Re: ETOPS Question

Date:         06 Oct 97 02:14:26 
From:         John Weiss <John.R.Weiss@Boeing.*NOSPAM*.com>
Organization: Boeing Commercial Airplane Group
References:   1 2 3 4 5 6
Followups:    1
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jf mezei wrote:
>
> When flying at 42k feet which seems about the ceiling for commercial
> passenger aircraft (except for Concorde), how long (max) would it take
> for an engine-less plane to drop back to sea level an become a boat ?
>
> In other words, how long would the backup electrical and hydraulic system
> really be needed for in the worse case scenario with a lightly loaded
> plane ? Are we talking 10 minutes, 30 minutes, an hour before the plane
> gets to sea level ?

In round numbers (which may or may not be representative of any
particular aircraft), assume:

Glide speed of 200 Kts
Glide ratio of 10:1

That gives a vertical speed of 20 Kts.  From an altitude of 42,000' (7
miles), there is about 20 minutes of glide time available.