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precise calibration of altimeter?

prkaye

Well Known Member
I installed a new Garmin G5 recently. I noticed that when i dialed-in the same altimeter setting on the G5 and on my Dynon D-180, the altitude reported by the two units differs by about 20 ft (when the ALTADJ offset of the D-180 s set to zero). In the Dynon there is simple way to adjust the altitude offset (I assume there is in the G5 as well).
Is there a DIY method for calibrating an altimeter precisely? I would need to know a precise elevation of the aircraft at some position on the ground, plus a precise and current altimeter setting. As for the former, I suppose the best way is to go to an airport that is absolutely flat, and use the published elevation. This would have to be an airport with a METAR and use a fresh altimeter setting just after a METAR is published. BUT - altimeter settings and physical altimeters are based on standard atmosphere... So on a day that is very warm or very cold (non-standard atmosphere), an altimeter set to current local altimeter setting might not read the true elevation (maybe off by tens of feet?). So many variables... is there a DIY way to fairly precisely calibrate our altimeters without specialized (expensive) equipment?
 
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I'd recommend going to an avionics shop and have them test your pitot/static system. They have calibrated tools that hook up to the pitot tube and static port that can accurately test your instruments.
 
The calibration process for the G5 must be done with a precision test box at 0/10k/20k altitudes. No shortcuts.
 
I'd recommend going to an avionics shop and have them test your pitot/static system. They have calibrated tools that hook up to the pitot tube and static port that can accurately test your instruments.

Yes, absolutely, I will be getting it in for the test later this summer. But my question in this thread is about what our best DIY methods are. More for interest than anything, and maybe to try to calibrate/check my equipment as best as possible before getting it into the shop.
 
The calibration process for the G5 must be done with a precision test box at 0/10k/20k altitudes. No shortcuts.

Ok that's good to know, thanks. The G5 is brand-new, so it's probably correct, and my 13-year-old Dynon D-180 is a bit off (by about 20 feet). It's interesting that the D-180 provides a user-adjustable ALTADJ setting for a single-point offset, where the Garmin does not. Setting the D-180 adjustment to -20 brings it in-line with hte G5.
 
For VFR:
Appendix F to Part 43 covers the semi-annual transponder checks. It doesn't cover anything regarding altimeters so I'm not sure what the legal requirements are.

For IFR:
Appendix E to part 43 covers the altimeter system checks. This calibration has to be done by a certified shop and logged every 24 calendar months. This is not something you should be adjusting in the Garmin or Dynon settings :)

Don't make assumptions about which of your altimeters is right. I go by the thought process that until they have been properly calibrated and checked by a service shop with the test equipment, they are both wrong :)
 
Altimeter-Encoder-Transponder

If you look at the FAQ section of Walt’s website (expaircraft.com) he answers all these questions for VFR & IFR.

In short, if your Altimeter is also your Encoder you need to get the altimeter checked even for VFR to ensure that it is providing correct altitude to the transponder. So if you are using an EFIS as the encoder, your need to get the Altitude checked.

See the above referenced FAQs for more detail.
 
Thanks guys. I do have a dedicated (certified) encoder for my (certified) Transponder.
My questions weren't meant to be about the regs - I understand the regs and, as i said, I have my airplane scheduled for the pitot-static test by an avionics shop in Aug.
My questions were more for educational value and wanting to understand and diagnose as much as I can myself before it goes in to the shop. It sounds like there's very little that I can do by way of builder-level testing of this stuff, except perhaps a leak test of of the pitot static system (which I plan to do).
 
……….BUT - altimeter settings and physical altimeters are based on standard atmosphere... So on a day that is very warm or very cold (non-standard atmosphere), an altimeter set to current local altimeter setting might not read the true elevation (maybe off by tens of feet?). ….
?

That’s not correct. What the guys in the tower actual do: They know their true altitude. They have a well-calibrated altimeter. They turn the Kollsman knob until the altimeter reads the true altitude. They read the Kollsman window - that is your altimeter setting.

You can often see this effect if you look at the Metar for a high altitude airport, say, KTVL. The metar data will have the altimeter setting in inches of Hg, and also the equivalent sea level pressure in millibars. Look up the conversion factor between in of Hg and millibars, and you’ll often see that the two numbers are different. The altimeter setting has been ‘fudged’ to make a standard altimeter read the airport elevation, despite the non-standard day.
 
Static port error

You can definitely check your altimeter range using a water manometer.

But remember, even with a properly calibrated altimeter by an avionics shop, you can still get incorrect altitude in the air. I understand that best way to check for that is a low altitude high speed speed pass over a runway, but have not tried that myself :)

Finn
 
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But remember, even with a properly calibrated altimeter by an avionics shop, you can still get incorrect altitude in the air. I understand that best way to check for that is a low altitude high speed speed pass over a runway, but have not tried that myself :)

Finn

This would be called a ‘static port placement error’, due to the static port being in a place where it senses accelerated air (top of the wing would be bad). It also covers changes in airflow induced by the port itself. Judging by posts on VAF, ‘fancy’ ports (machined pieces of aluminum looking like a quarter) seem to give erroneous results more often than vans’ hollow rivet!
 
That’s not correct. What the guys in the tower actual do: They know their true altitude. They have a well-calibrated altimeter. They turn the Kollsman knob until the altimeter reads the true altitude. They read the Kollsman window - that is your altimeter setting...

Interesting! There are so many nuances to this subject, and i dare say it is not well-understood by many pilots. Another wrinkle is the published airport elevation. My home airport is published at 418 feet. I asked the airport manager if he knows where exactly on the field this elevation is defined, and he had no idea. The airport appears pretty flat, but it would be intersting to the know the variation in elevation around the field. This would be a good number to put in hte Canada Flight Supplement for airports - max elevation, min elevation and exact location of the published elevation (a good choice would be wherever the compass rose is painted on the ramp?).
 
Here in the US the ‘airport diagram’ found in the ifr charts packet for an airport with an approach shows msl altitudes of the approach ends of runways, as well as the airport reference point.
 
This would be called a ‘static port placement error’, due to the static port being in a place where it senses accelerated air (top of the wing would be bad). It also covers changes in airflow induced by the port itself. Judging by posts on VAF, ‘fancy’ ports (machined pieces of aluminum looking like a quarter) seem to give erroneous results more often than vans’ hollow rivet!

And that error can be large. Like 5+ knots.
More importantly though it means your altimeter is wrong while moving as well.
Highly recommend everyone does the box method and use the NTPS spreadsheet.
I just did this for my G3X equipped RV10 for phase 1. It’s spot on. Within 1kt all the way to 180 true. Garmin pitot and cleaveland machined ports that mimic the pop rivet.
 
My D180 and G5 often differ by 20’. I regard that as remarkably accurate and well within the IFR tolerance!

Jack
 
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