Re: 737 NACA inlet with moving parts?

From:         "David G. Davidson" <gerhard@onramp.net>
Organization: http://members.aol.com/tristar500/l1011
Date:         11 Aug 96 00:03:57 
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Bill Urban wrote:
>
> On certain 737s, I've noticed that a NACA duct near the rear of the
> fuselage (right side, I believe) seems to have a mechanized piece which
> folds out away from the a/c, rather than the usual 'passive' design
> familiar from other planes (and race cars).  Is this the inlet for the
> APU?  How, when, and why does it operate?

It is indeed the air inlet for the APU. The door opens when the APU
start switch is placed to the START position (momentary) and after the
APU fuel shutoff valve has opened.

It closes when the APU switch is set to OFF or  because of a fire
warning shutdown.

The part that sticks out from the fuselage is called the "Vortex
Generator with Flap" in the training manuals. I'll have to check to see
exactly how that operates and when. I'll get back on this.

Dave