Limited panel depth for radios

The STC is the kicker. You can try to do a 337 field approval.

These days your FAA types aren't looking for a reason to say yes. They are predisposed to say no.

My avionics guy used to do 337’s a couple times a week. Then he got burned. He was working on a Bell helicopter that could have come from Bell with the option of a particular box. The original buyer didn’t and the current owner wanted to add the box.

It cost him a couple weeks fiddling with the FAA about a 337:feild approval that used to be routinely approved. They wanted the technical data that Bell didn’t want to share in order to approve.

Not all FSDO’s are that way. The workaround is to hire a DAR/DER

I just installed a Trig TY91 specifically because I wanted something that wasn't hanging down below the panel. The Trig radio works wonderfully. The control head fits a 2-1/4" hole and with the wires attached is probably less than 4" deep. The "main box" is also small and light (about 6" x 2" x 2") and was installed below the seat.

The TY91 does come with an install kit with all of the wire end connectors, pins, coax ends, etc. You will need to buy or borrow a D-Sub pin crimper and an RG-400 coax crimper . If you want to install the radio's 2-place intercom function, you'll also need a set of panel jacks (they're cheap). 

Wire, however, is not included. I installed "the works" with new headset jacks wired for mono intercom. I also installed 2 dedicated push-to-talk (red) buttons because I was tired of the messy dangling cords of the portable PTT button.

My wire shopping list was:

7ft RG-400 coax cable
11ft 20ga Tefzel wire
80ft 22ga Tefzel wire
32ft 22ga, 2-conductor Shielded Tefzel

Bonus: Get a half dozen of these Solder Termination shrink sleeves . Whenever you need to connect a wire shield to ground (at the dotted circles in my diagram) you just shrink these onto a bared spot in the shield and... presto! You have a six inch pigtail to connect to ground with no soldering or crimping required.

In the wiring diagram, you can ignore the "matching" TT22 Transponder which I installed at the same time.

 
I always feel better after drawing up a wiring diagram instead of using the book as I go. By the time I get the diagram looking neat and tidy I've drawn it a few times so it's dedicated to memory and less likely to get messed up. Feel free to use this diagram, or I can help if you want to alter it to your needs. 

 
The TC90 is the control head in the panel. Installing the TC90, I decided to get fancy and cut the specially-shape Trig holes, but the radio also comes with (surprisingly sturdy) "half moon" adapters to fit a round 2-1/4" hole.

 
Radio body is the one with cooling fins. The other is the transponder body.

The mounting tray was fabricated from .032" 2020T3 sheet using a miter saw. A regular wood blade will cut aluminum like a champ. I mocked up with cardboard until I had a shape that was fitting nicely, then used that as a pattern and riveted to the belly longeron and the diagonal support. You don't need a lot of strength here, just a stable platform for the Trig mounting tray. 

Did you mean fuel tank instead of firewall? I mounted my radio in the standard panel, but below the fuel tank. Also, don't forget that antenna leads and wiring coming out the back of the radio take up some room.

I would like to install a Radio in my panel instead of below the panel where it is difficult to see and control.    Most radios are 10 inches in depth.   My Coupe only has 6 inches from firewall to panel.   Any suggestions for a radio that will fit into a standard 2 1/4 cutout and require less than 6 inches depth?    A KRT2-S radio will fit but it is ETSO and FCC approved not STC.    A PAR200B radio is 6 inches in depth but requires a remote transmitter/receiver.   Would appreciate any advice how to select/install a radio in a Coupe Panel.    Jim K.   N87124