Re: 737 NACA inlet with moving parts?

From: (Keir Wallace)
Date:         11 Aug 96 00:03:59 
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Bill Urban ( asked:
>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?

Certainly is. The triangular plate hanging out in the breeze just forward
of the inlet is a vortex generator. [Looks like a T-tail assembly for those
who havn't seen one.] The vortex generator is attached to the inlet door
and when the APU is shut down the door closes and the vortex generator is
aligned with the local flow. Perhaps later model 737s with uprated APUs
needed more air and the vortex gen. was easier than a bigger or relocated
inlet. The aircon pack inlets below the wing root also have articulated
doors on the 73 but rather than vortex generators they have slush deflector
doors, just to keep the parts count up. ;-)

737 FO and power lurker.