Altimeter Certification Tests

[Edwardoc]

During the certification test of my newly installed Skyview system, as the altitude was increased from field elevation to 20,000 feet, the airspeed indication increased slowly to indicate over 300 kts at 20,000 feet and decreased as we descended.  It did this without any input to the Pitot tube from the test equipment.  The altimeter was spot on at every 2000 feet all the way up.  Where is the A/S input coming from if there was no input from the test equipment?  It was not hooked up to the pitot system at all.  When we did the A/S tests the indicated A/S  was within 1 kt,  at 50 kts, 100 kts, 150 and 200 kts.  Just wondering.

 

[jakej]

You need to 'balance' the system by connecting the static to the test box as well

Jake J

 

[dynonsupport]

This is normal. Airspeed is calculated from the pressure difference between the pitot input and the static port. By lowering the static pressure, the normal air pressure into the pitot makes it think it has airspeed.  (I think Jake meant "...by connecting the pitot to the test box...". But since there is a drain hole in the pitot line, that isn't the right thing to do.)

In short, your test worked correctly, and the airspeed is normal.

 

[Edwardoc]

Thanks, I'll pass this on to my avionics Tech to let him know the reason as he was curious about that.