Re: Thrust in idle engines

From:         skyesis@portal.com
Organization: InterServ News Service
Date:         03 Nov 95 04:23:14 
References:   1
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In article <airliners.1995.1626@ohare.Chicago.COM>, <MEZEI_JF@Eisner.DECUS.Org>
writes:
>
> If brakes are not applied, will the aircraft start moving right away with
> engines "idling" ?

Depends if the aircraft in question is light or heavy.  A heavy aircraft will
develope flat spots that keeps the aircraft parked at idle thrust even after
the chacks are pulled.  On a cold winter day you can feel the flatspots going
round on a Navajo until the tire heats up on its own.

> When idling, do the compressor blades spin slowly enough to
> be seen ?

No but some of the hubs have neat black white designs on them that you can see
going around, and around, and around, and around, ...

> Also, in a previous posting, someone said that without an APU, an aircraft
> would require power and "air" from the ground to start its engine. Are we
> talking about compressed air ? What for ?

Compressed air blown through the engine to get it rotating to the point where
fuel can be added and ignited to finish the start. Fuel+fire-rotation=meltdown.

> Also, why are DC9s and 727s capable of backing up from the gate without being
> pushed ? Is it a technical or legal consideration ? (eg: engines being far
> enough away from terminal for noise control etc).

I dunough.  But here is a sWAG at it:  Did you ever notice a yellow painted arc
infront of where the engine are on a 737 when parked at the gate.  That is for
IDLE thrust.  Anything that is not heavy or tied down is going to be sucked
through an expensive engine.  Now, push up the throttle to MAX Reverse an
imagine how big that area whould have to be.  Your million dollar engine would
probably be sucking up ground power cables, airconditioner ducting, gound crew
and anything else that happens to be in those bins under a movable (but
unfortunately nearby) jetway.

The DC-9 and 727 can get away with this procedure because 1)the FAA lets them,
and 2) there engine are Low Bypass turbofans that are high up out of the way.
And while I'm sure they are sucking lots of dirt and dust, they are not sucking
anything big enough to dammage the engine.  (I bet they have though)

Hope that explains it.

- Bill