AGL and MSL do not track

[Harold Roberts]

I have attached a screenshot of my AF-5000 as I was flying over my airport at 1,030' MSL. There should be about a 1,000' difference between the AGL and MSL. Note that the barometric pressure was properly set.

Instead I see 2,660' MSL and 550' AGL, which is about 1,000' off. I checked on Foreflight on my non-GPS iPad for the GPS altitude for comparison. It said 1,482' GPS MSL.

Questions:
1. Where does the AGL altitude come from that is displayed on the AF-5000? I have an ADS-B In GPS receiver from Garmin, the GDL-39. I have assumed that the AGL comes from GPS altitude minus the MSL of the ground below. But perhaps the AGL is simply a calculation by subtracting the MSL height of the ground below (based on map topo data) from the current barometric measured MSL? If the latter, then it is not possible to have a 1,000' error in the difference between MSL and AGL.
2. I have understood that by pressing and holding the BARO button while on the ground at the airport, the altitude of the airport is used for the MSL altitude and the AGL is set to zero automatically without need to set the BARO pressure. Is this true? The last time I tried this the reading on the ground was 100' AGL (incorrect of course) but pressing the BARO button did not do anything to the error or the barometric pressure, which was indeed incorrect on the display.
3. Non related to altitude, I was trying some functions and pressing buttons and knobs when I saw the compass rose change from a dim white to a bright white. I have always had a hard time reading the compass rose. See the photo attached of the PFD and you cannot read the degrees on the compass rose. Specifically in the photo you can barely see the W for West above the horizon or the 30 (for a heading of 300degrees). How can I permanently change it to the brighter white?

Best regards,
Harold Roberts

[Harold Roberts]

Hi Jonathon, could you take a look at this issue?

[Jonathon]

Harold the default AGL calculation is your GPS altitude minus the synthetic vision terrain altitude. It can switch to barometric altitude - synthetic vision terrain altitude if either GPS altitude is unavailable, or if it is set to only use baro altitude for the calculation (which should only be done when your only GPS installed is something like a CNX80/GNS480, which you definitely don't have). You should confirm that thte Altitude Source in the SET --> CAL --> Synthetic Vision menu is set to either AUTO or to GPS ONLY.

Your picture is too small to verify, but you can see which source is being used for the calculation in the AGL display, which after the AGL will either show a (G1) or a (B1) to denote whether the calculation used GPS or baro altitude, respectively.

If your baro altitude was really a thousand feet off at this time, then either
1) you were previously cruising at 2,660ft, your static system became clogged, and then continued to read 2,660 as you descended to ~1,500ft
2) your ADAHRS is not performing as expected and may need recalibration
3) The baro setting you entered was not correct for the atmospheric conditions encountered.

I have understood that by pressing and holding the BARO button while on the ground at the airport, the altitude of the airport is used for the MSL altitude and the AGL is set to zero automatically without need to set the BARO pressure. Is this true?

Pressing and holding the baro button will adjust your baro setting to whatever value will cause your barometric altimeter to display the same as your current GPS altitude. It will not adjust the AGL display, which is not pilot-adjustible except inasmuch as you can choose different sources to perform the calculation, as previously described.

I was trying some functions and pressing buttons and knobs when I saw the compass rose change from a dim white to a bright white. I have always had a hard time reading the compass rose. See the photo attached of the PFD and you cannot read the degrees on the compass rose. Specifically in the photo you can barely see the W for West above the horizon or the 30 (for a heading of 300degrees). How can I permanently change it to the brighter white?

The compass rose is designed to declutter by fading to the gray color after a few seconds, and this is not adjustible. The compass rose is primarily of value when you are actively making adjustments to your HDG, TRK, or CRS bugs. At all other times, the numeric magnetic heading display at the top of the rose should be the primary heading reference.

[Harold Roberts]

Jonathon,

Thanks. Looking at my photo you can see that the AGL has a (G1), meaning it is taking GPS altitude.

I had previously set the baro using a local ATIS so of the three theories for the discrepancy it leaves #1 and #2.

If your baro altitude was really a thousand feet off at this time, then either
1) you were previously cruising at 2,660ft, your static system became clogged, and then continued to read 2,660 as you descended to ~1,500ft
2) your ADAHRS is not performing as expected and may need recalibration

To diagnose whether #1 is correct, I plan to go up to a higher altitude, taking photos of the altitude tape. There should be ~1,000' difference between AGL and MSL. Then descending taking photos.

If the MSL gets "stuck" on the way down, I know I have a problem with my static system, i.e. #1.

If not and it is #2, what do I need to do to recalibrate the ADAHRS?

Thanks,

Hal

[cdeggz]

Jonathon, Harold,
I'm pretty sure there is a setting to make the compass rose always white. That is how I have mine set up. I tried both setting and preferred always being able to see the compass, and the clutter did not bother me.

[Jonathon]

You are right, Craig, forgot that was added a couple years ago. SET --> EFIS --> HDGC:W/HDGC:G to toggle between the always white or fade to gray after a few seconds.

[Harold Roberts]

Thanks Craig.

Jonathon, with respect to the altitude error, I observed the following yesterday on a flight:

1. The AGL is suspiciously low and remained pretty static after take off, reading something like 100' or 150' when I felt like I was above 500'.
2. I have an iPad with internal GPS but it is also connected by Bluetooth to the Garmin GDL-39. It was reading about 1,300' GPS MSL altitude. I assume that it used the GDL-39 GPS altitude rather than the internal GPS.
3. To check what the 'real' GPS altitude was, I used my iPhone and turned off the iPhone Bluetooth so that it would be forced to use the GPS built into the iPhone, not the GDL-39 Bluetooth input. The iPhone was reading 1,700' vs the 1,300' GPS altitude from the iPad, a 400' difference!
4. I landed and took off again. I noticed then that the GPS altitude between the iPad and the iPhone were tracking pretty closely and that the AGL from the AF-5000 was looking pretty accurate.

So whatever the problem is, it seems to sort itself out after some time passes on a flight. I have noticed this frequently, i.e. the AGL looks wrong and then seems to correct itself later.

Doesn't the AF-5000 get the GPS altitude from the Garmin GDL-39 rather than the ADAHRS? (recall that I confirmed that the AGL is followed by a "G1", indicating the GPS source). Given that the iPad was showing the wrong GPS altitude and that the iPad is connected by Bluetooth to the GDL-39, all fingers seem to be pointing to the GDL-39.

Your thoughts?

Harold