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Airspeed indicator calibration

MacCool

Well Known Member
I am suspicious of an error in my pitot system leading to incorrect airspeed indication. It does affect my G5, my EFiS, and my round gauge (all read the same).

I am taking the plane down to my avionics shop for IFR certification soon. Would that IFR certification process by an avionics shop make flying a GPS-assisted airspeed calibration redundant? Or would that flight-based process evaluation provide additional data that the IFR certification doesn’t?
 
If you have a plumbing leak in your pitot or static lines, it can be detected on the ground with proper equipment.

However, in my experience many airspeed indication errors in RVs are the result of static port problems, either due to improper location or port shape. Static pressure measurements can be very sensitive to the shape of the port. Since this is affected by airflow, it can't easily be detected on the ground.
 
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Flight Test Required to Determine Static Source Pressure Error

Matt is correct.

Flight test (GPS horseshoe runs or towing a static drogue) is required to determine static source error (which induces IAS and altimeter error). The standard Van's pop-rivet static port in the plans-specified location generally gives good performance; but the only way to ascertain what's going with multiple IAS sources is to flight test. A pitot/static check at an avionics shop can, however, help you determine if there are any leaks in the system. An in-expensive water manometer will actually work well if you want to experiment with your system: http://www.rstengineering.com/rst/articles/KP89JUL.pdf.

During flight test, you can bias static pressure using layers of tape in front or behind the port and if this reduces error, you can substitute a permanent "dam" (cut 960 washer, etc.) that is the same height and location of any type. Try a search for "static source pressure error" or "IAS error."

Drop an email or PM, happy to help if you want to discuss it further.

Cheers,

Vac
 
Thank you both. The static system ports are the ones the original builder put in 10 years ago and the plane had subsequently passed at least a few IFR checks since then in its 400 tach-hours. After an EFIS install that I did last year, the local avionics guy did a “quick and dirty” pitot-static leak test. It did show a leak “enough to prevent IFR certification but not enough to have a significant impact on VFR operation” (his words).

Let me add some complexity related to my problem but unrelated to my question. Relevant information that I didn’t mention…I recently did install a new updated EFIS, require installing and re-plumbing a new ADAHARS. That new ADAHRS (Dynon) required a substantial replumbing of the pitot-static system involving a couple of new PTF “T” connectors and capping of a couple of the abandoned lines. Sorry for adding that level of complexity.

So lately I’ve seen what I consider to be lower-than-expected true airspeeds. I want to check the ASI error and was planning a GPS-based calibration run, but recently decided to just go for all the marbles and take if for an IFR re-cert (different avionics shop) and have them find the leak and fix it. I’m a noob, and have not demonstrated a firm grasp of avionics practice to myself.

Anyway, the question is: Does an IFR certification process make the GPS-Assisted airspeed calibration process unnecessary?
 
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An IFR pitot static check does not “calibrate” airspeed indication. What it does do:
- Find plumbing leaks in both the static and pitot lines. Some also find leaks in th AOA line if you have an AOA type pitot.
- Will verify your altitude indication (whatever you choose to be for IFR work) is within tolerance limits. For extra money you can also get the various backup altitude instruments signed off (but that is overkill in my book).
- Verify your XPDR mode C is reporting the correct altitude.

What this check does not do:
- Give you any indication that your static port is the right shape and/or in the right spot.
- “Calibrate” your airspeed instrument.

If you want to do your own static system leak check to find leaks, here is a test rig made from Harbor Freight stuff.
F10-F8-C0-F-1-AFF-4721-AD29-4-FF09-EBECED1.jpg


While most EFIS airspeed indications are accurate, Van’s analog airspeed instruments are notorious for been poorly calibrated. Make a test rig to see what you have. This simple rig is amazingly accurate. Google details.
A7-E0-A5-E3-2967-410-A-A16-B-F8-D0-BE44633-C.jpg


Carl




Carl
 
Checks

I copied this from another website, Premier Aviation. It pretty succinctly covers the inspections

“ All transponder-equipped aircraft, both VFR and IFR, must have their transponder tested and certified every two years (see FAR 91.413). Aircraft operating under IFR must also have their altimeter(s), encoder and static system tested and certified every two years (see FAR 91.411).

In addition, FAR 91.217 requires that all transponder-equipped aircraft have their altimeter and encoder checked for data correspondence after initial installation or following any subsequent modification or maintenance.”

Notice the IFR check under 91.411 is inspecting the altimeters, encoder and static system. If your leak is in the static system the shop should be able find and correct it. It does not require a airspeed check, but the equipment can perform a comparison check.

If it were mine I’d do the GPS runs to determine the level of error. It would give you base data to make a comparison after the shop works on the system. There are other issues that can affect the system such as installation, choice of static port, and others.
 
Yeah, on reflection, and in light of Carl’s helpful clarification, I’m going to go fly that. Just finished putting a 360° route in Foreflight and I’m going to go out to the airport have the autopilot fly it. Thanks to you all.

Quite honestly this is a waste of time. You’ve already had a shop tell you your leak rate is unacceptable.
1. Use an apparatus similar to Carl’s first photo to draw a slight vacuum. Don’t forget to tape off or otherwise seal any additional static ports. Systematically hunt down and fix the leaks. Or pay a shop to do it for you.

After fixing all the leaks, go fly to test the static port placement/shape/etc. You can do this by flying a triangle, noting IAS and GPS ground speed, note OAT and pressure altitude so you can change IAS to TAS, find or calculate the formulas to compare the data over the three legs to eliminate the wind. OR (simpler IMHO): go to an airport with a control tower (can be closed). Land, stop, set your altimeter to read the field elevation. Take off, make a high speed pass level with the top of the tower, noting altimeter reading. Compare to published altitude (tower altitudes are in the AFD). They should agree to within your estimated error of how well you were at exactly the top of the tower. If they disagree, you have a static port shape/placement error.
 
Quite honestly this is a waste of time. You’ve already had a shop tell you your leak rate is unacceptable.
1. Use an apparatus similar to Carl’s first photo to draw a slight vacuum. Don’t forget to tape off or otherwise seal any additional static ports. Systematically hunt down and fix the leaks. Or pay a shop to do it for you.

After fixing all the leaks, go fly to test the static port placement/shape/etc. You can do this by flying a triangle, noting IAS and GPS ground speed, note OAT and pressure altitude so you can change IAS to TAS, find or calculate the formulas to compare the data over the three legs to eliminate the wind. OR (simpler IMHO): go to an airport with a control tower (can be closed). Land, stop, set your altimeter to read the field elevation. Take off, make a high speed pass level with the top of the tower, noting altimeter reading. Compare to published altitude (tower altitudes are in the AFD). They should agree to within your estimated error of how well you were at exactly the top of the tower. If they disagree, you have a static port shape/placement error.

Thanks Bob, I appreciate the excellent advice but I'm going to fly the calibration course anyway. Seems like as good a reason as any to go flying.

As for the leak that my local avionics guy asserts...I'll see what the other shop tells me once I get it down there and let them track it down and fix it while they have it, and then certify for IFR. I wish you lived closer.
 
Mac,

Welcome to experimental flight test :D.

Here's a spreadsheet that will help with data reduction for your GPS tests. It goes beyond the GPS math and also derives static source pressure error. The objective is to minimize that to the extent practical. There is data from a typical test run with my airplane so you can see how everything works. Just type over my data with yours.

The autopilot will probably handle the speed runs until you get down to L/Dmax (Vy) alpha/speeds, then you'll probably need to hand fly the slower portion of the speed band. See my note above on how you can bias pressure around a static port to mitigate pressure error.

Good luck and have fun learning--that's what experimental aviation is all about!

https://3c039af6-63d7-4703-82ff-4bf98735a5a3.usrfiles.com/ugd/3c039a_6e0390b2f8314af99ddc06b6d27dafb4.xlsx

Cheers,

Vac
 
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