Re: ETOPS Question

Date:         10 Sep 97 19:38:46 
From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
References:   1
Followups:    1 2 3
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In article <airliners.1997.2104@ohare.Chicago.COM>, k_ish
<kenish@ix.netcom.com> wrote:

> I have a basic (hopefully not dumb) ETOPS question:
>
> Why is there a time element to ETOPS, e.g. 180 minutes?  I have read a
> number of very technical articles on ETOPS, and understand the myriad
> factors determining ETOPS route of flight- but none of these articles
> address my question.
>
> It seems to me as long as you have enough range at the reduced ETOPS
> TAS, there should be no time limit.

The origin of ETOPS (Extended-range, Twin-engine, OPerationS) is a time
limit.  After several bad crashes of twin-engine piston airliners in the
1950s after they lost one engine and burned up the other one trying to
reach an airport, the FAA (or maybe it was still the CAA) created the
60-minute rule.  Any twin-engine airliner could be no more than 60 minutes
flying time (on one engine) from a suitable airport at any time during its
flight.  This rule was based on how long everyone figured the remaining
piston engine could be run at max continuous power before failing, and
it's still on the books today.  But with the advent of turbofan engines
and increasingly reliable airplane systems, it became pretty obvious that
the A300/B767 generation of twin engine jetliners could safely maintain
flight more than 60 minutes after an engine failure.  Also, it would be of
great economic advantage to the industry and the flying public if
twin-engine jetliners could be used on long routes, say over the Atlantic
and Pacific.  So "exceptions" to the 60-minute rule began to be granted,
but of course they inched the time limits up in increments.  So the first
ETOPS operations were 90 minutes.  Then 120 minutes, then 150 minutes, and
now 180 minutes.  The reason the whole thing is based on time is because
that's the only way you can measure the ability of a plane to continue
safe flight after the in-flight shutdown of an engine.  The fire
supression system must be able to keep a lower hold cargo fire from
spreading for 180 minutes, the backup electrical and hydraulic systems
must be able to keep the airplane's control systems functioning for 180
minutes, the battery(s) must be able to keep certain critical
communication and navigation systems functioning for 180 minutes, and so
forth.

C. Marin Faure
  author, Flying A Floatplane