Re: Squat interlock

Date:         25 Jun 99 01:32:49 
From: (Stephen H. Bradford)
Organization: Stargate Industries, LLC.
References:   1
Followups:    1
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JbTex wrote:
> I once heard pilots talking about a "squat switch" in the main
> landing gear of airliners.  They mentioned that it prevented
> the deployment of the thrust-reversers until the landing aircraft's
> weight settled onto the landing gear.  If this is true, are the
> armed spoilers also kept retracted by this switch, or must they
> deploy prior to the full "squat"?  Also, are there other systems
> controlled by squat/not-squat status?

Yes, you are correct.  On some aircraft the auto-spoilers are enabled by
the main gear squat switch and maybe backed up by a nose gear squat
switch and on the 737, a partial wheel spin up with the radio altimeter
reading less than 10 ft.  This is to allow auto-spoiler and thrust
reverser deployment in the event of landing on a very slick or snow
covered runway.

Other systems include engine idle and wing anti-icing deactivation.  On
Boeing and Mcdonald aircraft, a squat switch controls auto-scheduled
depresurization, nose-wheel steering activation and auto-brakes.

On the more advanced aircraft,757, Airbus, the squat switch controls
many computer functions.  An air mode transfer failure on takeoff, (the
airplane still thinks it's on the ground) can cause some very strange
indications and computer problems.  The aircraft must return and get the
swith fixed.

A ground school instructor once told me that the squat switch on the
737-200 does "33 wonderful things"  He was the only one who could
remember them all.  Thats why we have troubleshooting checklists.