ADAHRS calibration - and mag heading

[woneill]

Recalibrated my unit today because once again flying East my Skyview is 7 to 10 degrees different than my 2 Garmins + iPad. All 3 of them are within 1 degree at all times.

During today's recalibration I noticed for the first time there is a "mag heading" number under the "calibration" number. AHH! Further read on the bottom of the screen this "mag" reading isn't used in calculation. OK. But I also noticed that when my "calibration" was set 090 degrees the "mag" heading was 082 degrees??? I hit accept and saved all 4 directions anyway. Will fly tomorrow.

Don't know if I can get the 2 numbers closer or if this is just the way it is. All previous recalibrations were done just hitting the 4 cardinal directions and then SAVE. Never realized there was a "mag" number below. I've recalibrated 4 times in the 10 months I've owned the plane and flying East bound I've always been off 7 ~ 10 degrees Garmins to Skyview.

Any thoughts?

 

[mmarien]

The compass and GPS should be the same or very similar on the ground. The GPS also can be set to show true north as opposed to magnetic north which would always show the declination difference when compared to a compass. Are your cardinal directions magnetic or true? I believe they should be magnetic when setting the compass.

Also consider that the HSI is magnetic HDG while the GPS is ground TRK so there is always going to be a difference in the air when there is a cross wind. See the difference a crosswind will make in these two screen shots. The first on the ground and the second in the air with a cross wind.

 

[woneill]

Tech Support come up with any ideas?

 

[dynonsupport]

You might give us a call for a fuller conversation about this, but your GPSes and your magnetic heading will exactly match only in a perfect no wind situation. Most of the time, then, you should be expected to be pointing in a slightly different way than you're tracking (mag heading tells you which way the aircraft is pointing; all of your GPSes are telling you what direction you're traveling over the ground).

If it's always off in the same direction and in the same amounts, next consider if your calibration is being done with a good reference to magnetic north. You shouldn't be using the "uncalibrated" heading number that the calibration routine provides to line up - that's just there to give you an idea of what the compass's factory calibration is.