Re: Jet Engine Rev Ups for Take Off

From:         rdd@netcom.com (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         17 Aug 95 04:58:47 
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In article <airliners.1995.1241@ohare.Chicago.COM> Mark Wiklund <Mark_Wiklund@hmco.com> 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?

Standard procedure on most Boeing jets is to advance the thrust levers to
the vertical position, allow the engines to stabilize, then to set take-off
thrust.  The intermediate position is to allow  the engines to stabilize;
it also helps account for minor differences among the engines and to prevent
over-setting takeoff thrust on the first go.

It closes the bleed surge valves, and ensures that the same thrust is
available on each engine.  Since the bleed surge valves take time to close,
this can also affect takeoff performance.  The bleed surge valves open
at low idle RPMs; the idea is to provide venting so the engine is not
damaged during engine start.  When they are open, even if takeoff thrust is
applied, you will not get takeoff thrust until they are closed: this can
account for a 6-7 second lag between commanded thrust and getting what you
want.

As the crew of the A320 at Habsheim discovered, this can be a bad thing, so
in the air, at least, crews keep the engines running above this threshold on
approach.  This means there is only a couple of seconds between commanded
thrust and actually getting it.




--
Robert Dorsett                         Moderator, sci.aeronautics.simulation
rdd@netcom.com                         aero-simulation@wilbur.pr.erau.edu
                                       ftp://wilbur.pr.erau.edu/pub/av