Re: ETOPS Question

Date:         10 Sep 97 19:38:46 
From:         Antoin Daltun <adaltun@tinet.ie>
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At 02:03 08/09/97, k_ish 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.

This is based on a theoretical conception of a second engine failure, that
being more likely after the first has failed, and a statistically
acceptable level of risk.

Personally, this seems to me a relatively small risk.  Once an
airframe/engine combination is good enough to be trusted to make a take-off
and an approach with an acceptable level of risk, I am not clear that 180
minutes ETOPS has a risk sufficiently higher than 120 minutes ETOPS that it
is worth drawing the distinction.

I suppose it can be argued that exposure to dual (multiple) engine failure
near an airport is an unavoidable risk while a 180 minute ETOPS routing can
often be reduced to 120 minutes at minimum time/cost penalty.

I write this in a context of very low engine failure rates where twin en
route diversions are  said to be more often due to passenger incapacitation
than to technical problems.  [What about crew incapacitation?]

Brgds

Antoin Daltun