Heathrow crash

Andy Hawkins wrote in
:

Hi,

See the report into the Heathrow crash has concluded it was 'probably'
fuel line icing? Seems a bit vague to me, slightly worrying a definite
cause couldn't be found.


Well, the evidence would have melted away.

Interested in what the big iron drivers on here thought?

I've heard a lot of talk about this accident. this isn't the only 777
that has had thrust issues on approach after a long flight, but it's the
only one who had both wind down at the same time.
I've heard today that the report came out but I can't find it on the
net. I'd be able to comment more fully if I had it to read.

It isn't exactly fuel icing, but what's called waxing. The fuel gets
very gooey near it's freezing point and doesn't flow so good when it
gets into this state. A typical freezing point for fuel is around -39C
and we have a temp gauge to tell us what the temp is in at least one of
the wing tanks.
Long flights in temps of as low as -70C can get the fuel that cold.
Actually, the total temp including skin friction would be about 25
degrees higher than that.
The main lines of defence, mechanically, at least, are that the
hydraulic system radiators are in the fuel tanks. This keeps the
hydraulics form cooking and also warms up the fuel a bit, and the engine
oil is cooled by fuel through a heat exchanger.
Older airplanes, like the older 737s and 727s, had fuel heat. This used
bleed air for short periods of time, usually 30 seconds, and heated up
the fuel in a big way. It had a failing in that if it got stuck on you
had to shut down the engine. That happened to me twice in 727s, but it's
no big deal and the shutdown is precautionary.
For some reason they don't install the bleed air fuel heat on modern
airplanes. I have no idea why, but it would definitely have saved that
BA airplane. I wouldn't be surprised to find they're considering putting
it back in.
The BA flight was polar and very long, so the temps would have been very
low. The crew would have certainly been monitoring the temps. they have
**** all else to do on a long flight like that anyway, and if the temps
were getting low, they would have probably requested a lower altitude,
and/or increased the cruise speed to increase the TAT (total air temp)




Bertie