Re: ETOPS Question

Date:         09 Dec 97 03:54:25 
From:         Steve Lacker <look@the.sig>
Organization: Applied Research Laboratories - The University of Texas at Austin
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John van Veen wrote:
> > "Heating the APU normally" is descending into warm air and allowing it
> > to warm up due to the ambient temperature :-)
>
> When the combustion temperature, maybe 1100 degrees, of the fuel burning
> in the APU is considered, there isn't much difference between -45
> degrees and +90 degrees.  Both temperatures are very cold relative to
> how warm it will get.  Maybe someone who knows more about this has a
> comment.

OK- others have confirmed that APU's often have oil heaters, so lets
assume that there aren't any big issues with viscous drag from cold oil.
That *still* leaves the problem of sufficiently vaporising Jet-A in the
combustors for the ignitors to light it off... and THAT is a lot harder
when the inlet air is at -45 degrees rather than +90 degrees.

Similar effects can be observed with turbine engines just over the range
of temperatures encountered at sea level. I recently flew 2 legs on an
RB.211 powered 757. The first engine start was at an ambient temp of
about 70F, and I (as always) watched the tailpipe out the window. During
startup, I could tell exactly when the engine lit off by hearing a
gentle "thump" sound accompanied by heat shimmer suddenly appearing
behind the engine. Second leg: engine start on the ground took place at
about 30F. This time, when fuel was turned on, a big stream of
partially-vaporised fuel sprayed out the tailpipe (looked like white
smoke, smelled like raw kerosene when it got sucked into the ventilation
system) for about 1 second before the engine actually lit off (which
could again be identified by a gentle "thump" and appearance of heat
shimmer). I've noticed this on bigger fan engines for as long as I've
been flying. If startup is even *noticeably* slower at 30F versus 70F,
then there's a big problem (potentially) at -45F.

I don't know exactly what the APU builders *do* to aid this, but I
suspect that the fuel spray-bars (or whatever they are) were redesigned
to vaporise fuel efficiently, and ignitors are carefully designed to be
able to light colder, poorly vaporized fuel.

--
Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029
512-835-3286 slacker@arlut.utexxas.edu (Remove the extra 'x' to mail me)