GPS and permanent compass problem

[ross holz]

I recently purchased a recreational flying gps which I would recommend to anyone but it seems to put out quite a strong magnetic field and totally stuffed my compass. I set it up about 6" from compass and now compass is permanently stuffed, upto 30 deg out on some headings (ie gps not even switched on).

 

Tried the little compensating screws to adjust the error out but not enough adjustment. Other gps units have upset the compass but not permanently.

 

Anyone know if the compass can be re magnetised or what ever. Has anyone else had this problem? Battery only last about 10min. Iam wondering if this is a common problem also. Must draw a lot of power hence compass problem.

 

Other than that rec flying gps is a good cheap moving map gps. I use the geoscience 1:1mil .ecw map which is very similar to wac charts. Dont know the cost but maybe cheaper than combined wac chart collection. (Got a copy off my dad who used to go 4wd with oziexplorer) Also scanned in latest vtc etc. Stops the guess work when getting close to boundaries. Compiled a list of aust airstrips from various sources on the internet. If you tap and hold on a waypoint it will display field elevation, strip directions, etc if you store them in the description field which is handy.

 

 

[J170 Owner]

Well, that is weird. I simply do not understand that. My Garmin car GPS does a fine job when flying (in off-road mode) and has no effect on my compass, about 350mm away. (GPS is ONLY for backup). I suggest you remove the compass and place it in the middle of the field and with another compass and the runway, check its reading.

 

 

[ross holz]

I didnt believe it could happen either but it did. I think the way they initially magnetise them is in a strong electro magnetic field so I might have to send away for repair. I have taken it out and tried re magnetise it with a normal magnet but I dont think its strong enough. When its out of the plane it reads ok compared to other compasses so maybe its the liitle compensating magnets in the bottom that are stuffed. Iam not sure whats wrong with it. The error is not constant in the plane so it might be the compensating magnets.

 

I thought this may be a warning to others too if it is common to the recreational flying gps units. The compass in the car I noticed is now wonky too but other gps units didnt seem to effect it in the past.

 

 

[Guest Andys@coffs]

So the trick here is that any GPS for an aircraft must not have an onboard speaker. As you probably know speakers use a coil attached to the speaker diaphragm where the coil is surrounding a permanent magnet. Voltage into the coil creates a magnetic field where the field either attracts pulling the coil one way or repels pushing it the otherway depending on polarity of the applied voltage. The fixed magnet is the source of problems (assuming there is an inbuilt speaker) for the compass. If the GPS is based on a commercial tablet then it may even have 2 speakers for stereo.

 

The garmin Aera aviation GPS series has exactly the same problem because garmin in their search for even larger profits reused the hardware from a car system which of course has voice guidance rather than build hardware specific to the task. I expected better from Garmin, but like you will now always ask if a GPS has an onboard speaker, if it does move on to something else. In an aeroplane an onboard speaker is as useless as cow things on a bull with all the ambient noise we have....

 

When you say the compass is stuffed, do you mean when the GPS is removed from the aircraft, or when its still in? if the latter then no amount of adjustment is going to fix that.... If the former how was the GPS mounted, did you use any hardware (screws or brackets etc ) that have any iron content? If so they need to be removed and replaced with brass or plastic alternates. Bottom line is that there should be no ferrous material close to the compass if it can be avoided.

 

Andy

 

 

[facthunter]

Compasses respond to any ferrous object that may become magnetised. Even spring back binders etc should never be near a magnetic compass. When you install a compass in a different aircraft it has to be "swung" and they may even respond to the battery charging rate.. Andy's last para says it all. Nev

 

 

[ross holz]

Ah yes the speaker in the gps sounds like the culprit. Makes sense now why my old garmin gps plus 2 never gave trouble.

 

Yes the compass wont read correctly by upto 30 degrees, even with the gps removed from the plane and cant adjust enough with the compensating screws. Gps was mounted using suction cup on windscreen.

 

Compass reads ok out of the plane so maybe something in the dash has been magnitised. Might try mounting compass up high on windscreen like some later model Jabs.

 

Thanks for the help

 

 

[J170 Owner]

I didnt believe it could happen either but it did. I think the way they initially magnetise them is in a strong electro magnetic field so I might have to send away for repair. I have taken it out and tried re magnetise it with a normal magnet but I dont think its strong enough. When its out of the plane it reads ok compared to other compasses so maybe its the liitle compensating magnets in the bottom that are stuffed. Iam not sure whats wrong with it. The error is not constant in the plane so it might be the compensating magnets.I thought this may be a warning to others too if it is common to the recreational flying gps units. The compass in the car I noticed is now wonky too but other gps units didnt seem to effect it in the past.

If it is out of the plane and reading okay, then would not that suggest that something in the plane is causing the problem or am I being a silly person?

 

 

[Kyle Communications]

There would not be enough magnetic field in your gps to permanently corrupt the inbuilt compass in your aircraft. You are chasing your tail here looking for the wrong culprit. It was something else you passed into the aircraft or simply the compass crapped itself. The magnet in a speaker that size could never affect a compass magnet to that degree

 

Mark

 

 

[Guest Andys@coffs]

Mark/Ros

 

My Aera GPS definately affects my compass, to the point that it is useless when the GPS is within 2ft or so of it (sorta rules out the entire panel given the compass is in the midldle at the top). Also dont forget that compass failure doesnt have to involve massive errors, 5-10 degrees of error when your relying on compass clock and map alone will be enough to ruin your day. So while the speaker magnet may not be that powerful its that distance thing that wins everytime over a powerful but remote field.

 

That said, remove the GPS and the problem is pretty much removed. So i agree that the GPS in isolation shouldnt be the issue......

 

So, what about the power supply, did you have too much lead and coil it all up into a effective electromagnet near the compass? Is it possible that when you insert the power connector into the GPS it shorts just for a very short time (too short a period to blow the fuse), or the inrush current on initial plug in is enough to achieve the same outcome, in which case the coiled lead will generate a very strong magnetic field for the transient time involved and creating a resultant field in something ferrous near or inside it (you didnt wind the spare lead onto something metal?)

 

Ros, the benefit is that a compass itself can be used to find the source of the problem. Under normal circumstances it points north, unless there is a stronger source of magnetic field, in which case it will point at that. Might be easier with a simple N-S bar compass, just move it around and see where it is constantly pointing, where all the points intersect, then thats where your problem lies.Whatever it is that it points at has a strong unshielded magnetic field and needs to be removed and replaced or degaussed, an oldtimer TV repair tech who used to work on CRT tube displays can help.

 

Just as an aside, have you recently added a relay or solenoid to your power system? these can generate strong fields that affect compasses? But if it was that I would expect that the problem only occured when the main electrical system was powered on and everything was good when main power is turned off (assuming that everything is wired as it should be)

 

Andy

 

 

[ross holz]

Iam pretty sure it was the gps that stuffed the compass or at least magnetised something else near by because I have the same problem with the compass in the 4wd (a fairly good quality one with compensators) and the gps is the only thing thats come near them both.

 

As far as loops in the power lead, you only get about 2 inches of power lead with these cheap chinese gps units so that wasnt the problem.

 

I tried the hand held compass near the gps and the internal speaker gave a very strong indication.

 

No I havent added any solenoids etc and the wiring is standard.

 

All the gps units/electronic devices I've used effect the compass if close by, so if the field is strong enough would change the magnetic properties of a compass or what evers near by in my opinion. I was playing with a little hand held compass and changed the poles on that quite easily with another (speaker) magnet.

 

I held the compass up high on the windscreen away from the instruments etc and that seemed to solve the problem so perhaps something else has become magnetised. On the other hand I had a spare compass for the car and I could compensate that one but not the old one with the little adjusting screws after it became faulty. Either way I'll mount the compass up high on the windscreen which should solve the problem.

 

Thanks again for input