Pitot system - odd event

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.

The air doesn't actually move through the tube in normal flight. There is no flow to speak of. The column of air in the tube is compressed by the impact of the relative wind on the pitot tube. The airspeed is a pressure not a flow "sensor".

As you noted, the air can flow from the panel to the pitot tube, but that is what it should be able to do when the instruments are disconnected. It appears that is no blockage in the pitot line.

I've not seen anything like you describe in 6600+ hours of flying, both with mechanical and digital instruments.

On Wednesday, May 23, 2018 at 10:08:35 PM UTC-7, SoaringXCellence wrote:
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.

The air doesn't actually move through the tube in normal flight. There is no flow to speak of. The column of air in the tube is compressed by the impact of the relative wind on the pitot tube. The airspeed is a pressure not a flow "sensor".

As you noted, the air can flow from the panel to the pitot tube, but that is what it should be able to do when the instruments are disconnected. It appears that is no blockage in the pitot line.

I've not seen anything like you describe in 6600+ hours of flying, both with mechanical and digital instruments.

Thanks for the reply. Yeah I know air doesn't flow through tube in normal flight. I removed the airspeed instrument and forced air into the tube from the instrument end to see if air would flow out the pitot inlet back on the vertical stabilizer. That was to check if there was blockage. The pitot system also directs the ram air into the digital variometer. The digital variometer (an LX NAV S80) accepts the pitot input via a pressure transducer (also called a pressure sensor). No airflow through the system indeed: that would be bad. I believe my original post used the term "pressure sensor", not "flow sensor".

In my case, something happened to take out two instruments at the same time, both the diaphragm on the winter and the "pressure sensor" on the S80. It happened in flight, on tow. Most likely something caused an overpressure in the pitot system - I don't know what else might have done this. My problem now is to verify the integrity of the pitot pneumatics before connecting it to another expensive instrument.

On Thursday, May 24, 2018 at 3:42:38 AM UTC-7, Del Jensen wrote:
On Wednesday, May 23, 2018 at 10:08:35 PM UTC-7, SoaringXCellence wrote:
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.

The air doesn't actually move through the tube in normal flight. There is no flow to speak of. The column of air in the tube is compressed by the impact of the relative wind on the pitot tube. The airspeed is a pressure not a flow "sensor".

As you noted, the air can flow from the panel to the pitot tube, but that is what it should be able to do when the instruments are disconnected. It appears that is no blockage in the pitot line.

I've not seen anything like you describe in 6600+ hours of flying, both with mechanical and digital instruments.

Thanks for the reply. Yeah I know air doesn't flow through tube in normal flight. I removed the airspeed instrument and forced air into the tube from the instrument end to see if air would flow out the pitot inlet back on the vertical stabilizer. That was to check if there was blockage. The pitot system also directs the ram air into the digital variometer. The digital variometer (an LX NAV S80) accepts the pitot input via a pressure transducer (also called a pressure sensor). No airflow through the system indeed: that would be bad. I believe my original post used the term "pressure sensor", not "flow sensor".

In my case, something happened to take out two instruments at the same time, both the diaphragm on the winter and the "pressure sensor" on the S80. It happened in flight, on tow. Most likely something caused an overpressure in the pitot system - I don't know what else might have done this. My problem now is to verify the integrity of the pitot pneumatics before connecting it to another expensive instrument.

Sorry, I should have said "ram air signal to the digital variometer." Again I am well aware of the fact that no gas is actually transported from the pitot inlet to either instrument.