Next step to trouble shoot airspeed discrepancy ? (SOLVED)

Is this a production or E/AB aircraft?

I've been chasing a static port placement error ever since first flight. It's very frustrating.
 
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.
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This. When the ASI acts up, we go after the pitot system, forgetting that the ASI is a differential pressure gauge, measuring the difference between static and dynamic pressures. And many don't realize that accurate static pressure is MUCH harder to obtain than an accurate pitot pressure. Homebuilders run into this all the time, and the manufacturers of certified airplanes have to fool around quite a bit when testing a new design as they try to find the best source of static pressure.

Is that static port clean? Is there a flake of paint disrupting something? Or a ridge of paint left from masking during the last paint job? Has there been some mod that attaches something to the airframe ahead of the port? Is there supposed to be a dam ahead of the port that has been removed or damaged? Getting an accurate static pressure in a 150 MPH wind is not easy, and tiny things matter.

If there were no leaks in the two systems, the plumbing is fine. If the pitot's drain hole is within a few inches of the pitot opening, the loss of pressure through the hole is negligible. If that hole was three feet farther down the system it would make a much bigger difference due to the drag of the tubing ID.
 
This may not be applicable on a Comanche, but my Archer has a airpeed diaphram in parallel with the aircraft ASI that is used to disable the electric trim when it detects overspeed (145KAS). My airpeed diaphram had a hole in it so my aircraft ASI would read low by 20 knots. Replacing the diaphram solved the problem. I bet most avionics shops have no idea about this diaphram.
 
Baron62 said:
This may not be applicable on a Comanche, but my Archer has a airpeed diaphram in parallel with the aircraft ASI that is used to disable the electric trim when it detects overspeed (145KAS). My airpeed diaphram had a hole in it so my aircraft ASI would read low by 20 knots. Replacing the diaphram solved the problem. I bet most avionics shops have no idea about this diaphram.
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But a leak should have shown up when they tested the systems, if that Comanche had a leaky diaphragm somewhere. I still suspect there's a static source issue, right at the static port (or ports).

I did read, a long time ago, of an ASI problem caused by a burred edge on the pitot tube opening. It caused turbulence right at the opening that deflected the airflow and lowered the dynamic pressure.
 
Disconnected the ASI from the static system at the T junction going into the back of the ASI. 5300' TAS 157, IAS 144ish.

Now to figure out if its the line between the T for the two static ports and the ASI T or one of the static port legs.
 
Disconnected at the rear T, hooked up an air compressor and blew out the line just in case something was in there. Reconnected at the ASI and took it up to fly. 5000' MSL, TAS 152, IAS 138/139. So that's almost exactly what I would expect. I was going to reconnect at the rear T and tape over one of the static ports but didn't get a chance to finish before the food poisoning decided to resurface. Though at this point I don't think it is that, but that's another thread.
 
Went up today. Reconnected to the rear T. Blocked the left static port with an electrical tape patch. Took it up and flew it. Indicated as it did before disconnecting the tube. Same thing with the right static port. 2400RPM 24" MP, 5000MSL, <130 indicated. Disconnected from the T, 140+ indicated TAS 154.

I'm stumped at this point.
 
EdFred said:
Went up today. Reconnected to the rear T. Blocked the left static port with an electrical tape patch. Took it up and flew it. Indicated as it did before disconnecting the tube. Same thing with the right static port. 2400RPM 24" MP, 5000MSL, <130 indicated. Disconnected from the T, 140+ indicated TAS 154.

I'm stumped at this point.
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Now that you're back @EdFred, did you ever figure this out?
 
jsstevens said:
Now that you're back @EdFred, did you ever figure this out?
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I left the static disconnected at the T and my static port is my entire tailcone. I'm too big to be crawling around back there trying to avoid the battery and ELT and remote compass and cables going to the stabliator and trim tab and rudder and....

However, since I'm going to be grounded for a month starting Feb 7, I'm bringing the plane in for annual early, and will have my A&P run some air from the T junction out through the static ports, and reconnect and see what happens in March.
 
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EdFred said:
I left the static disconnected at the T and my static port is my entire tailcone. I'm too big to be crawling around back there trying to avoid the battery and ELT and remote compass and cables going to the stabliator and trim tab and rudder and....
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Well, I guess you don't have to worry about the static port clogging up...

This one bugs me (not as much as you, I'm sure) because it just doesn't make sense...
 
jsstevens said:
Well, I guess you don't have to worry about the static port clogging up...

This one bugs me (not as much as you, I'm sure) because it just doesn't make sense...
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Honestly, I had forgotten about it since my indicated numbers have been what they should be. Winds aloft calculations have been almost exactly as forecast. So it's something between the T, or the T itself, and the 2 static ports. What that is, I don't know.
 
another case of an owner not reading the FARs and understanding them and listening to people who make money fixing things. show me anywhere in those 2 fars that relate to the ASI and any certification. it only requires alt and static system checks. a airspeed calibration test is not required.
 
unsafervguy said:
another case of an owner not reading the FARs and understanding them and listening to people who make money fixing things. show me anywhere in those 2 fars that relate to the ASI and any certification. it only requires alt and static system checks. a airspeed calibration test is not required.
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I'm pretty sure 14CFR91.205 requires that I have a correctly functioning ASI.
Hey, what do you know it's the very first instrument listed under (b)
Where did I say they did an airspeed check on the initial test? I didn't.

You also have zero clue about my relationship with the shop. They are also a customer of mine, so there is no "lets try and sell this guy on stuff he doesn't need" going on

Another case of some guy on the internet attempting to sound smart while not knowing all the facts.
 
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EdFred said:
I'm pretty sure 14CFR91.205 requires that I have a correctly functioning ASI.
Hey, what do you know it's the very first instrument listed under (b)
Where did I say they did an airspeed check on the initial test? I didn't.

You also have zero clue about my relationship with the shop. They are also a customer of mine, so there is no "lets try and sell this guy on stuff he doesn't need" going on

Another case of some guy on the internet attempting to sound smart while not knowing all the facts.
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Well in post 18 you stated that the the 411 and 413 check is where they found it.

Those checks do not require the asi to be checked. If you requested it, great, but if the shop just does it and charges for it, then your getting charged for services not requested or needed by the far.

As to 91.205 it requires an airspeed indicator, not a "correctly funtioning" airspeed indicator. What "spec" was it out of? Far23.1323 spells out asi system certifications. Basically 5kts or 3% cas.

I'm glad you are making sure that all the systems in your aircraft are working properly, but it appeared from post 18 that the shop tested your asi as part of the static check which is not part of the requirement, and most avionics shops that do static checks are not equipped to properly check an asi, only an instrument repair shop can properly check an asi for calibration.
 
unsafervguy said:
Well in post 18 you stated that the the 411 and 413 check is where they found it.

Those checks do not require the asi to be checked. If you requested it, great, but if the shop just does it and charges for it, then your getting charged for services not requested or needed by the far.

As to 91.205 it requires an airspeed indicator, not a "correctly funtioning" airspeed indicator. What "spec" was it out of? Far23.1323 spells out asi system certifications. Basically 5kts or 3% cas.

I'm glad you are making sure that all the systems in your aircraft are working properly, but it appeared from post 18 that the shop tested your asi as part of the static check which is not part of the requirement, and most avionics shops that do static checks are not equipped to properly check an asi, only an instrument repair shop can properly check an asi for calibration.
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It's been coming up on two years since they did the test, but from what I recall the static test didn't initially pass due to a leak in the system. The ASI was bypassed and the static test then would have passed. They didn't do extensive testing on the ASI. They said they could send it back in for OH, I said and that will cost what 60% of a new one? Just get me a new one. I've had this shop with the same guys doing my 411/413 for the past 15 years, and they've never pulled anything on me. With the systems they install in other planes, the extra $600 or whatever it was for the ASI isn't even a whisper in a football stadium.

As far as 91.205...

(a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

Operable condition to most people means functioning correctly. They only did the ASI calibration and testing on the new one at my request, a year later.
 
OK, so a revisit.

Something is definitely up with the aerodynamics and the static ports. My guess is some sort of ram air happening. The new owner had the .411 static check done, but more extensive than just the regular .411 check a couple weeks back and had an alternate static installed. On the ground everything checks out perfect. Taking it up and Flying at 6500' with the alternate static on and somewhere in the neighborhood a little above ISA, indicated airspeed was about 140kts with calculations of 158 true, which is around what should be expected, maybe a shade high. (testing vs a GPS course was not done) Closing the alternate static put the airspeed down to just above 125 knots indicated, and the altimeter then indicated 200ft low. It is not an ASI/altimeter display error as the G5 install matches the analog displays, and pressure altitude matches the display on the transponder when ALT is used. And 125kts indicated is WAY low for what should be seen at that altitude and power setting. (full rental power)

The 1975 issue of the POH does say a higher airspeed is to be expected with Alt static, but does not say how much higher. A list of STCs on Webco does indicate that on later models, the ASI could change by about 8mph at that airspeed, but not 17mph. On approach with everything dirtied up, and even on take off (<90kts) the difference between the two sources is negligible, It's just going to remain as open. It seems that with this particular serial number the static ports might just be misaligned slightly causing a ram air issue and since the old ASI was out of spec, it probably countered it somehow???
 
1RTK1 said:
Without thinking it through are you sure the on is not off and visa versa.
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TAS based on gps course and TAS based off of IAS calculated with alt static match.
 
And the truth shall set you free!

Finally got up in a formation flight with a Commander and a Mooney today, not that the makes matter so much, but first chance I've had to test against other ASIs.

My indicated on the analog and the G5 agree with each other, but both the Commander and Mooney read 8 knots higher while we all maintaned formation.

Pitot tube has to be misaligned as everything else with the system has been checked and passes.
 
EdFred said:
My indicated on the analog and the G5 agree with each other, but both the Commander and Mooney read 8 knots higher while we all maintaned formation.

Pitot tube has to be misaligned as everything else with the system has been checked and passes.
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An 8kt error from the pitot side is huge, and pitot tubes are typically insensitive to misalignment. Did you note the altitude difference between you and the other airplanes?

Nauga,
and hit trailing cone
 
nauga said:
An 8kt error from the pitot side is huge, and pitot tubes are typically insensitive to misalignment. Did you note the altitude difference between you and the other airplanes?

Nauga,
and hit trailing cone
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Altitude difference was +/- 10 feet. And I'm not sure 8 is that big of a deal - in that it could be more. Was talking with hangar neighbor who has a Murphy and can get a 500fpm climb while his ASI reads 0.
 
EdFred said:
Altitude difference was +/- 10 feet. And I'm not sure 8 is that big of a deal - in that it could be more. Was talking with hangar neighbor who has a Murphy and can get a 500fpm climb while his ASI reads 0.
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8 kt source error correction is not necessarily 'that big a deal', but 8 kt due to a *pitot* side error is huge. If your indicated altitude was within 10ft of the other airplanes and you were level with them that would seem to rule out the static source error but something just isn't lining up. I'm interested to hear how far out of alignment your pitot probe is.

Nauga,
not thinking zebras.
 
nauga said:
8 kt source error correction is not necessarily 'that big a deal', but 8 kt due to a *pitot* side error is huge. If your indicated altitude was within 10ft of the other airplanes and you were level with them that would seem to rule out the static source error but something just isn't lining up. I'm interested to hear how far out of alignment your pitot probe is.

Nauga,
not thinking zebras.
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Honestly I'm not sure I'm going to worry about it now that I know for sure where the problem is. I suppose at next annual we can prop the plane level and see what the pitot tube angle is vs what it is supposed to be, and if it is where it is supposed to be...*shrug*
 
EdFred said:
And the truth shall set you free!

Finally got up in a formation flight with a Commander and a Mooney today, not that the makes matter so much, but first chance I've had to test against other ASIs.

My indicated on the analog and the G5 agree with each other, but both the Commander and Mooney read 8 knots higher while we all maintaned formation.

Pitot tube has to be misaligned as everything else with the system has been checked and passes.
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Normally, the difference between CAS and IAS is negligible at cruise speeds, but did you adjust the indicated airspeeds to calibrated on all three planes just to be sure?
 
To be sure of what? We were only comparing IAS. But to answer the non-pertinent question the Commander and the Comanche have the same correction at those airspeeds - a whopping 2 knots. So his 118 was actually 120 and my 110 was 112, still an 8kt difference. But to circle all the way back to the beginning of the thread the difference in IAS matches with what I *should* show based on calculated TAS off of GS readings.
 
EdFred said:
Yes, all within acceptable IFR tolerances.
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So here's a question. At about 120kts, how much airspeed error does 50' of altitude difference from static port error cause?

I'm guessing there's a formula or table somewhere.
 
donjohnston said:
So here's a question. At about 120kts, how much airspeed error does 50' of altitude difference from static port error cause?

I'm guessing there's a formula or table somewhere.
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Well the rough rule of thumb is~2% for every 1000'.

So 50/1000 * 0.02 or 0.0025%

There are a couple of online TAS (TAS at 0MSL and ISA = IAS) calculators and if you change the altitude by 50' the TAS doesn't change
 
EdFred said:
Taking it up and Flying at 6500' with the alternate static on and somewhere in the neighborhood a little above ISA, indicated airspeed was about 140kts with calculations of 158 true, which is around what should be expected, maybe a shade high. (testing vs a GPS course was not done) Closing the alternate static put the airspeed down to just above 125 knots indicated, and the altimeter then indicated 200ft low.
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Something FUBAR in the alternate static valve itself, or its connections?
 
donjohnston said:
So here's a question. At about 120kts, how much airspeed error does 50' of altitude difference from static port error cause?
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A 50' pressure altitude error at 5K ft is about a 3 psf (0.02 psi) static pressure error. That same 3psf error in dynamic pressure (Ptotal-Pstatic) is roughly a 3-4 kt airspeed error at 120 KCAS.

Nauga,
embracing the SSEC
 
nauga said:
A 50' pressure altitude error at 5K ft is about a 3 psf (0.02 psi) static pressure error. That same 3psf error in dynamic pressure (Ptotal-Pstatic) is roughly a 3-4 kt airspeed error at 120 KCAS.

Nauga,
embracing the SSEC
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Are you sure you are answering the question asked? Every 50' is a 3-4 knot error? I have never seen an ASI change by 3-4 knots over 50'.
 
EdFred said:
Are you sure you are answering the question asked?
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Yes, I am.
donjohnston said:
So here's a question. At about 120kts, how much airspeed error does 50' of altitude difference from static port error cause?
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A static source error that causes a 50 ft altitude error at 5000 ft MSL will cause a 3-4 kt indicated airspeed error at 120 kt.

Nauga,
formulaic
 
The way I read that is if I am at 4500 vs 4550 Im going to see 3-4 knots difference over 50'. I have never seen that in all my years of flying. A 500' difference would be 30-40 knots?

What am I missing?
 
Are you certain that the new ASI is calibrated properly ? I know that if the ASI in my Cessna 195 is replaced, it has to be calibrated to 195 specs before installation. Just because you installed a NEW ASI, that does not guarantee it is not the problem.
 
Doug Reid said:
Are you certain that the new ASI is calibrated properly ? I know that if the ASI in my Cessna 195 is replaced, it has to be calibrated to 195 specs before installation. Just because you installed a NEW ASI, that does not guarantee it is not the problem.
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Two ASI's in the plane - a G5 and the (then new) analog and those both match exactly, so it's not an instrument error, and that was also calibrated/tested before the G5 went in, so it was determined the ASI was not the issue.
 
If you have confirmed no ram air or static air leaks, alt air valve is not leaking, you have not altered the pitot tube or static ports and confirmed no water is trapped in static system, then I would possibly consider testing with analog ASI only to see if the problem is in an instrument. When you have two instruments in parallel, and one begins to leak, they are both affected to the same degree. Try to eliminate the G5 and new ASI..then the hunt is back on.

I suspect your old ASI had a leak which is why it was condemned...if it and the G5 were connected parallel, they would have read the same.
 
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