Re: 2 Vs 4 engines - CFM56 Engineering Dept Costs

Date:         10 Feb 2000 05:03:46 
From:         Richard Cochran <>
Organization: MindSpring Enterprises
References:   1 2 3 4
Followups:    1 2
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<> wrote:
> On 17 Dec 99 01:57:44 , Erik Verheijden <> wrote:
> >A closer look at statistics tells us that of all recent
> >incidents/accidents to large airliners, the number of engines on the a/c
> >wouldn't have made a difference in a single case.
> I would disagree. I think there is a reasonable chance the loss of the
> Lauda Air 767 might have been avoided had it been a 4 engine aircraft,
> however that is probably the only case.

Not exactly what you were thinking of, but...

UAL 232, July 19, 1989, Sioux City, Iowa.  The uncontained failure of
engine #2 on a DC-10 caused hydraulic failure and loss of control
authority, leading to a fatal crash.  This would not have happened if
the plane had been a twin, with no center engine.

The risk of problems due to uncontained engine failure goes UP as you
add engines, while the risk of total loss-of-thrust goes down.  A
simplistic exercise in statistics says that, assuming all the
probabilities are small, adding a third engine diminishes safety
unless three times the probability of an incident caused by an
uncontained failure is lower than the square of the probability of a
loss-of-thrust incident.  For a fourth engine to pay off, four times
the probability of an uncontained failure causing problems has to be
smaller than the cube of the probability of loss-of-thrust.

As the probability of loss-of-thrust gets smaller and smaller, those
squares and cubes get very very tiny, and it's hard to keep the
uncontained failure probability low enough to pay off.  For any given
set of failure probabilities, there is an optimum number of engines,
and adding more engines hurts safety.