G3X Airspeed Calibration Issues

[msaltzman14]

Hi All!

I recently got my panel upgraded to a full G3x kit. WOW is it great.

However, my True Airpseed is indicating 150 knots when my actual TAS (as calibrated using the three-leg GPS method) is 157.

I called Garmin and they said most likely culprit is the temperature probe reading inaccurately, but I just essentially debunked that possibility with the following logic:

Temp indicated was 0 degrees Celsius @ 8000 today. That temperature output a TAS indication of 150. For the actual TAS to be 157, the actual Temperature would have to be 23 degrees C (given 30.14 inches and 8000 indicated). It certainly wasn't anything close to that.

My avionics guys did a check and found no leaks in the pitot system.

Anyone else have this issue? Anyone resolve it?

Thanks!

[krwalsh]

I have a G3X as well, but I don't believe the problem is with your G3X. I think it is with your pitot-static system. First, you should have had an altimeter/transponder check done in accordance with 91.411 and 91.413. Follow the G3X manual and zero out all error as you go. Mine was perfect within 5ft right out of the box, so I doubt this is the issue.

You should read this:
https://www.ntps.edu/images/stories/...pec-method.doc

And this:
https://www.ntps.edu/images/stories/...s/tas_fnl3.pdf

And download this:
https://www.ntps.edu/images/stories/...s/GPS_PEC.xlsx

Which all come from the National Test Pilot School website.

Do a full sweep of airspeeds (and altitudes, if you want to be very thorough) and make the appropriate corrections to your static port(s).

I sort of wish that the G3X had a lookup table (LUT) so you could simply put in the GPS-verified speeds against the TAS and have the correction done in software instead of by modifying the hardware, but it is what it is.

Incidentally, you can verify the temperature probe by checking it on the ground against the field-reported ATIS. It should not be off by more than a degree.

[Richard Connell]

That?s a great post Kevin. Worthy of a sticky!
So many builders go through this process. Many more probably don?t.
I know of at least half a dozen here who have had various PEC issues due to different static port deviation from the vans pop rivet. Flush mounts / mounted externally / wrong rivet etc...
Anyway. I had also thought about some kind of software solution as well but I couldn?t see how one could avoid the associated altitude error associated with the static error. It all seems a bit circular to me.
Cheers
Richard

[g3xpert]

Agree. Since static source error affects both altitude and airspeed, putting a band-aid on airspeed would only address part of the problem.

It is better to correct the static port problem so both are accurate.

Thanks,
Steve

[jfrank71]

I had he same problem after my g3x touch install. It was a static port issue and I had flush mounted static ports. I used blue painters tape and secured a small flat washer over my static port on both sides with center of washer over port. Flight tested with much improvement. I ended up taking the vans rivet/static port, cutting off the rivet head and permanently attaching it to static port. I now had the small bump to disrupt airflow and give a true and accurate static reading. Problem solved and tas is accurate.

[Timberwolf]

Originally Posted by jfrank71

I had he same problem after my g3x touch install. It was a static port issue and I had flush mounted static ports. I used blue painters tape and secured a small flat washer over my static port on both sides with center of washer over port. Flight tested with much improvement. I ended up taking the vans rivet/static port, cutting off the rivet head and permanently attaching it to static port. I now had the small bump to disrupt airflow and give a true and accurate static reading. Problem solved and tas is accurate.

I did the same after my g3x install.

[msaltzman14]

Great advice everyone- I will be working it through in the next few weeks and will report back what I find. THANK YOU for the thoughtful help.

[msaltzman14]

Hey Guys-

Just to sense check-
My G3x shows correct field elevation when I punch in the ATIS barometer setting- does that not mean that the static port is reading correctly? Are we thinking this is a dynamic error only when the airplane is in motion? If that is the question, and I fly FAST right over the runway, we would expect the altimeter to read significantly differently than field elevation, right?

And because my TAS was reading LOW, that implies the static source is getting HIGHER than correct pressure, so the associated error in altitude would be for the altimeter to read LOWER than actual field elevation. Is that all correct logic? I can check it tomorrow morning without finding washers and tape...

[BobTurner]

Originally Posted by msaltzman14

Hey Guys-

Just to sense check-
My G3x shows correct field elevation when I punch in the ATIS barometer setting- does that not mean that the static port is reading correctly? Are we thinking this is a dynamic error only when the airplane is in motion? If that is the question, and I fly FAST right over the runway, we would expect the altimeter to read significantly differently than field elevation, right?

And because my TAS was reading LOW, that implies the static source is getting HIGHER than correct pressure, so the associated error in altitude would be for the altimeter to read LOWER than actual field elevation. Is that all correct logic? I can check it tomorrow morning without finding washers and tape...

Basically correct. Although the airspeed indicator is more sensitive to static source errors than the altimeter, so look for any difference, not just "significantly different".