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elt antenna and magnetometer both in empennage fairing?

Desert Rat

Well Known Member
Garmin recommends locating the GMU11 on the aft deck just forward of the h/s.

Also, lots of people mount the ELT antenna back there, but has anybody done both or does having the ELT in that location hose up the magnetometer?

I understand that there's an integrity check, but somebody must have already tried it. If it was a complete failure I'll save myself the trouble (but have to modify my pretty wire bundles and likely stick that ahrs shelf in the tailcone).

Before we get started, yes I know that some people puke all over that location for the elt antenna, but I'm not one of them.

Anybody?
 
Same plan

i am planning on the same thing in my RV7A using Vans ELT power supply bracket and the Garmin aft deck kit. No input on if they are compatible or not though.
 
If nobody else speaks up, please let us know if you find out before I do :)

For the antenna mount I just bent up a piece of .032 into a channel and pop riveted it across the big hole in the F-709 bulkhead.

If you do it this way the antenna fits nicely and there's about 5" separation from the magnetometer as shown. Won't know if thats enough until I do the interference check though unless somebody has gone down that path already.

As you can see, for my mockup gmu mount I'm using a pair of recycled organic composite structures with a mono-directional grain pattern.
 

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I personally do not recommend mounting any magnetometer on the shelf below the HS. When I purchased my RV7A from the original builder both my GRT and Dynon magnetometers were mounted on that shelf next to one another. There happens to be enough steel (spar) back there to screw up remote magnetometers and on the flight to its new home in Oklahoma from Colorado both my headings drifted badly (in opposite directions). Made navigation a little challenging.

After contacting both GRT and Dynon tech support I learned they recommend mounting their remote magnetometers in the wing tip away from any electromagnetic interference. I did that and my magnetic heading drift cleared up immediately.
 
For what it is worth.... not really the same but...

I'm not flying yet, and I may well have to move it but I "chose" my ELT antenna position at the end of a long day, and after I drilled the hole in the top of the fuselage I realized the antenna was going to be about an inch from magnetometer on the other side of said skin.

Its on my list to move to the wing, but I did get far enough along to try a calibration, and an interference check. In spite of my best-efforts including transmitting on a range of frequencies to try to excite the ELT antenna, nada. Nothing seemed to bother it, clean pass. Not flying so no guarantees, but so far so good.

On another plane I had the magnetometer two inches from a strobe cable. It didn't like that, but 5 inches made all the errors go away.

Your mileage may vary - considerably.
 
GMU11 location

I am currently installing a GMU11 and G5 in my RV-4, and plan to put the GMU11 on the AFT deck also. I believe many have successfully done that. We have an AVX shop on the field, and I will be having the "location survey" done in the next couple days before I drill any holes. They can hook up a simple device to the GMU11 at the desired location, then you power up the plane and operate everything possible to detect interference. I only have an old school tail light in the rudder fairing and electric trim to the elevator. If they don't show interference, its going in there. The tech said it only takes a couple inches to make a difference. The GMU can ignore stationary items like nut plates and such, but "sees" moving steel and electrical energy. I'm hoping for a good outcome, as I do not want to run additional wires to the wingtip and make a leveling bracket. My ELT antenna is in the cockpit and curved around the rear seat bulkhead, so its not a factor in my situation. I know it works there(at least when upright) based on the false trip that brought the CAP's attention.
 
If nobody else speaks up, please let us know if you find out before I do :)

For the antenna mount I just bent up a piece of .032 into a channel and pop riveted it across the big hole in the F-709 bulkhead.

If you do it this way the antenna fits nicely and there's about 5" separation from the magnetometer as shown. Won't know if thats enough until I do the interference check though unless somebody has gone down that path already.

As you can see, for my mockup gmu mount I'm using a pair of recycled organic composite structures with a mono-directional grain pattern.

I would highly recommend checking with whatever DAR you plan on using to make sure they are OK with ELT antenna mounted that way… mine said I had to mount it according to the manufacturers recommendation and would not allow it.
 
I am currently installing a GMU11 and G5 in my RV-4, and plan to put the GMU11 on the AFT deck also. I believe many have successfully done that.

-- snip --

The GMU11 works with my G5 very well in that location on my RV-6 and has for over 5-years.

We live close enough to each other that we can get the airplanes together to compare if necessary.
 
I personally do not recommend mounting any magnetometer on the shelf below the HS. When I purchased my RV7A from the original builder both my GRT and Dynon magnetometers were mounted on that shelf next to one another. There happens to be enough steel (spar) back there to screw up remote magnetometers and on the flight to its new home in Oklahoma from Colorado both my headings drifted badly (in opposite directions). Made navigation a little challenging.

After contacting both GRT and Dynon tech support I learned they recommend mounting their remote magnetometers in the wing tip away from any electromagnetic interference. I did that and my magnetic heading drift cleared up immediately.

that's really interesting, because Garmin specifically recommends this location for the GMU11. It's in the installation manual.

FYI- I don't recall there being any steel back there? If you build to plan, The h/s and v/s spars are definitely aluminum, not steel. I wonder if the builder of your plane went off plan?
 
I would highly recommend checking with whatever DAR you plan on using to make sure they are OK with ELT antenna mounted that way… mine said I had to mount it according to the manufacturers recommendation and would not allow it.


Thanks for the warning. I understand what you're saying, but here's my reality.

If you go with a full up fire breathing IFR system on something as small as an RV7, it's physically impossible to comply with all of Garmins antenna separation requirements without towing an antenna or two behind the airplane. I believe the elt in this faring represents a reasonable compromise.

Yes I understand it's TSO'd. yes I understand that horizontal installation doesn't meet the letter of the installation instructions. Ultimately, I'll move it if I have to.

But, if it doesn't cause problems with the magnetometer, that's where it's going to live unless, as you say, I'm forced to move it.
 
Yes I understand it's TSO'd. yes I understand that horizontal installation doesn't meet the letter of the installation instructions. Ultimately, I'll move it if I have to.

But, if it doesn't cause problems with the magnetometer, that's where it's going to live unless, as you say, I'm forced to move it.

But the problem is that it’s not up to you… but to each their own. I’d rather call and get clarification and not have to move it
 
But the problem is that it’s not up to you… but to each their own. I’d rather call and get clarification and not have to move it

Never ask a question if you don’t want to get the obvious “lawyer” CYA answer.

I built three RVs with the ELT antenna under the empennage fairing, never an issue for certification.

Keep in mind things have changed with ELTs:
- The primary connectivity mode is via satellite. These antennas have dual element for 121.5MHZ and 406MHZ. The radiation pattern at 406 is not necessarily looking for the horizon.
- I know of one 406 ELT going off by accident (antenna under the fairing). Even though the plane was on a IFR flight plan (and Flight Aware knew exactly where it was) the response was a call to his wife asking if she knew where her husband was.

While the antenna orientation means little at 406MHZ it could me a few db at 121.5, assuming you still had an antenna after the crash. In the photos of crashed RVs I looked at, the junction of the HS and VS was always intact. Flip the plane over and the top mounted ELT antenna is in the mud, or more likely gone.

If you are worried about your DAR, mount it inside the cabin like called for in the RV-14 plans. It will not work as well as under the empennage but the sea lawyers will be happy.

Carl
 
FYI- I don't recall there being any steel back there? If you build to plan, The h/s and v/s spars are definitely aluminum, not steel. I wonder if the builder of your plane went off plan?

No the HS and VS were built to plan! And the kit was a QB. When I called GRT and Dynon about mt heading drift problem they both told me there was too much magnetic material in that area, therefore they recommended installation of the remote magnetometers on the wing tip away from any electrical interference. I don’t know what Garmin’s recommendation is for their magnetometer location.

BTW, when I upgraded my old ELT for the ACK 406 ELT I did use some of that space for the new ELT antenna installation. As they said, it’s important for the antenna to be as close to vertical when you need it and in a crash scenario the aircraft is often (but obviously not always) found “nose down.” With it installed under the vertical stabilizer it will likely be “vertical” in a crash situation.
 
that's really interesting, because Garmin specifically recommends this location for the GMU11. It's in the installation manual.

I don't see a GMU 11 specific installation manual on the Garmin site. The G5 installation manual has several sections that refer to the GMU 11, and page 58 of that manual says "In general, wing mounting of the GMU 11 magnetometer is preferred."

I've pretty much decided to put the GMU 11 in the right wing, assuming it passes the interference check there.
 
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I don't see a GMU 11 specific installation manual on the Garmin site. The G5 installation manual has several sections that refer to the GMU 11, and page 58 of that manual says "In general, wing mounting of the GMU 11 magnetometer is preferred."

I've pretty much decided to put the GMU 11 in the right wing, assuming it passes the interference check there.

From the Garmin G3X installation manual, revision AB September 2021. appendix B, "Airframe Specific Installation Guidance."

B.2 Van’sAircraft
B.2.1 RV-7/9
The typical GMU magnetometer mounting location in RV-7/9 aircraft is in the aft fuselage, on the aft deck (F-714), just ahead of the horizontal stabilizer and just aft of the F-709 bulkhead. This location provides a level pitch and roll reference, and acceptable separation from excessive magnetic disturbances in typical RV-7/9 aircraft.
The wingtip can be used as an alternate GMU magnetometer mounting location for RV aircraft. Ensure adequate separation from steel aileron counterweights, transmitting antennas, and lighting fixtures, and follow the wingtip light wiring guidance for in Section 15.4 or Section 16.4.
 
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From the Garmin G3X installation manual, revision AB September 2021. appendix B, "Airframe Specific Installation Guidance."

B.2 Van’sAircraft
B.2.1 RV-7/9
The typical GMU magnetometer mounting location in RV-7/9 aircraft is in the aft fuselage, on the aft deck (F-714), just ahead of the horizontal stabilizer and just aft of the F-709 bulkhead. This location provides a level pitch and roll reference, and acceptable separation from excessive magnetic disturbances in typical RV-7/9 aircraft.
The wingtip can be used as an alternate GMU magnetometer mounting location for RV aircraft. Ensure adequate separation from steel aileron counterweights, transmitting antennas, and lighting fixtures, and follow the wingtip light wiring guidance for in Section 15.4 or Section 16.4.

I see.

Well...I'm not putting in a G3X just yet. But, 15-3 of that G3X installation manual does add a specific caveat to verbiage that mirrors what is in G5 manual where it says "In general, wing mounting of the GMU 11 magnetometer is preferred" but adds "(unless as noted in Appendix B)."

I have a 6, and for that Appendix B says "In RV-6/8/10 aircraft, the GMU magnetometer is frequently mounted on a rigid aluminum shelf suspended between the aft fuselage longerons, midway between the baggage bulkhead and empennage. The wingtip can be used as an alternate GMU magnetometer mounting location for RV aircraft.

For me, I'm 100% sure the wingtip will be easier. If I had a 7, I'd probably go with the aft deck.

I guess it boils down to putting it wherever it is easier and it works, but it makes me wonder why they don't mention the aft deck for the 6/8/10.
 
SNIP

I guess it boils down to putting it wherever it is easier and it works, but it makes me wonder why they don't mention the aft deck for the 6/8/10.

The SkyView system does not need a separate magnetometer. The two ADHARS modules are mounted just aft of the baggage compartment (RV-10 and RV-8), and in the left wing mount that Van’s provides in the RV-14 kit.

It would seem to me that mounting anything on the tail plate would be exposed to water and such. Perhaps this module is made for it. But I’m curious - why does the Garmin need this extra magnetometer?

Carl
 
I have a G3X with a GMU44 an an ELT antenna mounted in the empennage just as depicted by Desert Rat. No issues after 350 hours.
 
Seat belt cables

Has anyone ever had an issue with the shoulder harness cables interfering with GMU 11, when the GMU is located on a shelf just behind the baggage compartment bulkhead. I’m getting the “ magnetic anomaly “ warning.
 

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Has anyone ever had an issue with the shoulder harness cables interfering with GMU 11, when the GMU is located on a shelf just behind the baggage compartment bulkhead. I’m getting the “ magnetic anomaly “ warning.

Could it be the STEEL nut(s) and screw(s) holding the rail to the sliding canopy? They are a LOT closer than the shoulder harness cables.
 
I have a G3X with a GMU44 an an ELT antenna mounted in the empennage just as depicted by Desert Rat. No issues after 350 hours.

+1

Except that this identical installation in my RV-7A has worked flawlessly for 11 years and nearly 1200 hrs., including solid IMC actual IFR approaches down to minimums. I have much confidence in my non-touch G3X system and installation.
 
Forgot to mention, I replaced all nuts and bolts with stainless.

Be careful using stainless and depending on it being "non-ferrous". A lot of stainless steel hardware IS high in iron content. Always check with a magnet.
 
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