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Avionics shop can't test airspeed indicator


Seth1001

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I'm having a problem getting the IFR cert updated on my 77 M20J. The shop rigged up their pitot static dealio and they got the altitude and VSI and whatnot to work but they can't get the airspeed indicator to move at all. They thought it was broken but I took the thing flying and it works like a champ. I don't know enough about the systems yet to figure this out on my own. Does anyone have any idea why the airspeed works fine in the air but won't work with their test setup?

'

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Qué the dealio, amigo..?

Somebody needs to know how to use their test equipment...

The static systems seems to test well...

But the pitot system seems to not have been connected...

If they don’t connect the pitot system correctly, the ASI will read zero...

Did you get a sales pitch fir a new ASI with that?

:)
 

PP thoughts only, not an instrument guru...

Best regards,

-a-

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Yeah they said it indicated a clog in the pitot system but it works fine when I'm flying it. It picks up right about 20 knots and indicates spot on the whole time I'm flying. I wasn't there to see it but the shop manager says he double-checked the setup and everything was rigged correctly. I have no idea what the issue could be.

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2 hours ago, EricJ said:

Your pitot may (should) have a drain hole on the bottom.   It needs to be covered with a piece of tape or something for the ground test.

 

That would be my guess. The drain is a small hole on the bottom of the pitot tube just aft of the “bullet” nose. 

But, why were they even testing the pitot system in the first place? The cert is for the static system.

Skip
 

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3 hours ago, Seth1001 said:

I'm having a problem getting the IFR cert updated on my 77 M20J. The shop rigged up their pitot static dealio and they got the altitude and VSI and whatnot to work but they can't get the airspeed indicator to move at all. They thought it was broken but I took the thing flying and it works like a champ. I don't know enough about the systems yet to figure this out on my own. Does anyone have any idea why the airspeed works fine in the air but won't work with their test setup?

'

You can easily prove that it works.  Get a 4’ length of 1/4” surgical tubing, some vinyl electrical tape and 2 clothes pins.  About 3/4” back from the tip of the pitot head there is a small hole on the bottom, wrap a few layers of tape around and covering the hole.  Stretch the surgical tubing onto the tip of the pitot head, place one clothes pin at the end of the tubing, pinching the tube closed.  

Slowly roll the tubing around the clothes pin, this will increase the pitot pressure causing the airspeed indicator to rise. It takes very little pressure to move the ASI and little more to damage it.  Use the second clothes pin to pinch the tube, blocking the pressure in the system.  In a tight system when you stop rolling the ASI needle will stop moving and will hold steady. Typically you want to see less than 10 kits per minute loss rate.  Unroll the clothes pin slowly to avaoid damage to the ASI.

 If it falls you have a leak to locate and repair.  

Make sure that the gear selector switch is DOWN and the gear actuator circuit breaker is pulled.  If your plane has an airspeed safety switch and the gear switch is in the wrong spot the gear will retract.

Clarence
 

 

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43 minutes ago, PT20J said:

That would be my guess. The drain is a small hole on the bottom of the pitot tube just aft of the “bullet” nose. 

But, why were they even testing the pitot system in the first place? The cert is for the static system.

Skip
 

If they’re going to raise the static system high enough to check the transponder/ encoder correspondence they will be hooking to the pitot  head and will want it leak free.

Clarence

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I don't think the pitot systen is part of the IFR  check.  My shop told me I had a pitot leak but passed it anyway. Next time I had them fix it. it was a leak at the drain valve--100 kts per minute as I recall.  I couldn't see any difference in indicated airspeed in cruise afterward. 

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4 hours ago, 0TreeLemur said:

When my avionics guy tested the pitot-staic system in our M20C last summer, he showed me that the drain hole on my pitot tube is on the back side, not the bottom.  Then he put a piece of electrical tape over it.

There are two drain holes, the one at the back drains water from the electrical connection area, the one on the bottom drains the pitot opening.

Clarence

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3 hours ago, mooneyflyfast said:

I don't think the pitot systen is part of the IFR  check.  My shop told me I had a pitot leak but passed it anyway. Next time I had them fix it. it was a leak at the drain valve--100 kts per minute as I recall.  I couldn't see any difference in indicated airspeed in cruise afterward. 

Oddly we call it a pitot static test but there is no FAA requirement to test the pitot. Its really a transponder, encoder, altimeter test. Not sure where the term "pitot static inspection" came from.

 

-Robert

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11 minutes ago, RobertGary1 said:

Oddly we call it a pitot static test but there is no FAA requirement to test the pitot. Its really a transponder, encoder, altimeter test. Not sure where the term "pitot static inspection" came from.

 

-Robert

http://www.faa-aircraft-certification.com/43-appendix-e.html

https://www.govinfo.gov/content/pkg/CFR-2012-title14-vol2/pdf/CFR-2012-title14-vol2-sec91-411.pdf

 

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3 hours ago, RobertGary1 said:

Oddly we call it a pitot static test but there is no FAA requirement to test the pitot. Its really a transponder, encoder, altimeter test. Not sure where the term "pitot static inspection" came from.

 

-Robert

Yet, an airspeed indicator is required as part of TOMATO FLAMES for VFR flight.   Come to think of it when they tested my pitot-static system last year, they paid little attention to the airspeed indicator, mostly just to be sure that they didn't send it into unapproved pressure territory.   As they depressurize the static system to simulate a climb, they must also depressurize the pitot tube to prevent overpressure on the anaroid element or differential pressure transducer inside it, depending on type (analog or digital).

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45 minutes ago, 0TreeLemur said:

Yet, an airspeed indicator is required as part of TOMATO FLAMES for VFR flight.   Come to think of it when they tested my pitot-static system last year, they paid little attention to the airspeed indicator, mostly just to be sure that they didn't send it into unapproved pressure territory.   As they depressurize the static system to simulate a climb, they must also depressurize the pitot tube to prevent overpressure on the anaroid element or differential pressure transducer inside it, depending on type (analog or digital).

Yup, the ASI can be pushed well over redline during testing of the altimeter, so the pitot has to be hooked up and depressurized at the same time.   This is why they care about the leaks.   If a slow leak in the ASI turns into a big leak during the test, a bunch of instruments can be damaged, which is considered "bad".   

Below are pics of the drain hole for the pitot under the input, and the drain hole in the back for the electrical connector that Clarence mentioned.   This is a "leaky" pitot that was removed from my airplane a couple of years ago, but I think I just noticed what the issue may have been.   I'll test it again, it may actually be good.   $750 later.  ;)

20200102_125104.thumb.jpg.eb9afd2f126978a8bfe40feae8a46a92.jpg

20200102_125141.jpg

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When we redid the panel in our 'C, I wanted an airspeed indicator with kt on the outer ring, and mph on the inner.    So, I bought a really nice used one and to test it I bought a U-tube manometer cheap.   After verifying that the ASI was accurate, I sent it to an instrument company to verify have them paint '67 M20C arcs on it.   Love it. 

15 minutes ago, EricJ said:



20200102_125104.thumb.jpg.eb9afd2f126978a8bfe40feae8a46a92.jpg

 

Wow, Interesting.  I don't think the pitot tube on my a/c has a hole there.  I'll check.    Last Feb. or Mar. I posted on this forum regarding a "belching" pitot tube that seemed to not be draining properly.  At that time I looked it over real well and don't remember seeing a hole there.   I disconnected the pitot line inside the wing and blew back out through the pitot tube with compressed air to make sure that nothing was plugged up, as was suggested by one MS'er.   No problems.    This "belching" has happened a few more times when flying through rain.  The pitot tube on my a/c seems to be designed to occasionally need to "belch" to get the water out.

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When I landed at KLXV my altimeter read 9935 and my airspeed indicator read 0 after I stopped.  This is an example of where the static and pitot  must be the same.

Airliners (that still have analog instruments) have vibrators on the altimeters to help overcome the mechanical stiction for small pressure changes.

I was disturbed to see the avionics tech beating on my panel with his fist while doing the last IFR cert to make sure the altimeter had settled out at each altitude tested.

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5 hours ago, skykrawler said:

When I landed at KLXV my altimeter read 9935 and my airspeed indicator read 0 after I stopped.  This is an example of where the static and pitot  must be the same.

Airliners (that still have analog instruments) have vibrators on the altimeters to help overcome the mechanical stiction for small pressure changes.

I was disturbed to see the avionics tech beating on my panel with his fist while doing the last IFR cert to make sure the altimeter had settled out at each altitude tested.

I guess they would need vibrators.   We have 4- or 6-cyl. vibrators attached to our altimeters!  :lol:

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5 hours ago, skykrawler said:

When I landed at KLXV my altimeter read 9935 and my airspeed indicator read 0 after I stopped.  This is an example of where the static and pitot  must be the same.

Airliners (that still have analog instruments) have vibrators on the altimeters to help overcome the mechanical stiction for small pressure changes.

I was disturbed to see the avionics tech beating on my panel with his fist while doing the last IFR cert to make sure the altimeter had settled out at each altitude tested.

In fact the FAA official guidance says to tap the altimeter during the test. The idea is that our suck, squeeze, bang, spit engines produce enough of their own vibration. On the ground without the engine running your altimeter is expected to stick a bit.

 

-Robert

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21 hours ago, 0TreeLemur said:

When we redid the panel in our 'C, I wanted an airspeed indicator with kt on the outer ring, and mph on the inner.    So, I bought a really nice used one and to test it I bought a U-tube manometer cheap.   After verifying that the ASI was accurate, I sent it to an instrument company to verify have them paint '67 M20C arcs on it.   Love it. 

Wow, Interesting.  I don't think the pitot tube on my a/c has a hole there.  I'll check.    Last Feb. or Mar. I posted on this forum regarding a "belching" pitot tube that seemed to not be draining properly.  At that time I looked it over real well and don't remember seeing a hole there.   I disconnected the pitot line inside the wing and blew back out through the pitot tube with compressed air to make sure that nothing was plugged up, as was suggested by one MS'er.   No problems.    This "belching" has happened a few more times when flying through rain.  The pitot tube on my a/c seems to be designed to occasionally need to "belch" to get the water out.

Some have them just before the knee.  They can be hard to see, especially when plugged with baked gunk.  The ones before the knee are often thought to be leaking even more often, since the adapter does not go as far and requires the technician to tape over it.

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If they can't test the ASI during a static system check, they don't know what they are doing and will probably damage your instruments. This is a rookie mistake and is supposed to be covered in their Repair Station Manual training program. They can accidentally run the ASI backwards which usually damages it beyond repair.

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