Pitot system - odd event

Hi Del,
What kind of plane? Where is the pitot located (nose or tail, or somewhere else?), and where are the static ports?

Long ago, on a first flight in type, a friend had his airspeed turn into a "max speed" indicator. The needle would move up if you went faster than you had up to that point, but would not come down.

How old is the airspeed indicator? You have indicated that it is a Winter airspeed. Possible age related failure or manufacturing issue? If it was an overpressure and burst, I would expect the needle would have wound around a time or two in the process as the bellows pressure went up before rupture. I am more inclined to believe either age or manufacturing issue on the airspeed indicator.

Pitot is then connected to static through the failed bellows of the airspeed indicator would tend to make any airspeed system read low, and not high. If the static was somehow plugged, as you went up on tow at a constant airspeed, your indicated speed would drop. Only way I can think of to have the S80 reading high is that static pressure was made lower than actual.

You can safely leak check the pneumatic system by isolating it from all your flight instruments, and using a water manometer (no, you don't put any water into your pneumatic system!) or another working airspeed indicator.

I can't see how a bug impacting the pitot could cause that big a pressure bump, as there is considerable volume in the pitot lines and the bug would have to seal off the inlet and make considerable progress into the system to make the pressure go up that high.

Last obvious question. Had you made any panel changes or done any work that required access in the area since the previous flight when all was well?

As you said, many things don't add up.

Steve Leonard