Date: 06 Oct 97 02:14:26
From: malc@mci2000.com (Malcolm Weir)
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On 03 Oct 97 01:18:34 , jf mezei <"[non-spam]jfmezei"@videotron.ca> caused
to appear as if it was written:
>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 ?
As I recall, G-BDXH (BA's 747 glider) flew for nearly 30 minutes before they
got the engines back, and I think they started at 37,000ft. The engines
were restarted at about 10,000ft, as I recall. They even put the aircaft
into a dive (and so INCREASED the sink rate) at one stage due to loss of
pressurization and an inop. First Officer's oxygen mask.
Happily, G-BDXH never became a boat! In fact, in May 1996 it returned to
the limelight when a lightning strike caused chunks to the fin to fall off!
Of course, this aircraft had an APU running during the Jakarta Volcanic ash
emergency, but the point is that there is a LOT of time to discuss the
problem and prepare for ditching.
>If that time were to be lets say 10 minutes, why would 180 minute ETOPS
>certification require such truly backup system (when both engines fail)
>to operate for 180 minutes ?
There is a lot of interconnection between the primary systems. I suspect
that there are several systems that could, in theory, be affected by failure
of one engine. In this case, it is A Good Thing to have a backup system
capable of doing the work for as long as you need, while the primary system
is off-line.
Actually, the Gimli Glider itself illustrates this: one of the two fuel
computers was faulty, and when operating the defective computer prevented
the good one from providing fuel data. But when the circuit breaker was
popped on the faulty one, the remaining unit worked fine. But some
"obliging" character reset the CB to the faulty unit on the ground in
Montreal, so the aircraft had no fuel indication. Oops...
Malc.
