Re: Jet Engine Rev Ups for Take Off

From:         edgraf@pipeline.com (Edward Graf)
Organization: The Pipeline
Date:         17 Aug 95 04:58:48 
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In article <airliners.1995.1241@ohare.Chicago.COM>, Mark Wiklund writes:

>Having watched jet departures from the open air decks at
>Logan airport since a boy, and more recently having done
>much travel, I have always noticced that when a jet
>revs up for take off, it seems to go to half-power
>for a second or two before proceeding to take-off
>power - as if it was a two step process. Is this half-step
>process with all engines, or spooling up engines
>individually, or simply a misperception on my part?
>
>Thanks for the info. I've been lurking in this group a great
>while and, as a "mere" lover of airplanes, enjoyed your
>discussions greatly.
>
Mark,

I fly the 737-300.

Although not required, we use auto-throttles for take-off.  They are kind
of mindless in that the throttle can be all the way forward, yet the engine
is still close to idle.

Many times the two engines spool up at different rates  because of
differences in fuel controllers, age of engines, bleed valves closing at
different times, etc.  It is possible to have one engine nearly at idle
(but accelerating) and the other producing significant thrust, resulting in
lurching back and forth as one accelerates down the runway.

To minimize the potential thrust descrepancy, my company suggests manually
spooling both engines to 40% N1 (Fan speed) and allowing them to stabilize
before engaging the auto-throttles.  That seems to minimize the lurching
and seems to result in smoother acceleration, both engine and aircraft.  It
does result in a two-stage process.

Ed Graf