Re: 747 "smoke" trail

Date:         18 Jun 99 01:39:55 
From:         "J. W. Dawson" <>
Organization: Frontier GlobalCenter Inc.
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John Weiss <jrweiss@seanet*NOSPAM*.com> wrote in message <airliners.1999.630@ohare.Chicago.COM> ...
> The APU of a 747 is normally shut down before takeoff.  In some
> circumstances it may be left on to run one of the air-conditioning packs
> at low altitude, but it should not be emitting any visible smoke.  Just
> look at the 747s at the gate.  Most of them probably have their APUs
> running, with no visible smoke.

I believe the original issue was spotting smoke on arrival over the
destination and APU's are normally started sometime during the approach.
Exact timing varies from carrier to carrier and type to type.

"Smoke" can have a different appearance depending on sun angle, smoke
composition, humidity, etc. At low altitudes and normal temperatures
condensation clouds from the pressure change of the aircraft passing through
humid air commonly dissipates in seconds, as does jettisoned fuel. Smoke

And, having spend a good deal of time on airport ramps around running APU's
I can attest that they actually tend to be about as smoky as your average
diesel engine, maybe a little less - it just isn't particularly thick and is
often not noticeable - but it's certainly there.

On the other hand, when gas turbines light off there is often a puff of
lighter smoke - sometimes a lot more than a puff. RB211's seem to be prone
to smoky starts (having watched many L1011's belch a big blue-white cloud
when starting) and the versions used on 757's often emit a puff at start-up
and smoke after shut-down.

I've been told that this smoke is most commonly due to engine lubricating
oil that seeps through seals and burns off as the fuel lights off and the
engine heats up. It's normal and not at all uncommon. An APU is just a small
gas turbine and I believe that "burn-off" smoke is what's occasionally
briefly seen coming from the tail of descending airliners.