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Pitot system - odd event



 
 
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  #1  
Old May 24th 18, 04:02 AM posted to rec.aviation.soaring
Del Jensen
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Posts: 24
Default Pitot system - odd event

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.
  #2  
Old May 24th 18, 06:08 AM posted to rec.aviation.soaring
SoaringXCellence
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Posts: 305
Default 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.
  #3  
Old May 24th 18, 11:42 AM posted to rec.aviation.soaring
Del Jensen
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Posts: 24
Default Pitot system - odd event

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.
  #4  
Old May 24th 18, 11:50 AM posted to rec.aviation.soaring
Del Jensen
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Posts: 24
Default Pitot system - odd event

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.
  #5  
Old May 24th 18, 01:40 PM posted to rec.aviation.soaring
[email protected]
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Posts: 59
Default Pitot system - odd event

There might be a clue in AC43.13. Here's a caution note from the pitot static testing section ( Paragraphs 12-58 and 59)

"Do not blow air through the line toward the instrument panel. This may seriously damage the instruments. Be sure to disconnect the instrument lines so no pressure can reach the instruments."

"CAUTION: To avoid rupturing the diaphragm of the airspeed indicator, apply pressure slowly and do not build up excessive pressure in the line. Release pressure slowly to avoid damaging the airspeed indicator."


Sounds like over pressure or rate of pressure change has ruptured diaphragms before. For the electronic sensor I can't see how rate would do it which leaves over pressure. Probably more than the lungs or pitot ram air alone can make.

Any chance of compressed air, fast moving water, or ice in the pitot path?
Maybe somebody 'tested' or hosed off the glider between flights.
How long has it been since the last flight when it was working.

Does the tow rope hook up where it can affect the pitot port?

Was there any moisture (or bug parts ;-) in the pitot system when you looked at it?

Interesting detective project.

  #6  
Old May 24th 18, 04:57 PM posted to rec.aviation.soaring
Del Jensen
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Posts: 24
Default Pitot system - odd event

On Thursday, May 24, 2018 at 5:40:12 AM UTC-7, wrote:
There might be a clue in AC43.13. Here's a caution note from the pitot static testing section ( Paragraphs 12-58 and 59)

"Do not blow air through the line toward the instrument panel. This may seriously damage the instruments. Be sure to disconnect the instrument lines so no pressure can reach the instruments."

"CAUTION: To avoid rupturing the diaphragm of the airspeed indicator, apply pressure slowly and do not build up excessive pressure in the line. Release pressure slowly to avoid damaging the airspeed indicator."


Sounds like over pressure or rate of pressure change has ruptured diaphragms before. For the electronic sensor I can't see how rate would do it which leaves over pressure. Probably more than the lungs or pitot ram air alone can make.

Any chance of compressed air, fast moving water, or ice in the pitot path?
Maybe somebody 'tested' or hosed off the glider between flights.
How long has it been since the last flight when it was working.

Does the tow rope hook up where it can affect the pitot port?

Was there any moisture (or bug parts ;-) in the pitot system when you looked at it?

Interesting detective project.


Part of my preflight routine is to look at the inlet, mainly to make sure I remember to take off the cover.

Actually, we did look for bug guts when I got down! Couldn't see anything though.

I did blow from the instrument end to test for blockage. That was after I got down and removed the damaged instruments.

I remember back in the old days people puffing at the pitot inlet to make sure the gauge was working. Makes me shudder to think about it.

If I could figure out what happened, at least it would be an expensive lesson. As it is, it's just expensive.
  #7  
Old May 24th 18, 06:31 PM posted to rec.aviation.soaring
Charlie M. (UH & 002 owner/pilot)
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Default Pitot system - odd event

Cupping your hand around a pneumatic port and blowing into your hand has almost "0" chance of hurting instruments.
Putting your mouth (or worse, a compressed air gun) against any port may either hurt instruments, blow a line off, split the side of a line or more.

I can't see any normal glider speed/pressure, due to speed, hurting anything......unless you consider the US space shuttle in the same group. That will go a wee bit faster than most of us are used to.
;-)

Granted, a "cupped hand puff" only shows air moves, it does NOT show minor leaks. We did that in the spring once, everything moved correctly, I did first spring flight.
At about 100', the ASI was really low, basically no ram air instruments for the rest of the flight (ASW-20).
I did manage to do some miles and a few hours without the panel working.
  #8  
Old May 24th 18, 07:48 PM posted to rec.aviation.soaring
Steve Leonard[_2_]
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Posts: 918
Default 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
  #9  
Old May 24th 18, 08:20 PM posted to rec.aviation.soaring
jfitch
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Posts: 904
Default 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.


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?
  #10  
Old May 24th 18, 08:21 PM posted to rec.aviation.soaring
Dave Nadler
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Posts: 1,292
Default Pitot system - odd event

On Wednesday, May 23, 2018 at 11:02:30 PM UTC-4, Del Jensen wrote:
On tow the other day the airspeed indicator jammed and
the digital airspeed read about 20kts too high.


Are you sure this happened DURING tow, rather than before the flight?

Where is your pitot located?
 




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