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Date: Wed, 11 Jan 2012 12:28:52 -0500
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From: "Dennis Johnson" <pinetownd@volcano.net>
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Subject: Re: [LML] Re: Pitot static checks
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Hi John,

I use the same hand-held vacuum pump as Chris.  To check the static system, 
I tape over one of the static ports and use a suction cup attachment that 
came with the vacuum pump on the other static port.  As you pull vacuum, the 
altimeter will show higher altitude and the airspeed indicator will show 
higher airspeed.  (Because the airspeed indicator is measuring the 
difference between pitot and static pressures; pitot pressure stays the same 
as static pressure decreases.)  My airspeed indicator gets close to its 
upper limit way before the altimeter does, significantly limiting the amount 
of vacuum you can pull.

Close the valve on the vacuum pump, leave it attached to the static port, 
and watch to see how fast the airspeed indicator goes to zero and the 
altimeter returns to field elevation.  The faster they move, the bigger the 
leak in your static system.  Sequentially cap off each static line and run 
the test again each time until you find the one that's leaking.

I've never used the pump (which can also produce positive pressure as well 
as vacuum) on the pitot tube, but I would guess that it works the same way. 
You might have to fabricate an adapter to get an airtight seal on the pitot 
tube.

Be very careful, however, to avoid damaging your instruments doing this.  I 
haven't verified it, but a couple of avionics guys told me that a sudden 
change in pressure, such as would occur if your vacuum pump pulled away from 
your static port while under vacuum, can break the pitot/static sensors 
inside your avionics.  Replacing the Chelton GADAHRS, for example, would 
cost multiples more than a lifetime of pitot/static tests at an avionics 
shop.

Good luck,
Dennis
Legacy, closing in on 500 hours tach time