On Wednesday, May 23, 2018 at 8:02:30 PM UTC-7, Del Jensen wrote:
On tow the other day the airspeed indicator jammed and the digital airspeed read about 20kts too high. When I got down the digital airspeed was 30kts with the glider stopped, and turning off the unit (S80) and re-initialing the tas read 30kts. Upon examination the diaphragm was found to be ruptured in the winter, and auto zeroing the pressure sensors on the S80 left the tas navbox with no reading (---) indicating the transducer was likely damaged.

Something clearly sent a shock through the pitot system, perhaps a bug hit square in the inlet.

Air moves through the pitot tube when blowing into the tube from the panel end.

If any of you have experienced something like this before, I would appreciate your insight on this issue. I am reluctant to attach a new instrument to the system until I've taken basic precautions testing the system. Could there be a partial blockage that might shift and shock the system again? What procedures are there to establish the integrity of the pneumatics?

Many thanks for any ideas on this.

A possible explanation might be: something over the winter blocked the pitot or line from pitot to panel. On tow you gained altitude, the system measures the difference between pitot pressure and static. As you gain altitude, static pressure drops, raising that difference. At some point the difference was enough to blow out the blockage, but not before damaging the instruments. A 150 knot ASI is designed to measure about 3 psi (dynamic pressure at 150 knots). A 2000 ft tow is only 1 psi, so it would take a good thermal to exceed the range of the ASI. When the fault occurred was it sudden, or simply stick at the tow airspeed reading? Also, anything blocking the pitot would have resulted in the ASI misreading immediately, was that notices?