Re: 747 "smoke" trail

Date:         25 Jun 99 01:32:45 
From:         Art & Janet <>
Organization: AT&T WorldNet Services
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"J. W. Dawson" wrote:

> 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.

Garrett (Alliedsignal) APU's, with the exception of the ETOPS variety
such as the 331-500 (B777), are not started or run except on the ground.
The ETOPS variety normally is not run in the air except in the case of
single engine ops.

> On the other hand, when gas turbines light off there is often a puff of
> lighter smoke ...

> 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.

Nope, the puff is excess fuel that doesn't combust. If oil starts seeping by the seals, it means an
unscheduled engine removal.

-- Art
your mileage may vary