Next step to trouble shoot airspeed discrepancy ? (SOLVED)

EdFred

EdFred

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Last August I got my static system checked and the airspeed was out of spec. Option was to OH or buy new. Comparing the two prices, I went with new. Airspeed read slow. Came in like I normally did and floated foooorever. My initial thought was that the old airspeed indicator was the issue. I didn't get much flying done last fall and through winter due to schedule and weather. So finally this spring I really started to notice that my 2% for every 1000 feet numbers were coming nowhere close to shat I should be seeing - and I always seemed to have a tailwind or not much headwind based on GS vs (supposed) TAS - and I must have the worlds slowest Comanche.

So a few weeks back I finally got time to go out and do speed runs. Directly into the wind, and direct tailwind based on GPS speeds at the same altitude and power settings. 5000' and power settings provided an average GS of just over 150kts while indicating just under 130kts - or off about 10ktas. I contacted the avionics shop that did the install and took it over last week. They did all the static ground testing, everything checks out, no leaks in the pitot/static system. Calibrated air being injected directly into the pitot gave a difference of less than 1kt of between the test system and the ASI up to 160kias at which point there was 0 difference.

Take the plane up again saturday and did six 3-4 mile runs downwind and upwind at 5000' and I got GPS GS readings of:

167kts / 137kts
165kts / 139kts
169kts / 134 kts
Average GS/TAS comes out to just under 152kts. ASI was indicating 128/129. ~10% low seems to be consistent from approach speed (80mph indicated now, vs 90mph before) [and no I'm not confusing units - just easier to use mph ring on approach] all the way up to WOT cruise speed.

The only two possibilities I can come up with are:
Drain hole is allowing too much air to escape. (Drain hole was covered during ground test) or
Pitot alignment. The Comanche sits tail low/nose high on the ground and while on the ground the pitot tube is level, so in flight when I'm level, the pitot would be pointing downwards. The test tech said that should not cause the issue of a low indication, but I'm wondering if there might be some sort of 'cavitation' or eddie in the pitot system happening since it's not aligned with the AoA?

What say you?
 
Put some tape over the drain hole and see what it does for your numbers. I've seen this cause issues before on a 172 that someone installed a heated pitot tube where one without heat originally resided. Not sure what the owner ever did about it.
 
mondtster said:
Put some tape over the drain hole and see what it does for your numbers. I've seen this cause issues before on a 172 that someone installed a heated pitot tube where one without heat originally resided. Not sure what the owner ever did about it.
Click to expand...

That was what I was actually planning the next time up.
 
Next time you fly, do the box pattern at the same altitude and record results. An average of the 4 headings should provide a more accurate airspeed. As a comparison, my RV-7A at 8500' DA and 155 KTAS will indicate around 130 Kts.
 
Ray Eaker said:
Next time you fly, do the box pattern at the same altitude and record results. An average of the 4 headings should provide a more accurate airspeed. As a comparison, my RV-7A at 8500' DA and 155 KTAS will indicate around 130 Kts.
Click to expand...

Averaging like that can produce erroneous results.
Example: Wind straight out of north at 60kts, and plane flies at 60KTAS. (exaggerated but wouldn't matter if the wind were only 30kts)

Heading 360 produces GS of 0
Heading of 90 and 270 produce GS of 85
Heading 180 gives a GS of 120
120+85+85+0 = 290
290/4 = 72.5
Which is quite a bit off.
(using 30kt wind results in 63.5)
 
I wonder if something is going on with the static system, try alternate air during one of your runs.
 
If I understood correctly, this used to work correctly until your ASI wore out. You replaced it with new and now you have an issue. The calibrated ground test (forcing air in with drain hole plugged) says you don't have an issue. The alignment hasn't changed, right? The old ASI worked until it didn't, right? The only question in my mind is why would the drain hole size change? I'm wondering if you got the correct ASI replacement?

John
 
We are all stumped so far. Nothing changed but the ASI and the ASI is calibrated based on ground tests. Havent been able to get up to cover the drain hole. I didnt think which ASI mattered.
 
Maybe I am missing something, but depending on temperature and pressure, I would not be surprised to see the IAS at 130 with a TAS of 150.
 
Topper said:
Maybe I am missing something, but depending on temperature and pressure, I would not be surprised to see the IAS at 130 with a TAS of 150.
Click to expand...

Even when using the TAS calculator in the 430 where I put in the IAS, OAT, pressure, heading, and altitude it does not match up to what my 'true' TAS is. (calc off ground speed run)

I would expect to see 130 in around the 8000ft range, not sub 130 at 5000ft.
 
I have been spoiled, my aspen in my 182 gave me TAS and the G1000 does it in the C400. TAS matches the GS correcting for the wind that is also displayed. I don't remember what the typical difference between IAS and TAS is (especially as low as 5k, I almost never fly that low), but there is a difference. I may fly tomorrow and will note the difference between True and indicated.
 
What was the symptom that lead to the replacement of the original ASI?
 
Topper said:
I have been spoiled, my aspen in my 182 gave me TAS and the G1000 does it in the C400. TAS matches the GS correcting for the wind that is also displayed. I don't remember what the typical difference between IAS and TAS is (especially as low as 5k, I almost never fly that low), but there is a difference. I may fly tomorrow and will note the difference between True and indicated.
Click to expand...

About 10% difference between IAS and TAS at 5000'. I'm off by around double that.
 
jsstevens said:
What was the symptom that lead to the replacement of the original ASI?
Click to expand...

91.411/91.413 check.

When they did the cert the ASI was out of spec (I didn't ask details, it may have been leaking too much, or it may have been indicating too far off). The cost to OH vs replacement, steered me towards replacement.
 
EdFred said:
91.411/91.413 check.

When they did the cert the ASI was out of spec (I didn't ask details, it may have been leaking too much, or it may have been indicating too far off). The cost to OH vs replacement, steered me towards replacement.
Click to expand...

I guess I'd assume it (the old ASI) was out of spec according to a bench test (removing the rest of the system from the test)? And you were not having any other of the symptoms you noted (like floating on landing) before changing it?

Based on symptoms described here (and I'm no A&P) my main suspicion would be on the new ASI. That's the piece that changed.

If it was the orientation of the pitot, it would have been doing it all along. If it was the size of the drain hole, how did it change (or it would have been doing it all along)? If it was leaks in the rest of the system, why didn't the calibrated ground test find it?

Is it possible (again, I'm no A&P) that the new ASI works fine but is the wrong ASI for this plane? My question is: is the Comanche ASI calibrated for the orientation of the pitot and the size of the drain hole while the new one is not?

John
 
So I got up today did another Speed Run 154 based on the ground speed average and with the drain hole taped over with Gorilla Tape I had an indicated between 135 and 140 closer to 135. So that's weird.
 
Topper said:
I have been spoiled, my aspen in my 182 gave me TAS and the G1000 does it in the C400. TAS matches the GS correcting for the wind that is also displayed. I don't remember what the typical difference between IAS and TAS is (especially as low as 5k, I almost never fly that low), but there is a difference. I may fly tomorrow and will note the difference between True and indicated.
Click to expand...
So you're saying that then the GPS takes a ground speed and a calculated TAS to determine winds, those winds somehow verify the TAS calculation?
 
A pitot tube can be off-angle by a considerable amount (like tens of degrees) before having a noticeable effect due to alignment. Static port, OTOH, is a lot more susceptible to errors - anything happen upstream of your static port(s) recently? Can't imagine how the drain hole is affecting it unless (a) it is oversize, or (b) there's partial blockage somewhere else. Anyone drilling out your drain these days?

Does your GPS record data? There's a fairly easy way to get a good airspeed cal if you're interested(*). By 'good' I mean 'better than averaged legs,' but that kinda goes without saying ;)

(*) It does require math beyond what an E6B can do for you o_O

Nauga,
and his circling wind cal
 
nauga said:
A pitot tube can be off-angle by a considerable amount (like tens of degrees) before having a noticeable effect due to alignment. Static port, OTOH, is a lot more susceptible to errors - anything happen upstream of your static port(s) recently? Can't imagine how the drain hole is affecting it unless (a) it is oversize, or (b) there's partial blockage somewhere else. Anyone drilling out your drain these days?

Does your GPS record data? There's a fairly easy way to get a good airspeed cal if you're interested(*). By 'good' I mean 'better than averaged legs,' but that kinda goes without saying ;)

(*) It does require math beyond what an E6B can do for you o_O

Nauga,
and his circling wind cal
Click to expand...

That's what I think too, maybe a kink in the static tube to the sensor? But I guess even a partially obstructed the tube would still eventually get to the correct reading. Or maybe a leak somewhere in the static system? Switching to alt air would quickly tell if that was an issue. Could also be a leak in the pitot system tube, maybe a nick or a loose fitting, causing air to leak, a small leak might be relatively insignificant at low speeds but increase in error at higher speeds/ more pressure.
 
PaulS said:
That's what I think too, maybe a kink in the static tube to the sensor? But I guess even a partially obstructed the tube would still eventually get to the correct reading.
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Yeah, a kink wouldn't show a significant error at steady speed unless it's a really bad one and you're climbing of descending.
PaulS said:
Or maybe a leak somewhere in the static system? [...] Could also be a leak in the pitot system tube...
Click to expand...
He did say it passed the ground checks.

Nauga,
with static cling
 
MauleSkinner said:
So you're saying that then the GPS takes a ground speed and a calculated TAS to determine winds, those winds somehow verify the TAS calculation?
Click to expand...

Not sure how it is done, the aspen (and the g1000) uses what they call an air data computer that calculates TAs, wind speed and direction. I will show a direct crosswind that is not affecting GS.
 
Topper said:
Not sure how it is done, the aspen (and the g1000) uses what they call an air data computer that calculates TAs, wind speed and direction. I will show a direct crosswind that is not affecting GS.
Click to expand...
It uses ground speed, track, and true airspeed to calculate the wind.

An air data computer is a built-in electronic e6b that's ties directly into the appropriate systems to get its readings and does continuous calculations.

If you were to fly with your pitot plugged, you'd see a direct tailwind equal to your ground speed.
 
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The drain hole is probably the size of 28GA wire. Pretty damn small. The pitot tubing crack I considered, I may pull the pitot tube off and check. As far as static port issue - I have two of them, but my airspeed error is at all altitudes, so I'm not sure the static side is the issue, especially since it tested fine and passes static check on the ground. Also, I don't have alternate static and don't feel like replacing my VSI.
 
EdFred said:
The drain hole is probably the size of 28GA wire. Pretty damn small. The pitot tubing crack I considered, I may pull the pitot tube off and check. As far as static port issue - I have two of them, but my airspeed error is at all altitudes, so I'm not sure the static side is the issue, especially since it tested fine and passes static check on the ground. Also, I don't have alternate static and don't feel like replacing my VSI.
Click to expand...

It also seems to me that if the pitot line is cracked it would fail the ground test. (I'm imagining they blow calibrated air into the pitot on the plane for the ground test.)
 
jsstevens said:
It also seems to me that if the pitot line is cracked it would fail the ground test. (I'm imagining they blow calibrated air into the pitot on the plane for the ground test.)
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Yeah, but they sealed the drain hole during that test, and when I covered the drain hole I also got a bump in IAS. It's just weird.
 
EdFred said:
Yeah, but they sealed the drain hole during that test, and when I covered the drain hole I also got a bump in IAS. It's just weird.
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OK. But if the tubing was cracked and leaking wouldn't that show in the ground test? I'm still of the opinion that the ASI is not calibrated correctly for this installation. I'm not and A&P and may be ignorant of the realities, but it's the only piece that changed.
 
jsstevens said:
OK. But if the tubing was cracked and leaking wouldn't that show in the ground test? I'm still of the opinion that the ASI is not calibrated correctly for this installation. I'm not and A&P and may be ignorant of the realities, but it's the only piece that changed.
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Not necessarily, if the tubing was cracked inside the pitot mast and the mast itself is sealed, with the drain hole covered, the only place for the air to go is in through the tubing to the ASI. With it open, it can escape. The avionics guys are just as confused as the rest of us. I am leaning towards you on the ASI, with the exception that the ground test checked out. Which is what makes it odd.
 
EdFred said:
Not necessarily, if the tubing was cracked inside the pitot mast and the mast itself is sealed, with the drain hole covered, the only place for the air to go is in through the tubing to the ASI. With it open, it can escape. The avionics guys are just as confused as the rest of us. I am leaning towards you on the ASI, with the exception that the ground test checked out. Which is what makes it odd.
Click to expand...

I see. So the potential crack would leak out the drain hole but it's plugged for the ground test. That makes sense. This is a stumper. The other thing I have no idea of is how big a leak would it take to make a noticeable difference?
 
EdFred said:
As far as static port issue - I have two of them, but my airspeed error is at all altitudes, so I'm not sure the static side is the issue, especially since it tested fine and passes static check on the ground.
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The ground test won't show disrupted air flowing into the static port (AKA 'source error'), it will only show leaks and gauge error, which is why I asked about stuff upstream of the static port. Your cracked pitot sounds plausible but would be pretty unusual. Did your ground test check both static sources separately or suck both at the same time? A single clogged static port could cause some unusual indications but probably not very consistent.

Nauga,
source, error, correction
 
nauga said:
The ground test won't show disrupted air flowing into the static port (AKA 'source error'), it will only show leaks and gauge error, which is why I asked about stuff upstream of the static port. Your cracked pitot sounds plausible but would be pretty unusual. Did your ground test check both static sources separately or suck both at the same time? A single clogged static port could cause some unusual indications but probably not very consistent.

Nauga,
source, error, correction
Click to expand...
I was not present during the test I was sitting out in the lobby waiting I will double check that.
(Class C airport on Signature ramp they get weird)
 
EdFred said:
Averaging like that can produce erroneous results.
Example: Wind straight out of north at 60kts, and plane flies at 60KTAS. (exaggerated but wouldn't matter if the wind were only 30kts)

Heading 360 produces GS of 0
Heading of 90 and 270 produce GS of 85
Heading 180 gives a GS of 120
120+85+85+0 = 290
290/4 = 72.5
Which is quite a bit off.
(using 30kt wind results in 63.5)
Click to expand...


Only method I know to get this right is by using three headings and a calculator.

Averaging the speed on four cardinal headings is wrong as you point out. And averaging the speed on an upwind and reciprocal downwind leg can be wrong as well, you don't know the exact wind direction aloft.

http://www.eaa62.org/technotes/speed.htm
 
dell30rb said:
Only method I know to get this right is by using three headings and a calculator.

Averaging the speed on four cardinal headings is wrong as you point out. And averaging the speed on an upwind and reciprocal downwind leg can be wrong as well, you don't know the exact wind direction aloft.

http://www.eaa62.org/technotes/speed.htm
Click to expand...

If you keep turning until you reach min speed or max speed, you will be (almost) exactly upwind or downwind. When your heading changes ~3 degrees from that and your speed either starts to to back up (or down) you have the wind direction aloft. I wasn't just picking a random heading and flying it.
 
dell30rb said:
Only method I know to get this right is by using three headings and a calculator.
Click to expand...
There are lots of ways to do it. With a GPS that records groundspeed and track I like flying a circle at steady airspeed. There's a little math involved beyond a spreadsheet (or at least my skills with a spreadsheet) but it's pretty good, at least in steady air, which any straight-leg method requires as well.

Nauga,
and all the wind that's fit to print
 
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