SVE AGL

[330drvr]

Am trying to use the new SVL AGL feature of revision 6c. With GPS as the selection, on the ground it reads zero as I would expect. Inflight the readout is pressure altitude! Makes no sense. As a test, I went out over the Gulf of Mexico. At 2500 MSL on the altimeter, the Garmin 480 GPS showed 2550 feet as the GPS altitude. But the readout on the PFD SVE was 2200 feet. That was the pressure altitude. Selecting GPS, Baro made no change. Selecting Baro on the settings did show 2500 feet. That will do, but I would prefer to get the GPS height over terrain correct. That would be safer to protect if I had an incorrect altimeter setting inserted. Any ideas? Do I have some other GRT selection set incorrectly? Thanks

Dual HX
Garmin 480 GPS navigator

[GRT_Jeff]

I don't think the 480 sends the GPS altitude in its serial data message, so the EFIS should display the height relative to baro-corrected pressure altitude, indicated with a "B", or not show the SVE height at all.

[330drvr]

Thanks Jeff: That explains it. I will make "Baro" the SVL setting and move on! It is a great safety feature. Appreciate it.

[330drvr]

Jeff: Once more comment for your information data base. This explains why whenever I change the altimeter setting on the PFD, I get a GPS dif xxxfeet message below the altimeter setting. That message is zero when the altimeter is set to 29.92 (QFE). In essense, the current altitude is being compared to pressure altitude. I assume the HX gets a wee bit confused by the lack of a GPS altitude message from the Garmin 480. One further observation, when the 480 is in simulator mode and simulates being at 5000 feet, the SVL was perfect with GPS selected in the HX menu.

[GRT_Jeff]

That is strange. You should only get the GPS Alt Diff box if the EFIS thinks it's receiving a GPS altitude. The GPS estimate of altitude is usually a good comparison for the baro-corrected altitude. Maybe the 480 is sending pressure altitude, but I haven't heard another report like this.