Question Altitude difference between G3X and G5?

[Neal]

My last FX-3 had a 50-60ft difference if I recall and I had it recalibrated under warranty and that got it within 10-20ft difference. My new FX-3 it appears was calibrated, the altitudes matched but now a year and a half later they seem to be drifting apart with the G5 now 30-40 ft different. As I have no intentions of flying IMC/IFR I don't particularly care but have noticed the delta. I'm curious what others are seeing between the G5 and G3X altimeters?

 

[ve6yeq]

I have had the same experience with the calibration initially being the same 10-20 ft, and after a winter, the drift is now more like your 30-40 ft. Maybe when the weather warms up, the difference will shrink?

Given that the calibration tolerance for a barometric altimeter is like +-75 ft according to the FAA AIM 7.2.3 a), I guess we have to be happy with 30-40 ft until everything switches to GPS altitude. As an engineer working on my PPL, these altimeter errors worried me, but my instructor pointed out that the absolute value for barometric altitude really does not matter as long as all the other aircraft agree on the same error and tolerances. This is exactly the same as all the tolerances in magnetic headings and the movement to true north. I do all my flight planning and navigation with GPS altitude and true north bearings (i.e. ForeFlight), and then convert to barometric altitude and magnetic headings only when communicating with ATC / other pilots.

 

[Cactus Charlie]

I saw a similar drift in altitude between GSU 25C (G3X Touch system) and my G5. It annoyed me so I developed an experimental airdata test set and ran the G5 calibration procedure. Altitude difference is now 10 ft or less.

It's unfortunate that Garmin does not allow a baro setting offset or altitude correction to be entered by the pilot. G5 and G3X Touch baro settings are slaved in the normal G5 installation and only independent when the G5 is configured as a "stand alone" instrument.

The calibration procedure is easy but best done with a calibrated airdata test set.

I found the G5 error increased with time and was independent of ambient temperature.