Re: Jet Engine Rev Ups for Take Off

From:         arch6@inlink.com (Archibald McKinlay)
Organization: McKinlay & Associates
Date:         17 Aug 95 04:58:48 
References:   1
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In article <airliners.1995.1241@ohare.Chicago.COM>, Mark Wiklund
<Mark_Wiklund@hmco.com> wrote:
>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...

All aircraft, if flying with checklists, will use a similar medium-to-high
power pre-takeoff check.  Some can be done at the "Hold Short" immediately
before taking the runway, others must run-up on the runway itself.  This
is ususally because of several factors, primarily only the runway may be
cleared enough for the higher power run-ups and taxi-ways may be too
congested or littered.

Props run-up before taking the runway to check magnetos, manifold
pressure, etc. at a reasonable throttle.  This usually poses no threat to
bushes and other aircraft so they can do it along taxi ways etc.  Even so,
once on the runway some props cannot hold brakes at full power so they'll
run-up to a less than takeoff power and, after gauges settle down, go full
throttle, release brakes, and roll, with clearance of course.

Jets usually runup on the runway due to the higher thrust and sometimes
FAA requirements.  Runup on taxi or marshal areas tend to tear up property
and cause FOD to other engines.  Jet runup doesn't check magnetos but
thrust, fuel flow, hydraulic pressures, computers, etc.  This is usually
at around 70% to 85%, again due to brake holding.  Once cleared, the
release of brakes and full throttle require more checks of the engine
speed, pressures and fuel flow, then speed at each marker and rotation or
flight.  Jet refusal speeds and weights are higher so that refusal can be
pretty serious.

Some jet aircraft, if fully loaded, can go to full power and hold brakes
without skidding the tires.  Most cannot.  You can blow tires here but it
is rare.

BTW, the military jets of the late sixties had some engines that required
the jet to be tied down and run at full power on the ground for three
minutes.  This was due to the fact that most engines came apart in the
first three minutes of flight, so if you made that wicket you were good to
go!  Actually the probabilities did fall off rapidly and this procedure
was justified.  Any Corsair pilot can verify this. Lost one, the only
engine in the jet, myself, but htat is another story....

--
Truth arises from disagreement amongst friends, D. Hume (Scotland)
       eine Flucht nach Vorn machen, make a retreat forward
Loved and Missed, so Work Together and Rejoice, Phillipians 4:1-13
Archibald McKinlay, VI    Booz€Allen & Hamilton/McKinlay & Associates
Software Safety Engineering and Management           arch6@inlink.com