Re: ETOPS Question

Date:         10 Sep 97 19:38:46 
From: (Stephen H. Westin)
Organization: Program of Computer Graphics -- Cornell University
References:   1
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k_ish <> writes:

> 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.

In designing for safety, you must think not only of a backup system,
but what happens when the *backup* fails. Think about it: how long
would *you* want to be flying on a single engine?

Assuming failures are random, the longer you fly, the more chance of
an engine dying. When that engine is the only one you have left, you
would like to minimize that time and thus the probability of having to
glide to the nearest airport. I assume the various ETOPS time limits
represent a tradeoff between route flexibility (you don't want to have
to fly near a handy airport) and the increasing reliability of
aircraft engines.

Stephen H. Westin