electrical power, but so does an
ETOPS twin.
> >Actually, there are less risks in an ETOPS airplane because not only does
> >the plane have additional backup capabilities in the critical systems, but
> But why do they need additional backups if the basic components are so
> reliable?  :-) We can go a looooong way with this kind of logic.

Because as far as the engine and airframe manufacturers are concerned,
there is no acceptable level of airplane loss. As overused a cliche as it
is, it's better to be safe than sorry.  So the manufacturers sit around
and come up with everything they think could possibly go wrong, and then
figure out a way to keep those situations from endangering the people on
the airplane.  We do this despite the fact that the engines and systems
components are incredibly reliable, and we do it on every airplane,
regardless of how many engines it's going to have.

C. Marin Faure
   author, Flying a Floatplane