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 (Remove the extra 'x' to mail me)