Re: longest flight?

From:         graemec@ibm.net (Graeme Cant)
Date:         10 Oct 96 11:29:28 
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In article <airliners.1996.1978@ohare.Chicago.COM>, Alan.Wong@anu.edu.au (Alan
Wong) says...

>Mmmm. This will also depend on how the aircraft is configured. For instance,
>Qantas 767s were (probably still) not allowed to fly to Europe from Asia over
>Afghanistan because their emergency oxygen packs were too small. If the plane
>lost cabin pressure over Afghanistan, there wasn't enough oxygen to last till
>the plane cleared the mountains. (Source: Australian Aviation) Other 767s
>such as Lauda and Lufthansa/Condor had no such problems.

Yes - but there's a bit more to it than that.  There are two rules
that come in here.  I know the Australian ones but most countries
are similar.

1.  An RPT aircraft must have sufficient fuel to ensure it can
reach a suitable airport from any point on the route if it suffers
an engine failure or a decompression.

2.  An unpressurised RPT aircraft is limited to 14000ft if oxygen
is available or 10000ft if it isn't.

The decompression requirement means that at the critical point,
there has to be enough fuel to allow a descent to 14000ft, cruise
at that altitude to the limit of available oxygen followed by
cruise at 10000ft to the 'suitable' alternate.

The fuel flows at 10000ft and 14000ft differ sufficiently that it
is usually a good trade to carry (say) 200kg extra oxygen if that
allows 1000kg less fuel to be carried.  With the 747-400
depressurisation fuel is often required on the Pacific so they have
large oxygen supplies.

The Qantas 767 is different.  They were purchased with 90/120 mins
ETOPS in mind.  They were always going to have enough fuel to reach
a suitable airport - there had to be one within 120 mins.  Carrying
extra oxygen to allow cruise at 14000ft rather than 10000ft
is no benefit so they only carry enough for the descent to 10000ft.
Decompression fuel may become a problem with 180 mins ETOPS, however.

Alan's right that 767s can't reach Europe over the Himalayas with
the small oxygen pack.  The escape routes from the Karakorams and
Hindu Kush routes that are currently flown (the western end of the
Himalayas) do not allow an immediate descent to 10000ft because of
terrain.  The aircraft must be able to cruise for some time up to
14000ft to reach the escape airports - Tashkent, Delhi, Karachi or
Kabul - and the lack of oxygen stops the Qantas 767 doing this.

The real problem in the area for all aircraft is Tibet rather than
the Himalayas themselves.  A chart I have shows that in the region
bounded by 75E-105E and 30N-50N the lowest safe altitude is 15800ft,
most MSAs lie above 20000ft and the highest is 31400ft.  Even
unlimited oxygen will probably not allow RPT flight over this area
where a decompression might require one or two hours unpressurised
cruise above 20000ft.  I know a finer grid would show lower heights
but the number and complexity of the escape routes needed to the
few airports available are probably more than any regulatory body
would accept (unless 'affordable safety' returns to CASA - in-joke
for Australians :).

Finally, AFAIK (and it's not my area) the 757 is the only modern
Western aeroplane certified to takeoff and land at altitudes above
10000ft.  An emergency landing by a 747 could produce a great new
restaurant at Lhasa!

Graeme Cant