Stupid Garmin

[Major Tom]

I bought a Garmin GPSMap 396 on EBay - that's an aviation GPS with XM datalink.

A word on the difference between aviation GPS and an automotive one. In a nutshell, aviation GPS contains a lot of aviation-specific functions and information. Very briefly, that includes things as runways, communication frequency, terrain and obstacle proximity warnings, airspace information and warnings, function to find the nearest airport, etc.

The 396 has an XM antenna that is mainly used to get weather information through XM (subscription required, obviously). It can also receive music, but that's quite a distraction. The weather information is extremely valuable, especially the automated weather reports at airports and the radar map.
Now to the stupid part.
Since the same type of XM antenna is used both for cars and for planes, and in cars it attaches to the dashboard using a magnet, the XM antenna has two strong magnets in it.
Why is that stupid? Because how airplanes navigate. Most airplanes have a DG - directional gyro - which is used as a "compass". I write "compass" in double quotes since it's not really a compass, you initially set it to the North, and using a gyro (typically vacuum driven one) it keeps pointing North (with certain precession). To set it initially and re-align it every 5-15 minutes, you use a magnetic compass. Why do you need two of them? Because the magnetic compass, unlike the DG, points North whenever you're flying in a constant speed. If you accelerate or turn, it won't show the right heading (which becomes a real challenge in turbulent air).

Back to the XM antenna. If you put it too close to the magnetic compass, it affects it. The plane isn't too big to begin with, and the magnetic compass is pretty much in the middle. That makes you put the antenna within 40 cm from the compass or so. If it moves during flight and gets closer to the compass, you might be &*#(%# (especially if you're in IMC).

This evening I opened the XM antenna (theoretically voiding the warranty, though since I got it used, I don't think I had warranty to begin with) and removed the two strong magnets in it. I achieved two things - no more issues with the compass, and a new toy for my son.

Major T.

[drew]

I bought a Garmin GPSMap 396 on EBay - that's an aviation GPS with XM datalink.

A word on the difference between aviation GPS and an automotive one. In a nutshell, aviation GPS contains a lot of aviation-specific functions and information. Very briefly, that includes things as runways, communication frequency, terrain and obstacle proximity warnings, airspace information and warnings, function to find the nearest airport, etc.

The 396 has an XM antenna that is mainly used to get weather information through XM (subscription required, obviously). It can also receive music, but that's quite a distraction. The weather information is extremely valuable, especially the automated weather reports at airports and the radar map.
Now to the stupid part.
Since the same type of XM antenna is used both for cars and for planes, and in cars it attaches to the dashboard using a magnet, the XM antenna has two strong magnets in it.
Why is that stupid? Because how airplanes navigate. Most airplanes have a DG - directional gyro - which is used as a "compass". I write "compass" in double quotes since it's not really a compass, you initially set it to the North, and using a gyro (typically vacuum driven one) it keeps pointing North (with certain precession). To set it initially and re-align it every 5-15 minutes, you use a magnetic compass. Why do you need two of them? Because the magnetic compass, unlike the DG, points North whenever you're flying in a constant speed. If you accelerate or turn, it won't show the right heading (which becomes a real challenge in turbulent air).

Back to the XM antenna. If you put it too close to the magnetic compass, it affects it. The plane isn't too big to begin with, and the magnetic compass is pretty much in the middle. That makes you put the antenna within 40 cm from the compass or so. If it moves during flight and gets closer to the compass, you might be &*#(%# (especially if you're in IMC).

This evening I opened the XM antenna (theoretically voiding the warranty, though since I got it used, I don't think I had warranty to begin with) and removed the two strong magnets in it. I achieved two things - no more issues with the compass, and a new toy for my son.

Major T.

I can relate.

[Greybeard]

a new toy for my son.

Major T.

Maybe my memory is wrong, but isn't your son still rather young? You've explained what not to do with them...?

[Major Tom]

Maybe my memory is wrong, but isn't your son still rather young? You've explained what not to do with them...?

He's 5 (in his words - "I was a baby, but I grew. Now I'm medium size"). My plan is to first have a little talk with him explaining that whatever he does, these things don't go in his mouth. The magnets will be for supervised play.

Major T.

[Phonedave]

Maybe my memory is wrong, but isn't your son still rather young? You've explained what not to do with them...?

He's 5 (in his words - "I was a baby, but I grew. Now I'm medium size"). My plan is to first have a little talk with him explaining that whatever he does, these things don't go in his mouth. The magnets will be for supervised play.

Major T.

In his mouth, on the CRT, over the flash drive, dropped into the slots on the stero receiver ......


As an aside, in a car, what exactly are the magnets supposed to be attracted to on the dash? My dash has very little metal in it. Is it mean to go on the outside of the vehicle?


And it looks like Garmin has corrected itself according to these posts on the Piper forums.



08/18/2009 10:17AM
Posts: 25 Garmin XM Antenna Magnet Issue - gone?


I'm about to purchase either a 396 or 496 (replacing a panel docked 296) because I want XM weather. In trying to decide I came across a number of sources that claimed that the 496 was improved as Garmin removed the magnets from the XM antenna mount (obvious possible compass issue).

My question is simple, did they fix/change this across all their portables, or just the 496? My guess is that they all have the same XM antenna now, but want to confirm before I order my unit. Anyone?

My preference is to just buy the 396 and save some $$$...


Options: Reply To This Message•Quote This Message


gsengle
08/18/2009 02:45PM
Posts: 25 Re: Garmin XM Antenna Magnet Issue - gone?


Let me put it in this way. Evidently in 2006 or so with the introduction of 496 they stopped putting magnets in the xm antenna. I can't find out if they did the same across the entire product line at that time. My guess is they did - since the Garmin lists the same part for both items as the GXM™ 30A smart antenna which is probably magnet free - just looking for someone with a newer 396 to confirm

[Major Tom]

Garmin at least got wise. Good. I have the GXM 30 though... I'd expect the FAA to check for unnecessary magnets during certification, but what can I say.

I don't *know* what it should attract, but I *suspect* they have a mount that they put on the dash which has a piece of metal or a magnet that the XM antenna attaches to.

Are you sure flash drives are sensitive to magnetic fields?

Major T.

[Phonedave]

Garmin at least got wise. Good. I have the GXM 30 though... I'd expect the FAA to check for unnecessary magnets during certification, but what can I say.

I don't *know* what it should attract, but I *suspect* they have a mount that they put on the dash which has a piece of metal or a magnet that the XM antenna attaches to.

Are you sure flash drives are sensitive to magnetic fields?

Major T.

Now that I think about it, I would say no - a magnet should not effect a flash drive. Solid state memory is not a magnetic storage meduim.

Maybe in some strange way it might - I don't know if you magnitized any ferrous metal in the device what would happen, but as far as erasing it, I am going to say no.

I assume the magnets are pretty strong. I took a look at the Garmin site and they show them mounting the antenna to the outside of the plane, with an aerodynamic cover on it. Same thing on the car units.

-dave

[Major Tom]

Now that I think about it, I would say no - a magnet should not effect a flash drive. Solid state memory is not a magnetic storage meduim.

Maybe in some strange way it might - I don't know if you magnitized any ferrous metal in the device what would happen, but as far as erasing it, I am going to say no.

I assume the magnets are pretty strong. I took a look at the Garmin site and they show them mounting the antenna to the outside of the plane, with an aerodynamic cover on it. Same thing on the car units.

-dave

You might be right with respect to mounting it on the outside of a car, I don't know.
I don't think it would work on a plane - I think it's made mostly of aluminum, which is paramagnetic (and not ferromagnetic as iron is), and I definitely wouldn't let it mount on a plane without being properly attached. I don't know what Garmin use for their non-portable units - maybe some permanent installation of the same antenna.

Major T.

[Major Tom]

I just found a page where Garmin say they'll exchange a GXM 30 with GXM 30A for "mere" $125. Good thing I removed those magnets myself.

[Major Tom]

Time for my next Garmin-related complaint.
It seems that if I wasn't using the GPS for a few hours, it comes up with the wrong time and takes too long to acquire reception. Reason - there is a 3V button-type battery that is used for that purpose and apparently reached the end of its useful life.
Garmin don't write about that anywhere and even don't tell their tech support personnel about it. The battery is soldered to the board. They would charge you around $200-250 if you want to send it in every four years or so to have it replaced.
Here is how to replace it. Yuck.

Major T.