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Airspeed and Altitude Calibration Procedure and Spreadsheet

Vans101

Well Known Member
I made a similar post on the Dynon forum however there are a number of really smart people who do not have a Dynon EFIS and this stuff is the same so I wanted to post here to get help...

I am FINALLY getting around to doing the airspeed and altimeter calibration in my airplane.



Does anyone have a procedure and an associated spreadsheet that I can use to do this test?



I have seen procedures where you (I) fly one leg on a given TRACK (and some use a HEADING) lets say use 000 (Due North at level altitude and do not make changes in power and let the IAS stabilize (at about the 30 second point should do it) and then I record the GPS Speed and the TRACK (or HEADING) degrees in some place on the spreadsheet…then make a turn to lets say left heading (or track?!?!?) 120 degrees and fly that track for 30 seconds then record GPS speed and track …then fly a track of 240 degrees…



Got it....this compares GPS speed to ground speed and the three legs cancel out for the effects of the wind...some people say to do four legs so I presume that would be mo-betta...



So now is the part(S) I don’t get…



What I want to do is to ensure that my Calibrated Air Speed is close to my Indicated Air Speed and that my Indicated altitude is close to True Altitude (MSL) not just at the one speed and altitude I just did these three legs but I want to ensure that the CAS and True Altitude (MSL) is accurate for speeds from VS+5 all the way up to whatever my engine can get the airplane speed and I want to do this from 1000 MSL up to about 17500 MSL so I should do this same test at the various speeds (VS+5, 80, 100, 120, 140, 180, 200) at 2000 MSL and then go up to higher altitudes (5000, 10000, 15000, 17500) and then do the same steps again…



Then I need to plug and chug this data into a spreadsheet and then that spreadsheet will squirt out the data so I can compare my Dynon altimeter and IAS and then make whatever adjustments I need to make the Dynon match what it really should be.



Different air speeds may really affect the static source pressure and that might screw around with both IAS and True Altitude. Example: I do these three legs at 70 KIAS and the GPS altitude shows 2000 feet and then when I do the 180 KIAS legs the GPS altitude might be showing 2800 feet due to poor static source location



The only way to ensure that the indicated altitude when set to the local altimeter setting is close to True Altitude MSL is to actually compare the Indicated altitude to the GPS altitude at the various airspeeds.



So I cant imagine that I am the first person to want to check this data so someone out there in Cyberspace has most likely come up with a spreadsheet that has lots of data blocks to record data such as the local altimeter setting for the location I am doing my triangles over, the OAT for where I am triangulating…and the spreadsheet that will do this fancy Greek math for me and work the EFB backwards to come up with all of this stuff...



Then I presume the Dynon will have places to make these calibration changes and I will study the manual for that however the first step is to determine what procedure to use, what data to collect, and then what spreadsheet to use...



Anyone out there that can help...Please???



THANKS!!!
 
1. Airspeed. In principle if you fly 3 legs (doesn’t matter if you choose heading or ground track, but you do need to use slightly different formulas depending on which you choose) then you have enough data to calculate wind speed and direction, and true airspeed. This assumes the wind remains constant in direction and magnitude. If you fly 4 legs, the problem is mathematically over-determined and you will need to do some sort of averaging to get the best fit to the data. As I replied in the other thread on this subject, I agree with another reply: the chance of the wind varying over the time it takes to fly 3 legs probably is worse than the simple averaging method, where you have the same issue of the wind varying but it’s quicker, making that a bit less likely. Just determine the wind direction (by slowly turning while watching your gps ground speed). Direction where ground speed is minimum is directly into the wind. Fly into the wind, note gps ground speed; do a 180, fly with the wind, note ground speed. Average the two speeds, that should be your TAS. Record density alt and OAT, use calculator to change TAS to IAS. Compare to what your cockpit IAS says. Repeat at faster and lower speeds.
2. Altitude. You’ve made an incorrect assumption here: a properly functioning barometric altimeter will match True Altitude (essentially gps altitude) only if you have a ‘standard atmosphere’ day, which is rare. Fortunately, they are not as sensitive to static port errors as the IAS is, so if there are no large errors in the IAS check, the altimeter can be calibrated on the ground.
 
Some thoughts:
- I don't know of any EFIS system that lets you tweak airspeed. This is a straight forward pressure delta. I did however use a simple manometer to make sure the SkyView airspeed was correct, and to see how far off my analog airspeed indication was indicating. This test also verified no leaks in the pitot system. The SkyView was dead on, and the analog airspeed read ~5kts fast at the low end. I'll take it apart later to adjust at some point.
- This leave static errors to be the biggest issue with airspeed. Get your pitot/static check done. During this check you can adjust the SkyView to read exact altitude.
- If flying you see a big IAS error, then that points to a bad static port install. If so, there are fixes for that as well.

I have a PDF file that has a manometer scale that reads directly in knots. If interested PM me your email.

Carl
 
Is your OAT in a good location and not reading high because of engine heat that too can cause for inaccurate TAS. You have a lot of variables each one needs to be calibrated to get the proper answer.
 
Getting the spreadsheet is just part of the problem.

Who Cares?

I have done more than a dozen of these in the past 3 years, lately after paint.

1. get the excel spreadsheet that does all the calculations.
2. pick a smooth day, keeping in mind that the airmasses can and do change between the legs invalidating some runs.
3. warm the engine, I found it takes 30 min to get a good stable run.
4. I use Track, as GPS ground speed will be compared to TAS from the EFIS. Also the SS I have uses track, the GPS ground speed is why.
5. Be sure you watch the IAS to ensure it is stable. It takes me 15 sec after a turn to stabilize the speed again. I don't run at a fully lean condition, it helps the power stability.
6. Watch the panel time - write that down for the start and finish for good data for each leg. This way you will know where to work in the data sheet.

I download the Garmin EFIS data, highlight the columns and rows for the correct times. 1 point per second. To ensure good data, average 15-45 seconds of data. Look at the VS and altitude for each point and ensure it is not wandering. This means approx 2 min per leg.

I have seen EFIS calculated winds aloft change in a leg. You can look at windy.com winds (at your altitude) to see of there are significant air mass changes due to small gradients in pressure. They are always there.

Built into all of this the assumption that the TRACK and TAS position/calculation is accurate from the panel.

BTW - after paint, I had to do three runs before getting one without issues. It turned out the result was within .3 kts of my original error vs speed chart; well within run-to-run variance. Conclusion: Paint, in my case, made no difference

Norman is right, the OAT must be valid, but that is another story of validation.
 
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GRT has had the capabilities of inputting an airspeed correction on their air speed calibration page since the introduction of their first EFIS
 
Wow...good information...very grateful for your help!!!

>get the excel spreadsheet that does all the calculations.

BillL would you be kind enough to share the spreadsheet?


>I don't run at a fully lean condition, it helps the power stability.

Never would have thought of that

Thinking and typing out loud…I think the best way to do this is to fly over an uncontrolled airport that advertises the altimeter setting and fly at 1000 AGL to do the triangles because that altimeter setting will be close enough (unless here is a huge lapse rate change or an inversion) and as long as the GPS altitude does not change significantly from Vs +5 test to the max IAS that the engine can produce leg then you know that the static source is not affected by airflow over the airplane surfaces or compressed by the wing.

The pitot should be a reliable indication because they seem to be rather forgiving to installation angle errors so I think the IAS will be really close with this fancy new electronic stuff…but there is only one way to find out so maybe I will do the triangles at 1000 AGL over a known altimeter setting and then do a 10000, and 15000 triangle and just watch the GPS altitude…

I’ve been really wanting to do some fuel consumption performance tests at different altitudes and speeds anyway so this is a good opportunity...IF...my static system is reliable...Oh I got to make sure that the Fuel Flow numbers are accurate.
 
For lower speeds land on a runway keep the speed up and compare the airspeed indicator with the groundspeed on the GPS. If the windsock indicates calm then the difference between the airspeed indicator and the GPS will be your low speed error.
 
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For lower speeds land on a runway keep the speed up and compare the airspeed indicator with the groundspeed on the GPS. If the windsock indicates calm then the difference between the airspeed indicator and the GPS will be your low speed error.

No, this doesn’t work. Airspeed will read indicated, gps true (if no wind), speeds. This only works at sea level, standard day. Also keep in mind that CAS and IAS can be significantly different at high (er) angles of attack due to the fixed pitot.
 
Thinking and typing out loud…I think the best way to do this is to fly over an uncontrolled airport that advertises the altimeter setting and fly at 1000 AGL to do the triangles because that altimeter setting will be close enough (unless here
.....

I think this is a poor idea. Down low is where you are most likely to get significant wind variations, due to terrain, totally messing things up.
If you are concerned with static port variations messing with your altimeter, here’s an easy test: Fly up to LVK. Land and, in the 25R runup area, set the altimeter to exactly 400’. Ask the tower for a high speed low pass down 25R. Fly it at exactly 463’ on the altimeter. Mid-field, look to the right. You should be at the same height as the top of the control tower. If you are, then you have no significant static port placement errors. As I previously mentioned, the airspeed is more sensitive to these errors, but at least you’ll know they are small enough for the altimeter to work reliably (assuming it passed the ground check with a good pitot-static test rig).
 
Airspeed and static calibration

As already noted, A/S pitos with an EFIS generally are fairly accurate straight away, and easy to check with GPS and various spreadsheets. One recommendation is to pick a relatively calm wind day to reduce the influence of and concern about wind variations during flight.

Static is another story. It appears most people are pretty happy with the Vans designation of static port location and method. However, static pressure accuracy and resultant altitude data are extremely susceptible to errors from choice of port location as well as port "design", such as the Vans use of rivets. I wonder how many RVs out there do actually deliver accurate altitude info, and how many folks have no idea how far off they are, because verification of altitude is significantly more difficult than A/S.

I wrote an extensive article for Kitplanes some time ago about doing static location and calibration. If it helps you, it can be found at:

https://www.kitplanes.com/static-port-location-and-altitude-calibration/

The article includes references to Kevin Horton's excellent work on the subject, including his spreadsheet for determining A/S and altitude from flight data.

Reinhard Metz
 
>I wrote an extensive article for Kitplanes some time ago about doing static location and calibration. If it helps you

YES...it does help!!! THANKS for doing this work.

Looking at the spreadsheet I see the following:

Each "run" is a fixed airspeed flying three tracks using TRK mode...easy squeezy

IAS and "IAS protected for instrument error" is pretty much the same thing because I do not know what the instrument error is because it is EFIS.

Indicated Pressure Altitude is what my ALT HOLD was set to and it will remain constant and same as above with EFIS there will be no error so it is the same thing.

I see you did six different airspeed at approximately the following: 100, 80, 150, 160, 142, 145. Is there a reason why you did this pattern?

With the power of the spreadsheet it does not matter if the IAS is 142.6 knots or 140 KIAS so as long as I pick a power setting and the air is smooth.

What I did see on your data was that the wind was changing significantly so were these 6 runs completed on the same day within an hour?

If the airplane has EFIS would it be beneficial to record the ships TAS, and wind direction and speed so as to compare to the math?

Thanks again for your help and this spreadsheet was exactly what I was looking for
 
>
I see you did six different airspeed at approximately the following: 100, 80, 150, 160, 142, 145. Is there a reason why you did this pattern?

With the power of the spreadsheet it does not matter if the IAS is 142.6 knots or 140 KIAS so as long as I pick a power setting and the air is smooth.

What I did see on your data was that the wind was changing significantly so were these 6 runs completed on the same day within an hour?

If the airplane has EFIS would it be beneficial to record the ships TAS, and wind direction and speed so as to compare to the math?

There was no particular reason for the specific air speeds or order. The main thing was to check at least at landing, midrange, and cruise speeds to verify that A/S and altitude held correct at all of them, as they can be right at some speeds and wrong at others due to the influence, among others, of angle of attack.

Runs were done on different days. Yes, if you have EFIS wind data, it is nice to correlate it to indicated A/S data, and it will usually be right on. But that's only correct data if your system is already calibrated and correct, because the EFIS wind data uses your measured data, so if it's wrong, so will your wind data be.
 
>
IAS and "IAS protected for instrument error" is pretty much the same thing because I do not know what the instrument error is because it is EFIS... for

Calibrated airspeed (CAS) is IAS corrected for instrument errors. However ‘instrument’ means the entire system, and the biggest errors are usually not the efis, but rather the static port design and placement, and, at high angles of attack, the pitot placement (because it’s no longer pointed into the relative wind). In doing these tests, you are mainly checking the design and placement of the static port. You are also checking the air pressure sensor built into the efis, although these days they seldom fail and are usually quite accurate.
 
The only way to ensure that the indicated altitude when set to the local altimeter setting is close to True Altitude MSL is to actually compare the Indicated altitude to the GPS altitude at the various airspeeds.

Keep in mind that Altitude is the *least* precise information you can get from a GPS... True altitude and GPS altitude can be way off.
 
static port placement

Has anyone looked at other locations for the static port that are perhaps more representative of true static air pressure? Perhaps somewhere not in the airstream, like inside empty cavities in the wing, or hstab, or vstab? I'm aware that inside the cockpit is subject to pressure variations due to open/closing the air vents, etc. but perhaps there are other places that are better than the side of the fuse...
 
Keep in mind that Altitude is the *least* precise information you can get from a GPS... True altitude and GPS altitude can be way off.

Actually, a waas gps is pretty precise, and true altitude and gps altitude should closely match (otherwise LPV approaches wouldn’t exist). Barometric (indicated) altitude, otoh, depends on a standard model of the atmosphere, and can differ significantly from true altitude.
 
Keep in mind that Altitude is the *least* precise information you can get from a GPS... True altitude and GPS altitude can be way off.

What BobTurner said is absolutely correct. Drives me nuts when someone makes an emphatic totally wrong assertion here. The WAAS specification requires a position accuracy of 25 feet or better, for both lateral and vertical measurements, at least 95% of the time. Actual performance has been shown to be better than 5 feet vertical. While barometric/indicated altitude can and usually differs significantly due to non-standard atmospheric conditions, what is important is that everyone flies with a properly calibrated pitot system so that everyone shows the relatively SAME, though not necessarily absolutely correct altitudes, for collision avoidance.
 
Your ASI will read correctly if your static port is located correctly. You can verify your static port by making three passes over the runway at different speeds and at a fixed height (I chose 50' and 70 Kts, 100 Kts, and 140 Kts.) If your altimeter reads the same for all three passes, then your static port is good.
 
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