What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

ELT antenna

wirejock

Well Known Member
I mounted the "Yak" (ACK) ELT on a Vans mount per plans just behind the F706 baggage bulkhead. That was enough for the day. Dufficult job. Next up is the antenna. The manual says not to pass through any bulkheads.

Does that mean not to actually route the coax through a snap bushing or don't cross a bulkhead at all?

My 7A is a slider. The only place I can mount the antenna to comply with the DAR and Yak manual is centered top aft skin right in front of F708 bulkhead. That means passing F707. The F707 bulkhead is wide open. The coax can pass F707 without touching anything. Is that ok?

Should the coax be secured or just left hanging? If I tie it off, it could get severed in an accident.

I installed nutplates in all the locations on the mount for future use. So many builders have problems with the Yak, I will probably replace it. That mount is not coming out so hopefully the holes match other ELTs.
20230516_153519.jpg
 
My understanding is if the coax is routed through a bulkhead (whether a snap bushing or rubber grommet) it could get severed if the bulkhead deforms during a crash. So routing it around a bulkhead, and secured sufficiently, is preferrable.
 
My understanding is if the coax is routed through a bulkhead (whether a snap bushing or rubber grommet) it could get severed if the bulkhead deforms during a crash. So routing it around a bulkhead, and secured sufficiently, is preferrable.

What Terrye said.
 
What would be the preferred method of securing the coax when routing around a bulkhead?
 
Getting around a bulkhead that is covered by a baggage bin cover panel that will also need some sort of hole drilled with a snap bushing or something similar, doesn’t seem like it would alleviate the potential shearing of the antenna cable in a crash. To take the bulkhead question out of the equation, you would need to mount the antenna through the skin of the compartment the ELT is in - like through the top/side/bottom of the turtle area skin? What happens when you flip over or that tail cone skin is sacrificed? It doesn’t seem like some non-standard installation would be worth the effort. Is there some data/information that the per plans installation hasn’t worked?
 
Securing cables

What would be the preferred method of securing the coax when routing around a bulkhead?

I can't speak as an expert, but I hate zip ties with a passion. Not a single one on my airplane.
They hold great till they get brittle. New ones wouldn't release in an crash. Brittle ones just fall apart.
My plan is to leave service loops, aka crash loops secured with self fusing silicone tape. One near the antenna. One at the F707 bulkhead. One at the ELT. It holds very well and tears under pressure. Normally, I would put a tie of waxed lashing string, but that stuff will cut wire. Not a good idea.

If someone has another product, I'm listening.
 
Bulkheads

Getting around a bulkhead that is covered by a baggage bin cover panel that will also need some sort of hole drilled with a snap bushing or something similar, doesn’t seem like it would alleviate the potential shearing of the antenna cable in a crash. To take the bulkhead question out of the equation, you would need to mount the antenna through the skin of the compartment the ELT is in - like through the top/side/bottom of the turtle area skin? What happens when you flip over or that tail cone skin is sacrificed? It doesn’t seem like some non-standard installation would be worth the effort. Is there some data/information that the per plans installation hasn’t worked?

I'm not passing coax through any bulkhead with a hole. The ELT is AFT of the baggage bulkhead per Vans on a Vans designed ELT mount. The only bulkhead the coax will pass is a huge open area. ELT and antenna are both aft of the fuse/tail cone seam. Antenna is recommended on top. Nothing I can do it if it goes wrong side up. Same problem if not worse to place it on the bottom or a side skin. Plans warn against placing it in wingtips or under fairings. It has to be a certain orientation from vertical.
Per Mel, DAR, it has to be installed per manual or be in violation.
Maybe I'm missing the point.
 
Maybe I’m being cynical but if a crash is sufficient to deform a rear bulkhead and severe a coax cable, then the forward compartment probably could be used for an aluminum coffin.

But to keep in the spirit of things, perhaps a loop in the coax between the bulkheads to provide the “slack” needed to prevent stretching and potential severing of the coax in the event of a crash.
 
Loops

Now there's an idea!

Maybe I’m being cynical but if a crash is sufficient to deform a rear bulkhead and severe a coax cable, then the forward compartment probably could be used for an aluminum coffin.

But to keep in the spirit of things, perhaps a loop in the coax between the bulkheads to provide the “slack” needed to prevent stretching and potential severing of the coax in the event of a crash.
See below. Already planned

<snip>
My plan is to leave service loops, aka crash loops secured with self fusing silicone tape. One near the antenna. One at the F707 bulkhead. One at the ELT.
It holds very well and tears under pressure. Normally, I would put a tie of waxed lashing string, but that stuff will cut wire. Not a good idea.

<snip>
 
Last edited:
Snap bushings

If memory serves, this means that all RV-14s built per plans will not pass a DAR.

Carl

Yes, plans call for penetration through 2 bulkheads complete with snap bushings for the 14. I think a "crash loop" would help but that I'm pretty sure was not in the plans.
 
Maybe I’m being cynical but if a crash is sufficient to deform a rear bulkhead and severe a coax cable, then the forward compartment probably could be used for an aluminum coffin.

Absolutely not! A certain amount of "crush" is designed into many tail cones to relieve pressure on the shoulder harness and avoid back injuries.
 
Attached is the seminal report on ELT crash performance. Earlier studies showed as many as 15 percent of ELT failures in survivable crashes were due to antenna/cable issues. However during full scale crash tests roughly half resulted in damage to antenna connectors themselves. None were from cable shearing. It also found that many of the cable connectors provided by ELT vendors were not fabricated to MIL-DTL-17 and failed with 1/5th of the mechanical load. The report concluded that the problem could indeed be solved by adding a 10 inch length of cable at both ends using MIL-DTL-17 cable.

https://ntrs.nasa.gov/api/citations/20170002586/downloads/20170002586.pdf

Mike
 
Last edited:
Misinterpretation?

From the Installation Manual for the ACK ELT-04, page 4:

"...and should avoid crossing aircraft production breaks (i.e. riveted fuselage sections)..."

I don't see how drilling a hole in the webbing of a bulkhead, insulating it with a plastic snap bushing, violates the letter or spirit of the installation guidance.

However, for argument's sake, refer to RV-7 DWGs 21 and 26. Drilling a routing hole in the area where the bulkhead halves overlap and are riveted together, wouldn't be recommended (e.g. Detail B, DWG 21).
 
No good solutions?

So, it seems that all the official manufacturer's advice for mounting antennas assumes they will be mounted externally. Nobody here seems to want to do that, which I understand.
So, either the antenna goes aft and under the empenage fairing where it will be horizontal (wrong according to the manufacturer) but very survivable and no bulkhead's need be penetrated, or it can go vertically mid-cockpit (correct, according to the manufacturer) but must past through several bulkheads to a less survivable location. Also, there is ZERO advice on exactly how much of this antenna must be exposed above the canopy rail, since Artex assumes the entire antenna is external.

It feels like everyone is lawyering up around this, sticking to their official and impractical decrees, and no one is speaking realistically or practically. Frustrating.
 
Larry,

Here are a couple of pics that show what I'm doing. It is a work in progress but I think you can get the ideal. I'll loop the coax under the bulkhead and continue forward with this method as soon as I make a few more of these little brackets. Once I get the entire route figured out I'll distribute any left over cable into loops as available.

I'm sure others may have comments on my approach, but its the way I'm doing it so...
 

Attachments

  • IMG_2565.jpg
    IMG_2565.jpg
    212.8 KB · Views: 48
  • IMG_E2585.jpg
    IMG_E2585.jpg
    89.5 KB · Views: 57
Back
Top