Re: What could replace the P-3?

Date:         26 Apr 98 03:44:32 
From:         "Robert J. Carpenter" <rcarpen@erols.com>
Organization: Erol's Internet Services
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Neil Gerace wrote:
>
> Now, consider a hypothetical situation where the RAAF had the money to
> replace its venerable P-3C Orion ASW planes with some new model. Say it
> wanted to convert an exisiting but up-to-date airliner to fill the role.

> Now my question is, which one? In my 'International Dictionary of Civil
> Aircraft' which purports to list every type of civil plane still in
> service -- the Lockheed Electra which the P-3 came from is still in
> there -- there doesn't seem to be a 4-engined airliner around any more
> that's about the P-3's size.

> I'm assuming such a plane would need four engines for safety during long
> patrols at sea, not merely because the P-3 has four engines. Is this
> right?

As others will point out, there are two quite different causes of engine
failure.

1) a sick engine,

2) fuel exhaustion, stupid errors, etc.

In general, "reason 2" applies roughly equally to 2 and 3/4 engine
aircraft, since they often affect ALL the engines.

If the failure is due to "reason 1", one has to look into the urgency of
seeking a landing place. A two-engine plane flies adequately on one
engine; that was a certification requirement. Three and four-engined
planes do OK with one engine out.

"Reason 1" engines failure are essentially uncorrelated, so more engines
means a higher likelyhood of a single failure. It is twice as high on a
4-engined plane than on a 2-engined plane, everything else being equal.

But what happens if another engine were to fail? I wouldn't want to ride
in a 3-engined plane with only one working engine. A 4-engined plane
with only two engines on the same side wouldn't be very nice, either.

If the likelhood of failure of the remaining engine on a two-engine
plane is P, then the likehood of failure of one of the two remaining
engines on a 3-engine plane (with one out) is twice as high, 2P.
Likewise, the likelyhood of a second failure is 3P on on 4-engined plane
with one out, due to "reason 1".

So, 3 or 4 engines doesn't buy you as much as you might think. I'll bet
a failed engine is a worrying situation on those planes as well.

	Bob Carpenter