Compass Calibration: Looking for Alternate Method

[Lon]

I replaced my ADAHRS and therefore had to do a fresh compass calibration. The method described in the SkyView Installation Guide requires pointing the plane North, South, East and West, and pressing a button after the plane is oriented in each direction. The Guide recommends using an airport compass rose to do that, but after my home airport, KSMO, shortened the runway, its compass rose has been blocked by taxiway end lights. So instead, I used the compass in my iPhone. With the canopy closed and engine on, as required by the Guide, the iPhone compass is not steady and not accurate. The FAA Airport Diagram for KSMO shows that the exact heading for Runway 21 and a parallel taxiway, but the heading is 212.7 degrees, not pure N, S, E or W. Is there a way I can point the plane directly down the runway or taxiway, and then "tell" the Dynon the plane is facing 212.7 degrees and it should calculate N, S, E and W itself?

 

[JohnAJohnson]

I don't know the answer to your question, but have you considered taxiing N, S, E, & W, using a GPS? On the ground, track = heading, and even a handheld GPS (or tablet/phone) would be a whole lot more accurate than a tablet/phone compass. Google Earth shows a pretty good sized ramp on the SE corner of the airport, and you might be able to establish reliable, accurate tracks on the cardinals while taxiing in that area.

 

[airguy]

The method mentioned by John above is probably the best way if you have limited space and don't have a compass rose - just taxi on a ramp area or fuel pump area and do the best you can. Then go fly - and do the airborne calibration which requires at least two 360-degree circles. In my experience two circles, one in each direction, is enough to get you pretty close but if you'll double that you'll get really good accuracy.

 

[swatson999]

There's a compass rose over the hill at KCMA.

 

[andresmith76]

I replaced my ADAHRS and therefore had to do a fresh compass calibration. The method described in the SkyView Installation Guide requires pointing the plane North, South, East and West, and pressing a button after the plane is oriented in each direction. The Guide recommends using an airport compass rose to do that, but after my home airport, KSMO, shortened the runway, its compass rose has been blocked by taxiway end lights. So instead, I used the compass in my iPhone. With the canopy closed and engine on, as required by the Guide, the iPhone compass is not steady and not accurate. The FAA Airport Diagram for KSMO shows that the exact heading for Runway 21 and a parallel taxiway, but the heading is 212.7 degrees, not pure N, S, E or W. Is there a way I can point the plane directly down the runway or taxiway, and then "tell" the Dynon the plane is facing 212.7 degrees and it should calculate N, S, E and W itself?

Lon - To answer your question - no, you cannot offset the Dynon as you described. I recently calibrated my ADAHRS unit and used the method described by JohnAJohnson. With the engine running, I slowly moved & turned my plane until the Dynon EFIS heading indicated 360°, then stopped and pressed the button to calibrate. I did this for all 4 headings. I spoke to Dynon tech support (Jeff?) and he approved using this method. Airguy has the correct idea though, the ground calibration gets the unit close but the in-air method is what gets the electronic compass fine-tuned.

Andre'

 

[Lon]

Lon - To answer your question - no, you cannot offset the Dynon as you described. I recently calibrated my ADAHRS unit and used the method described by JohnAJohnson. With the engine running, I slowly moved & turned my plane until the Dynon EFIS heading indicated 360°, then stopped and pressed the button to calibrate. I did this for all 4 headings. I spoke to Dynon tech support (Jeff?) and he approved using this method. Airguy has the correct idea though, the ground calibration gets the unit close but the in-air method is what gets the electronic compass fine-tuned.

Andre'

Andre,

It never occurred to me that the PFD's Magnetic Heading Indicator gets its heading from a different source than the Moving Map's Current Ground Track Readout. But your answer to my question made me think about it.

Is this an accurate description of the way the HDX works to display headings?

1. The PFD's Magnetic Heading Indicator gets its headings from the ADAHRS, and that's why a compass calibration has to be done when a new ADAHRS is installed.

2. The Moving Map's Current Ground Track Readout gets its heading from the GPS (not the ADAHRS), and that's why it can be used to do a compass calibration.

Lon

 

[Lon]

I don't know the answer to your question, but have you considered taxiing N, S, E, & W, using a GPS? On the ground, track = heading, and even a handheld GPS (or tablet/phone) would be a whole lot more accurate than a tablet/phone compass. Google Earth shows a pretty good sized ramp on the SE corner of the airport, and you might be able to establish reliable, accurate tracks on the cardinals while taxiing in that area.

John,

The SE corner of the airport used to be a tie down area. When the city began making the airport less desirable for pilots, plane owners left the field. Planes in the SE corner tie down were moved elsewhere, and the SE corner was fenced off and is no longer accessible by planes. The SW corner tie down area is still accessible and has some planes tied down there. But there is enough room there to do a compass calibration. I just needed to figure out how to do it, without using an iPhone. Andre's method sounds like it'll work perfectly.

Lon

 

[skysailor]

Andre,

It never occurred to me that the PFD's Magnetic Heading Indicator gets its heading from a different source than the Moving Map's Current Ground Track Readout. But your answer to my question made me think about it.

Is this an accurate description of the way the HDX works to display headings?

1. The PFD's Magnetic Heading Indicator gets its headings from the ADAHRS, and that's why a compass calibration has to be done when a new ADAHRS is installed.

2. The Moving Map's Current Ground Track Readout gets its heading from the GPS (not the ADAHRS), and that's why it can be used to do a compass calibration.

Lon

You are correct on point 1. As for point 2, the moving map has both heading and ground track. Heading comes from the ADAHRS just like the PFD. Ground track comes from the GPS 250/2020 or your actual GPS source. The difference between the two is shown as wind while airborne. The wind indication will only be accurate if you have a valid and accurate compass cal.

 

[Bill Putney]

I replaced my ADAHRS and therefore had to do a fresh compass calibration. The method described in the SkyView Installation Guide requires pointing the plane North, South, East and West, and pressing a button after the plane is oriented in each direction. The Guide recommends using an airport compass rose to do that, but after my home airport, KSMO, shortened the runway, its compass rose has been blocked by taxiway end lights. So instead, I used the compass in my iPhone. With the canopy closed and engine on, as required by the Guide, the iPhone compass is not steady and not accurate. The FAA Airport Diagram for KSMO shows that the exact heading for Runway 21 and a parallel taxiway, but the heading is 212.7 degrees, not pure N, S, E or W. Is there a way I can point the plane directly down the runway or taxiway, and then "tell" the Dynon the plane is facing 212.7 degrees and it should calculate N, S, E and W itself?

A compass rose is a very special thing to set up. I tried to get one done at my airport and I gave up. There are companies that will come out and survey prospective areas and give you a certification if they find one that you can build a compass calibration circle on. Short of that, it really won't be an FAA approved calibration circle.

You can paint a compass rose on any blank piece of pavement but if you really want to have it work for compass calibration it has to be free of things like electrical conduits, metal pipes and rebar running under it. All those things will throw your magnetic compass off. Knowing the runway heading doesn't insure that your compass is going to line up if the runway has any ferrous metal or electrical wiring under the pavement.

 

[Lon]

You are correct on point 1. As for point 2, the moving map has both heading and ground track. Heading comes from the ADAHRS just like the PFD. Ground track comes from the GPS 250/2020 or your actual GPS source. The difference between the two is shown as wind while airborne. The wind indication will only be accurate if you have a valid and accurate compass cal.

About point 2:

Is the ADAHRS-derived heading displayed on the moving map?

What provides the Current Ground Track Readout (at the top of the Heading Arc): the GPS or the ADAHRS?

It seems to me that the Current Ground Track Readout can be used to calibrate the compass only if the GPS provides the Ground Track Readout.

?

 

[Lon]

A compass rose is a very special thing to set up. I tried to get one done at my airport and I gave up. There are companies that will come out and survey prospective areas and give you a certification if they find one that you can build a compass calibration circle on. Short of that, it really won't be an FAA approved calibration circle.

You can paint a compass rose on any blank piece of pavement but if you really want to have it work for compass calibration it has to be free of things like electrical conduits, metal pipes and rebar running under it. All those things will throw your magnetic compass off. Knowing the runway heading doesn't insure that your compass is going to line up if the runway has any ferrous metal or electrical wiring under the pavement.

The KSMO compass rose has been there for a long time -- long before I began flying out of there -- so I imagine that once upon a time, it passed muster and was an FAA approved circle. When Santa Monica shortened the runway, the city wanted to rip out the compass rose and replace it with grass, apparently to give birds a comfortable spot from which to watch, and fly along with, planes taking off. The local 99s Chapter persuaded the city to leave the compass rose in place, because it has historical significance. But taxiway end lights prevent planes from actually getting to it.

 

[andresmith76]

Andre,

It never occurred to me that the PFD's Magnetic Heading Indicator gets its heading from a different source than the Moving Map's Current Ground Track Readout. But your answer to my question made me think about it.

Is this an accurate description of the way the HDX works to display headings?

1. The PFD's Magnetic Heading Indicator gets its headings from the ADAHRS, and that's why a compass calibration has to be done when a new ADAHRS is installed.

2. The Moving Map's Current Ground Track Readout gets its heading from the GPS (not the ADAHRS), and that's why it can be used to do a compass calibration.

Lon

Lon -

Based on this discussion, I called Dynon today and again spoke with Jeff in tech support. He confirmed what you stated above for #1, "The Heading Indicator is driven by the Magnetometer". He then said "The GPS provides position only and not the ground track". So my calibration procedure that I posted earlier was not correct. Jeff did say that performing an in-flight calibration is the most accurate method to calibrate the electronic compass, whether it's the remote magnetometer or the one included in the ADAHRS. He also mentioned either electronic compass could be off as much as 10 degrees and the in-flight calibration would be able to correct. Sorry for the confusion!

Andre'

 

[Lon]

Lon -

Based on this discussion, I called Dynon today and again spoke with Jeff in tech support. He confirmed what you stated above for #1, "The Heading Indicator is driven by the Magnetometer". He then said "The GPS provides position only and not the ground track". So my calibration procedure that I posted earlier was not correct. Jeff did say that performing an in-flight calibration is the most accurate method to calibrate the electronic compass, whether it's the remote magnetometer or the one included in the ADAHRS. He also mentioned either electronic compass could be off as much as 10 degrees and the in-flight calibration would be able to correct. Sorry for the confusion!

Andre'

Thanks Andre, though I confess this is a disappointment. Where do you suppose the moving map gets the Ground Track it displays? Do you think it calculates the plane’s Ground Track using the last few GPS-derived positions? And if that’s what it does, couldn’t I use the moving map to do a ground calibration simply by taxiing for a bit while the moving map Ground Track shows 360, 090, 180 and 270? I’ll do the in-flight calibration too. But right now, I think my PFD compass may be off by more than 10 degrees.

 

[andresmith76]

Thanks Andre, though I confess this is a disappointment. Where do you suppose the moving map gets the Ground Track it displays? Do you think it calculates the plane’s Ground Track using the last few GPS-derived positions?

According to Jeff, "GPS provides position information only". He also said specifically "GPS does not provide ground-track", so I'm, assuming it derives this info from the magnetometer.

And if that’s what it does, couldn’t I use the moving map to do a ground calibration simply by taxiing for a bit while the moving map Ground Track shows 360, 090, 180 and 270? I’ll do the in-flight calibration too. But right now, I think my PFD compass may be off by more than 10 degrees.

That's how I thought the system operated when I first performed my ground compass calibration but Jeff yesterday said this is incorrect. He did say using a hand-held compass for setting heading reference was good enough to get you close and then perform the in-flight calibration to fine-tune.
He also said you can see how far off the Skyview electronic compass is by lining up parallel to a runway and comparing how your heading reading is to the runway number.

Maybe someone from Dynon can help clarify with a post to this thread?

 

[Lon]

Andre, It looks as though we were partially correct, though not in a way that helps with an on-the-ground calibration. The HDX Pilot's User Guide explains that the moving map ground track is based on GPS information (just as we first thought) but only if the plane is moving "fast enough." If the plane isn't moving "fast enough," the ground track is based on magnetic heading information which I believe means the ADAHRS determined heading. The ground track source is indicated by the color of the border of the digital ground track readout -- magenta for GPS, and white for magnetic heading. I've looked at photos I've taken of my plane's display while flying and while on the ground, and sure enough, while flying, the border of the readout is magenta, and when on the ground, the border is white. The User Guide doesn't say how fast is "fast enough" so I don't know whether I can taxi fast enough to get a GPS magenta border, but I'm guessing not.

 

[Eugr]

While map view on the ground will display compass heading, the compass calibration menu will have GPS ground track in it though, so you can use this one. Also, you can use ground track from any other GPS in your plane (e.g. Garmin 430W).

 

[JohnAJohnson]

The KSMO compass rose has been there for a long time -- long before I began flying out of there -- so I imagine that once upon a time, it passed muster and was an FAA approved circle. When Santa Monica shortened the runway, the city wanted to rip out the compass rose and replace it with grass, apparently to give birds a comfortable spot from which to watch, and fly along with, planes taking off. The local 99s Chapter persuaded the city to leave the compass rose in place, because it has historical significance. But taxiway end lights prevent planes from actually getting to it.

Compass roses can only be presumed good when they are laid out. Back when transport category aircraft had a flux valve (magnetic azimuth detectors, or flux gates, etc.) in each wingtip, to install one, the aircraft was brought down to a compass rose, which was certified annually to be free of magnetic interference, and the N-S line was truely magnetic N-S, with any errors documented. The aircraft was placed on said line using plumb bobs, then, the four hour procedure to install and align the flux valve was done using precision theodolite equipment. It was very common to see movement of the N-S line over a years time, and it could be something as trivial as "the car salvage yard two miles east of the field closed and got rid of their cars".

So today, I believe the best way is to get a reliable N-S-E-W ground track and do the ground calibration to them. When I say "reliable", it is not enough to simply turn to a heading and stop. You should taxi at least 100' on the ground track to accept it as reliable, since GPS has some error, and the ground track is based on where you were, vs were you are now. Example: A 30' GPS Position Error to the left of your track line at the start, and a 30' GPS Position Error to the right of your track line at the end nets you a 45 degree error if you only taxi 30'. And you need to do it in a "clean" area away from any suspected interference. As an example, we had a parking spot where the #2 compass (running off the flux valve in the right wing) differed from the #1 compass (left wing flux valve) by six degrees. The error was caused by a 6" water main running under the the right wingtip, about six feet below one foot of concrete ramp.

 

[Eugr]

You should taxi at least 100' on the ground track to accept it as reliable, since GPS has some error, and the ground track is based on where you were, vs were you are now.

And you need to taxi as straight as possible, as some GPS’s (like Dynon’s built-in ones) are sensitive to even small deviations. And faster you taxi is better.

 

[Dynon]

To clear this up: Anytime you see the words Ground Grack - it's ALWAYS provided by GPS (in SkyView and all GA avionics at least. Airliners and equipment with "true inertial" positioning can do it via those platforms). So the map's GPS ground track is indeed provided by the GPS. The heading on the HSI is magnetometer/ADAHRS.

A technical aside: GPS only ever knows instantaneous position natively, so any GPS-based calculation, whether it's direction (ground track), speed, etc, are all derivations that use recent history (ie, a series of very recent position indications) to derive those values.

 

[Ricardo Godoy]

I still haven't located the source of electromagnetic interference that changes the reading of the compass by 37 degrees. For example, at HSI, 335 degrees, gps 012 degrees. DEVICES gps ok. no wind to alter the bow. this is intermittent.