Re: Engine questions.....

Date:         06 Oct 97 02:14:27 
From:         "JCSFlyboy" <jcs@flyboy.com>
Organization: Why? Network Dial-Up Customer
References:   1
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David A. Stuart, Sr. <dstuart@mail.bcpl.lib.md.us> wrote in article <airliners.1997.2289@ohare.Chicago.COM>...
> 1- Today, I took a relative to to National (DC). While waiting for her
> plane to arrive a 757 was waiting at the adjacent gate to off load / on
> load pax. The 757 had its on board ground power unit operating (evident
> from the hot exhaust gases) and was possibly connected to electrical
> power from the gate.
>
> The entire time the aircraft was on the ground the right engine was
> slowly (60-100 rpm) turning. I assume it was bleed air from the gpu. The
> questions is.....why?  Is it for lubrication purposes ? Was a bleed air
> valve left open or was it a faulty valve allowing blow-by?

1. Do you remember if it was very windy that day? When the wind is strong
enough (as little as 10mph) it can turn those fan blades on most all jet
airliners (not sure about military fighters). And of course, the aircraft's
orientation can explain why only one of the engine's blades where turning.
It has nothing to do with the APU. Bleed air? Let me quote the 757 OM: "The
valves [bleed air] are spring-loaded closed and electrically opened based
on pneumatic demand. System logic prevents valves from opening until the
engine is running and closes them when the engine shuts down."  "The APU
drives a generator to supply electrical power and provides bleed air for
the pneumatic system."