The Cessna 120-140 Association Forum

I’m in the process of installing a new transponder and need to replace the antenna cable. For some reason the old antenna is mounted in the last bay of the tail section so I’ve gotta get in there to replace it and the coax. Any suggestions on what to use for support and also any trick to get around the control cables?

It'll help a lot to get the tail up to level the "trip". I recall someone had found a way to place a board as a "catwalk" (cat crawl? ), but I never saw pics. A few years back I convinced my wife to crawl back there to help with ELT antenna R & R. She informed me that it was her last time...and, I'm confident that the promise still stands...so I don't ask. When I've ventured part of the way back I've just used lots of old towels to cushion my knees and help push/protect cables out of the way without (hopefully) breaking anything. That worked out OK, 'though my worst fear was sudden cramps or back "going out" -- fortunately neither occurred. FWIW.
Mac

AS Mac has said, level the aircraft. A lot depends on your personal "configuration" (weight, height, etc). I did the board trick, (may have written about it years ago). Take two pieces of wooden 2x2s or a 2x4 ripped in half approx. 15" or so long that will fit between the formers and position them long wise up the sides of the fuselage that will allow a cross board to be high enough to clear the cables, and nail on each side to a piece of plywood or OSB, etc, forming a platform. I didn't pad them but one could if concerned about scratching the fuselage wall. Possibly make two to fit each side of formers. Stuff towels in between covering the formers and cables.

Another item, look for some long pieces of cardboard i.e. water heater or refrigerator box or just tape two pieces together and cover the platform/s - just lay them in position and you'll be surprised how easily one can slide their body into position.

Another item I have used for years is a set of outdoor lawn chair cushions. Neighbors daughter was discarding a complete set of about 6 pieces, some triple fold and some double. They can be easily formed to fit in various aircraft to get under instrument panels, etc. I keep one in my basement to facilitate changing the A/C filter. On Cessnas, one can take out the front seats and "fill up" the floor board "cavity" to make under instrument panel more comfortable. Used them many hours in my Bonanza doing annuals and mods to my Bonanza

Oh yeah, if the battery box is in the way too much, removing a couple bolts gets it out of the way.

My granddaughter is an A&P and at 5'2" or so (she says she is 'fun-size') scampered back there to replace my ELT. Made me very jealous of her youth and flexibility!

On second thought, unless required at that location by the new transponder manufacturer, why not just relocate your new antenna under the seat area, of course, bottom of the fuselage: New location would facilitate future biennial transponder checks and also shorter cable would reduce longer cable loss.

However, I suggest, if you relocate, make your new installation first and flight check transponder for ok performance, then remove old antenna installation. Just in case for some odd reason the rear antenna location was at that specific location.

Reason I suggest this, on my other plane, an Ercoupe LSA, I bought post heart attack, had been rebuilt and has a center stack three radio configuration, King flip/flop com on top , king com&gps combined next and a Collins TDR 950 transponder on bottom. The aircraft had a known transponder problem when I bought it - previous owner with the problem, had bought another transponder and had it bench checked before installation but the problem still existed.

I witch- hunted the system to no avail and I have an electronics background. Finally, up flying one day, turned all avionics "OFF". Don't have a separate avionics power buss, so usual procedure was, after engine running, turn avionic "ON, top down. That day, changed procedure and turned avionic "ON" from bottom up -BINGO- the transponder started working with "reply light" flashing!! Obviously one of the upper radios, probably the com/gps would block the transponder operation. Final fix, KISS principal, log book entry and placard panel!

I discussed this problem with King and my transponder repair station but they had never heard of such a problem! Others may never hear of such a problem in a life time but I pass in on just in case. I apologize for the length. FWIW

My son (who is building time for commercial rating) convinced me to buy a tail pull handle. I said sure as long as you crawl in…
Yes, leveling the plane on sawhorse (under a bulkhead of course) and some padding material make it easier. As well as someone who treads carefully.
We teased him the whole way cause his pants started to slide off. Wish I had a video of him getting past those initial cables, wires (didnt know he knew how to cus like that) and battery box. Plus he is skinny. I’m too robust and old to be that nimble.

While your back there you might as well clean it out good.

Just remove the current X-ponder antenna and relocate farther forward? Where it's easier to work on?

In the picture above with the owners son stuffed in the tail cone, you see a doubler on the top of the fuselage just aft of the battery?
That is where I put my GPS antenna. The Garmin GDL 82 was mounted on the side of the fuselage, to the right of the battery box, but aft of the baggage compartment. (left side of the fuselage looking forward)

The antenna was below the GDL 82, at about the 7 O clock position with a 90 degree coax connector so it wouldn't interfere with the control cables.
Big Edd said "KISS principal" by all means!

The battery box is a serious pain in the patoot. I carefully drilled out the rivets and replaced them with nut plates. It now takes 10 minutes to remove the box and gives excellent access to the cables, pullies and antennas.