Fooling a magnetometer

Jim K

Jim K

Final Approach
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Have a GMU-11 magnetometer in my tail. Getting vectors to final this afternoon, the autopilot kept kicking out of heading mode into roll mode as the magnetic heading was unreliable, indicated on the G5's by turning yellow.

I haven't had this problem before, and after landing, I realized it might have been due to the ~60lbs of steel parts I had picked up, which was the purpose of the flight. They were sitting in the rear passenger footwell, probably 8-10 feet from the magnetometer, and it never occurred to me beforehand that they would interfere.

I just thought this was interesting and wondered if anyone else had seen similar interference.

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That should be at least fastened in place with a seat belt.


Friend was flying some Fairbanks Morse scale parts from the factory in his Cessna 182, all seats removed except pilots. W/B details not available, he did not get up, crashed at the end of the long runway. Fortunately, all well secured. If not secured, some of it would have come forward, and joined him in the pilots seat.

He rented a Cherokee 6/300, and flew the parts to the sales barn, and business resumed. He did not make a profit on that cargo flight.
 
Haven’t seem this before, but it’s not unrealisitc.
 
Jim K said:
wondered if anyone else had seen similar interference.
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Could be because those steel parts are also magnetized. We fought a flux valve direction problem and were about to start changing big parts when we called an old-timer. He said to get a gauss meter and check the area. Found the steel tail skid mounted 6 inches behind the valve pegged the meter. And once it was degaussed, all was well.
 
Bell206 said:
Could be because those steel parts are also magnetized. We fought a flux valve direction problem and were about to start changing big parts when we called an old-timer. He said to get a gauss meter and check the area. Found the steel tail skid mounted 6 inches behind the valve pegged the meter. And once it was degaussed, all was well.
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Gauss meter. There's a tool I didn't know existed.
 
Do you have a portable compass (scouting type, for example) that you can use to test whether that steel part happened to be magnetized? Hold the portable compass near the steel part, and see if the needle deflects.

If that part wasn't magnetized enough to deflect a needle visibly at a distance of half a foot, I'd be surprised if it would meaningfully affect a compass or magnetometer located 8 feet away.
 
NoHeat said:
Do you have a portable compass (scouting type, for example) that you can use to test whether that steel part happened to be magnetized? Hold the portable compass near the steel part, and see if the needle deflects.

If that part wasn't magnetized enough to deflect a needle visibly at a distance of half a foot, I'd be surprised if it would meaningfully affect a compass or magnetometer located 8 feet away.
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So I took a small compass outside, and sure enough, when I bring it close to the part, particularly the ends of the tubes, the compass points to them.
 
Jim K said:
So I took a small compass outside, and sure enough, when I bring it close to the part, particularly the ends of the tubes, the compass points to them.
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Well done!

I'd bet your test explains what happened with your avionics. Not a very common occurrence, I'm sure.
 
Jim K said:
So I took a small compass outside, and sure enough, when I bring it close to the part, particularly the ends of the tubes, the compass points to them.
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How close? If it doesn't affect it from the same 8-10' away the magnetometer is from the parts, then that's probably not your problem. But how does the G5 discern that the heading is "unreliable"?
 
My Aspen went 30 degrees off when I put a large metal dog crate in it. The mag is in the roof, so only the tall crate makes it happen.
 
I’ve never had a problem but all three GMU11 I have mounted myself are in wingtips.
 
Gary Ward said:
I bet this is why they put some of them out on the wing tip?
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Possibly. With increased use of digital processing in AHRS, FOGs, RLGs, magnetometers, etc. trying to fit those technologies into "legacy" aircraft designs has required different approaches based on the sensitivities of those systems. All my experience with this is on helicopters where there are no "wing tips" to put things. But through software tweaks and other installations practices they are making things work.
 
Dana said:
How close? If it doesn't affect it from the same 8-10' away the magnetometer is from the parts, then that's probably not your problem. But how does the G5 discern that the heading is "unreliable"?
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To answer your question, pretty close, like a foot, but the only compass I had handy was a cheap little one about the size of a penny that I bought to put in the top of a walking stick. I'm pretty certain the gmu11 is a LOT more sensitive.

As to what causes the g5 to decide it doesn't like the heading...I have no idea how that works.

Gary Ward said:
I bet this is why they put some of them out on the wing tip?
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bnt83 said:
I’ve never had a problem but all three GMU11 I have mounted myself are in wingtips.
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I questioned that when it was installed, as the old King magnetometer was in the wingtip. The shop said they put them in the tail because they have interference problems with strobes/led wingtip lights.
 
Jim K said:
I just thought this was interesting and wondered if anyone else had seen similar interference.
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I and my team spent many hours (way more than you'd believe) at a very remote and austere site trying to get couple of magnetometers to calibrate and function if/when they took a calibration. I can't remember what led us to try it but eventually someone handed me a small magnet and I noticed it stuck to the magnetometer mounting screws used in installation. I yelled at the guys who installed it, but they yelled back that they had followed the work order (and in their defense they were *very* good and would not have done any less). We dug up the drawings and saw that manufacturing had spec'd ferrous mounting screws. We swapped them out with some non-metallic fasteners and the team cleaned up the mess from my head exploding, and what do you know, the mag calibrated easily. I went someplace private to call home to explain what needed to be done to the drawings and install kits, and can neither confirm nor deny being hoarse the next day.

Nauga,
and that scene from 'Scanners'
 
When I had it done they said the wing tip was recommended by garmin if possible.
They said it was tough getting a custom made shielded cable(canbus I think?) through the wing. showed me a couple custom tools they made to get wiring through cessna wings. Made a custom bracket they showed me to mount it up there under the wing tip.
I had stock strobes and stock nav lights when it was installed about 800 hrs ago. Then I did put WAT led nav lights in the wing tips this past summer. I have flown it about 90 hrs since with no interference that I have noticed. I flew with the new led nav lights on full time for about 2 weeks so I didn't find a problem at night.
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Gary Ward said:
When I had it done they said the wing tip was recommended by garmin if possible.
They said it was tough getting a custom made shielded cable(canbus I think?) through the wing. showed me a couple custom tools they made to get wiring through cessna wings. Made a custom bracket they showed me to mount it up there under the wing tip.
I had stock strobes and stock nav lights when it was installed about 800 hrs ago. Then I did put WAT led nav lights in the wing tips this past summer. I have flown it about 90 hrs since with no interference that I have noticed. I flew with the new led nav lights on full time for about 2 weeks so I didn't find a problem at night.
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Well, the shop that did mine cut some corners and screwed up some other stuff, so I wouldn't be surprised if they went the cheap/easy route and lied to me.
 
Jim K said:
pretty close, like a foot, but the only compass I had handy was a cheap little one about the size of a penny that I bought to put in the top of a walking stick. I'm pretty certain the gmu11 is a LOT more sensitive.
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Sensitivity doesn't matter. A compass or magnetometer can only indicate the actual (total) magnetic field direction at the device's location, regardless of how sensitive the device is. That magnetic field is the vector sum (not sure about the exact math) of the earth's magnetic field strength and that of any other magnetic objects nearby. A more sensitive device won't make the other object's contribution to the field any more pronounced.

However, depending on the direction of both fields, the summed strength of both fields is greater when they're in the same direction and less when they're opposite (i.e. straight north or south). If the magnetometer is reporting the field strength and it's below some threshold, that might be what triggers the error. If you're heading east or west, it's like a crosswind, won't affect the field strength (as much) but it will affect the field direction. The error may also be triggered by variation in the field strength as it turns.

The right way to check it is to lay the compass right on top of the magnetometer with the airplane pointed east or west, and observe it while you move the metal parts into and out of the plane.
 
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I've installed them in wingtips with no strobes and in wingtips with LED anti-collision lights. All of them were calibrated on an actual compass rose that all the transport category aircraft use.

Obviously, carelessly installing magnetic hardware to attach the wingtip later down the road by other people would mess it up.
 
Jim K said:
…I questioned that when it was installed, as the old King magnetometer was in the wingtip. The shop said they put them in the tail because they have interference problems with strobes/led wingtip lights.
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Our GMU-11 was originally installed in the right wingtip and ended having to be relocated for the same reason.
 
were u flying here?

IMG_3053.jpg


also, the FAA says to stay away from manhole covers. was u flying over manhole covers???

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what was your whiskey compass doing, was it also whacky?
 
I have a GMU-22 out in the wing of the skylane and sometimes rebar will be enough to affect it. There is a concrete pad at a taxiway entrance at my home field and I'll get a "MAG ANOM" flag every time I go over it.
 
eman1200 said:
were u flying here?

View attachment 112846


also, the FAA says to stay away from manhole covers. was u flying over manhole covers???

View attachment 112847


what was your whiskey compass doing, was it also whacky?
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Actually I did notice that it seemed to be 10-20 degrees off, but I had about a 40 knot crosswind so i rationalized it being due to that. It's a vertical card compass, and it's always seemed kind of slow and not very precise.
 
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