On Thursday, May 24, 2018 at 12:43:52 PM UTC-7, Dave Nadler wrote:
On Wednesday, May 23, 2018 at 11:02:30 PM UTC-4, Del Jensen wrote:
Something clearly sent a shock through the pitot system

No... These exact symptoms can be caused....

Donkeys years ago I happened by a gliderport where I was assaulted by
an angry new glider owner. "Your instruments don't work worth crap!
P.O.S. !!! Go fly this glider and tell me what's wrong". And so forth.

I strapped on the shiny new plane and took off, noting incorrect ASI
on tow and general odd behavior. Enjoyed a nice flight and gave the new
owner some time to reflect on the error of his ways. Very peaceful
with no audio or instruments to distract from the pleasure of a nice
soaring day and the new-glider smell. Eventually I flew back home and
noted that as I rolled to a stop the ASI still read 45 knots.
New owner ran up and I pointed at the mechanical ASI...

The glider had been hosed down, and water was trapped in the low part
of the tube between the pitot probe and instruments. With enough airspeed
and agitation bubbles of air could get through the water, then the remaining
column of water acted like a water manometer. IIRC there was more than
a pint of water in the tube.

There was certainly no "burst diaphragm" in this instance but all other
symptoms match (including failure to correctly re-zero the electonic
ASI). Drying everything out (no compressed air EVER) cured all.

Please tell us the location of your pitot and the type of glider!

Hope that helps someone,
Best Regards, Dave

Thanks guys. The glider is a glasflugel 304 (original version, 1980). Pitot is on vertical fin, static port on side of boom about 3/4 toward tail. Location is Arizona. Neither TE nor static was affected.

Glider is in hangar. I lost the pitot cover a couple of months ago, and have been meaning to get that taken care of. Maybe I should get on that.

It was a great soaring day, but I just puttered around locally for about an hour and a half after the airspeed failed, since I had to leave myself enough time to get behind the panel and pull the instruments once I got down.

Although I try to check things over one last time after belting in, I suppose it's possible the instrument was reading falsely at take off. I usually start rolling with flaps at -2 to get early aileron effectiveness, ease them back to neutral at about 45kts, so I usually take a glance at the airspeed well into the take off roll. I don't recall anything odd at that point. But who knows? That's a busy time, and most of take off is done by "feel," like everything else.

Landing without the instrument was a non-event. In some ways easier since I didn't feel any obligation to glance at the airspeed - easier to focus purely on attitude and wind noise, which is the we are supposed to do it. Landing with airspeed covered should be a part of training in my opinion.

P.S. I love my 304! The trailing edge spoiler/flap system is wonderful.