Next step to trouble shoot airspeed discrepancy ? (SOLVED)

Doug Reid said:
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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The G5 was never connected with the old ASI.

The old ASI was replaced with a new ASI, at that point the new ASI indicated lower than the retired ASI.
All lines have been checked, purged, each port was sealed off, etc, etc, etc, etc.
Calibrated air into the pitot on the ground. What the ASI is supposed to read during that test is what it read exactly, and has continued to read since the install.

After all of that was done, a G5 was installed. That agreed with the analog ASI. So it's not a display error, it's not a leak error, it's not a water in the system error, it's not a static error. Finally get it up with 2 other airplanes that had matching ASIs while all at the same altitude, and the one in the Comanche was the odd man out. The only thing that hasn't been checked/inspected/tested is the alignment of the pitot tube. Everything else has been torn apart and looked at.
 
EdFred said:
The only thing that hasn't been checked/inspected/tested is the alignment of the pitot tube. Everything else has been torn apart and looked at.
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Pitot misalignment sufficient to cause an 8 kt error in cruise would be immediately apparent at a glance for a normal Pitot tube.

Nauga,
bent
 
Doug Reid said:
Can't fix it if it aint broke :).....good luck !
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Yeah, I know. The whole system passes ground tests. But TAS based on GS across a course vs what the IAS should read shows reading 7-8kts low, which was corroborated by 2 other ASI.
 
donjohnston said:
Is there a formula used to determine that?
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I did the estimate earlier with standard tables but equations work just as well.

Empirical ambient pressure vs. altitide From USNTPS FTM-108 eqn 2.7 or any number of other references:
Amb. Press = (Sea Level Press)*((1 - 6.8755856e-6*(pressure alt in ft)))^5.255863

Assuming conditions are 5000ft MSL and 120 KCAS.

Ambient Pressure at 5000ft = 1760.8psf
Ambient Pressure at 4950ft = 1764.1psf
So the static pressure error in a system at 5000 ft but reading 4950 ft is 3.3psf

Most any aero textbook, or USNTPS FTM-108 eq. 2.13 rearranged
Dynamic pressure = 1/2(density at sea level)*(equivalent airspeed)^2
but at these conditions, calibrated airspeed is the same as equivalent airspeed (compressibility correction ~0)

Using all the proper unit conversions
Dynamic Pressure at 120KCAS = 48.8psf, so a Pitot-Static pressure difference of 48.8psf will indicate 120 kt (neglecting instrument corrections)
If there's a 3.3 psf error at the static port in that same system the pressure difference sensed would be 48.8-3.3 = 45.5psf.
Rearranging the dynamic pressure equation:
airspeed = sqrt(2*dynamic pressure/sea level density)
At 45.5 psf pressure differential the Airspeed = 116 kt, a 4kt airspeed error resulting from a 50 ft altitude error.

Nauga,
QED
 
So why does my IAS not drop 4kts when I climb 50ft ?
 
EdFred said:
So why does my IAS not drop 4kts when I climb 50ft ?
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Presumably because you're trimmed and flying to maintain calibrated (or indicated) airspeed.

Nauga,
closed-loop
 
This one has me flummoxed. I wonder if there is some type of restriction in the pitot tube that would cause the system to pass pressure tests, but be off when measuring ram air. Post some pics of the wing and pitot tube when you get a chance.
 
nauga said:
Presumably because you're trimmed and flying to maintain calibrated (or indicated) airspeed.

Nauga,
closed-loop
Click to expand...

But if I was 50ft higher or lower than the other aircraft in formation my ASI would read 4 kts different? That makes no sense.
 
EdFred said:
But if I was 50ft higher or lower than the other aircraft in formation my ASI would read 4 kts different? That makes no sense.
Click to expand...

Maybe their ASIs were wrong.....? :D
 
EdFred said:
But if I was 50ft higher or lower than the other aircraft in formation my ASI would read 4 kts different? That makes no sense.
Click to expand...
Airspeed is a comparison between the ram air and the static air. The static air is not the only pressure that's changing as you go up/down 50 feet, but the ram air as well, so the delta between the two will be the same.
 
EdFred said:
So why does my IAS not drop 4kts when I climb 50ft ?
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Because your static port climbs with you. Nauga is showing you the math to convert an altitude error to the equivalent airspeed error assuming a consistent static source error. I think his point was that if you were flying in formation and only saw only a 10' difference in indicated altitude, you likely don't have a static source error of sufficient magnitude to cause an 8 knot airspeed error. At least that's what I'm getting.
 
Lindberg said:
Because your static port climbs with you. Nauga is showing you the math to convert an altitude error to the equivalent airspeed error assuming a consistent static source error. I think his point was that if you were flying in formation and only saw only a 10' difference in indicated altitude, you likely don't have a static source error of sufficient magnitude to cause an 8 knot airspeed error. At least that's what I'm getting.
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And I interpreted it the opposite way, that if we were 100' different we would see the 8kt difference, and that's why the ASI's don't match.

When I've run the GPS course tests, and then run the numbers back through the TAS calculators to get what my ASI should read, it's been off 7-8kts. I went out an flew formation and got the same 7-8kts difference. Then he posted the whole pressure altitude difference thing, and I took that as a reason as to why our ASI's wouldn't agree.
 
EdFred said:
When I've run the GPS course tests, and then run the numbers back through the TAS calculators to get what my ASI should read, it's been off 7-8kts. I went out an flew formation and got the same 7-8kts difference. Then he posted the whole pressure altitude difference thing, and I took that as a reason as to why our ASI's wouldn't agree.
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My 'whole pressure altitude difference thing' was a response to @donjohnston's questions. My response to you has been and still is that the Pitot misalignment to cause an 8kt error at GA speeds would be immediately apparent by looking at it.

Nauga,
probing
 
OK.

This has finally been solved with the 2 year static check done last night. The previous place that did the static check used enough clay to fill about 1/2 a can' of Play-Doh™ to seal off the static ports and standoffs in front of the ports. Well, it appears that it doing so, it actually sealed off the leak that was there as well - and may have been there for the plane's entire life. Doing the check last night much less clay was used covering only the port and none of the stand off adjacent to the port and the static system still had a substantial leak. I don't have a wind tunnel to examine what's exactly going on aerodynamically while in flight but something is affecting it. Disconnected at the T in the tailcone, ran the check from there and it passed with no leak at all.

Took it up afterward leaving the tubing disconnected from the T and using the entire tailcone as a static port. Altitude checked with ATC, and airpseed was dead on what it was supposed to be.
 
Why is it always the last thing you check? JK!
Good on the persistence and find.
 
donjohnston said:
If the altitude ATC shows is what your transponder is sending out. And your transponder is getting that altitude from your altimeter. How would it not check?
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If there was pressurization (or depressurization) in the tail cone due to aerodynamics, then the altimeter would be reading off that and ATC would show something different. Just like if my kollsman window was off by .5 compared to actual airpressure. Or what would the altimeter show if the static port was inside a pressurized aircraft, or if the static port iced over?
 
My understanding (which is by no means authoritative) is that the static system provides pressure altitude to an altimeter/encoder. The encoder provides pressure altitude to the transponder which reports that altitude to ATC.

When I was doing my Phase I testing, I never got a peep that my altitude was any different than what I was seeing. But then I discovered a static port location issue which resulted in me actually flying about 300' higher than my altimeter (and ATC) thought I was flying. So my altimeter showed 5,500' and that's what ATC saw.
 
donjohnston said:
My understanding (which is by no means authoritative) is that the static system provides pressure altitude to an altimeter/encoder. The encoder provides pressure altitude to the transponder which reports that altitude to ATC.

When I was doing my Phase I testing, I never got a peep that my altitude was any different than what I was seeing. But then I discovered a static port location issue which resulted in me actually flying about 300' higher than my altimeter (and ATC) thought I was flying. So my altimeter showed 5,500' and that's what ATC saw.
Click to expand...

I have foggy brain this morning, but I thought there's a sensor in the encoder that compares to what the static lines are reporting and kicks out that difference to ATC. Their computers do what they do for local altimeter settings and that's how they check altitude. But I don't know, I'm so tires this morning, if someone asked me what 2 + 2 was, I'd say green.
 
Yes, there is quite a bit of confusion in this area. I learned a lot while fixing my static port problem. Most people I talked to tended to say things like: "If it passed the Pitot/Static/Transponder check, then it's accurate." When I explained that just verifies that the altimeter shows the correct altitudes at specific pressures and that those numbers are transmitted correctly by the transponder. And that there is no test which can be done on the ground to verify that the static port is not in a low or high pressure area (without a wind tunnel). I either got a blank stare or an "Oh!".

The only single aircraft way I came up with to verify airspeed was to fly a three (of four) course heading, record the ground speeds and plug them into a spreadsheet or online calculator. That will spit out the actual TAS. For altitude I noted the GPS altitude at the runway and then made low passes at high speed and low speed then compared the difference between GPS and altimeter. If the altitude difference is the same at different speeds, then the static port is probably in a null pressure area. Some people suggested flying past a tower of known height. To get close enough for that to be accurate made me uncomfortable.
 
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