Avionics Retrofit: Pre-digital Aircraft


August 2003


This month’s Shop Talk takes on the daunting task of avionics retrofit for pre-digital avionics aircraft. Before 1979, most general aviation aircraft came with no digital avionics. In those days a 360 channel radio was not uncommon and a 720 channel digital radio was top of the line.



Times have changed; in 1997 the FCC & FAA implemented a rule that all VHF aircraft radios used to transmit must meet a 30 parts/million frequency tolerance and utilize 25 kHz spacing with 720 or 760 channels. A list of approved radios was published that are acceptable for use and those radios that can be updated with an upgrade kit. All other radios are not approved for transmitting after 1997.


Today, a completely upgraded radio stack with GPS can cost $25-35,000 at your local avionics shop. That is not affordable for most people with a $30-60,000 aircraft. Fortunately, there is a way to get a GPS and a COM radio installed in your aircraft for about $5,000. Follow along as we take you though the time and cost of installing a Garmin GNC250XL COM/GPS unit. It seems nowadays everyone is talking about the Garmin 430-530 or UPS units. These are great systems, but are they what the average weekend VFR flyer needs? The Garmin GNC 250XL is also a GPS and COM all rolled up into one neat package. At around $3,000, you can purchase this compact radio/GPS that measures 6¼" wide 5½" long and 2" tall with an easy to read display screen that’s 4" wide and 1¼" tall. This radio can be seen in bright daylight and will fit into your radio stack easily.


So what else do you need that is not included with this unit? You need a COM antenna (not provided as many installations will use an existing one), antenna coax for GPS and radio, connectors, shielded multi conductor Tefzel wiring, a radio blower fan (optional), a 10amp circuit breaker, a bunch of special crimpers and pin extractors and a GNC250XL installation manual. You will also need a manual for your audio panel and transponder so the GNC 250XL can be interfaced to the audio and get altitude input from your blind encoder. One more thing you will need is PATIENCE!


Once your favorite A&P mechanic (you don’t always need a radio shop) starts disassembly of your instrument panel you will probably discover extra wiring, circuit breakers and other junk that was left installed from previous avionics jobs. If this excess is not connected to anything, now is the time to pull it out! If all the wiring looks like Medusa’s latest ‘do, maybe it is time for a complete rewire. Eventually the radio rack and electrical panel will look as though you will never ever be able to fly that airplane again! Don’t despair; this is how it’s done.


After all the old wiring, circuit breakers and other related junk is removed, the design of the radio rack can commence. Take some time to sit in the pilot’ seat and visualize the radio stack. Another useful technique is to stack the units (carefully) on a table and run some sample in-flight scenarios. This will help insure a logical arrangement. Placing the primary NAV, COM & GPS systems in the center stack eliminates the parallax that occurs when you have to look all the way to the right side of the instrument panel. Mounting primary radios and the GPS display high in the instrument panel keeps you from having to look down and then back up to see out the windshield.


Two wiring harnesses and one interconnect harness for the blind encoder and the GPS/COM radio will have to be fabricated. If you are not good with small parts and detail work then have your radio shop do this work. They do harnesses every day, so if you provide the correct lengths, for them it’s a no­‑­­­brainer and can save you considerable time. (Time or money – altitude or airspeed: take your pick.)


With most of the radio rack and electrical panel opened up, it is a good time to install a radio blower fan and hoses to all the chassis that have a fresh air hose connection. This will provide forced air for equipment cooling to the backside of your radios and will extend the life of the electronic components. Lone Star Aviation at 817-548-7768 makes a great little blower unit called the Cyclone and can supply the hoses and fittings.


Once your radio racks are in and the harnesses are installed, clean up the circuit breaker panel. Remove any unused breakers and install the new ones for your radio and blower. For circuit breakers, I recommend using the Klixon pull type. They can be pulled out to de-energize a system and likewise, an overload is easily spotted with the breaker physically extended. Always re-label the breakers with the name of the protected system.


Using the respective antenna installation manuals, find a good location for the GPS antenna and possibly a location for a new COM antenna. Try to maintain 36" distance between both COM antennas and GPS antennas. When in doubt, ask your avionics shop.


Route the GPS and COM antenna wires and install the antennas using the manufacturer’s installation data and manuals. Before connecting cables or racking the radios, with circuit breakers open, perform continuity checks of all new or changed wiring. It’s very easy when working with shielded wire to get the main center conductor wire grounded to the shielding. Take the time to ohm it out now before you close up and you will save a bunch of time (and smoke) in the end. After a complete functional check of the GPS and COM radios, install the interior and close the airplane up.


So now you have completed your functional testing and closed up the airframe, how do we return the job to service? GPS installations are done as a FAA field approval for VFR and IFR certification. You will need an IA mechanic to complete a Form 337 and get the FAA to sign it (block 3).


Do you need an avionics shop? No, but they do this work all the time and should have the correct wording on a 337. The 337 must reference the location datum point of the units installed: antennas, blower, GPS and COM. Additionally, what Mil-Spec of wire was used, how the clamping of the wire bundles and circuit protection are installed, the installation manual(s) used, part numbers used and the sections of AC43.13/2A (radio and electrical) that apply by paragraph. If you have never done one of these then enjoy the vertical learning curve. If a recently approved 337 is available, you can get an idea of what your FSDO wants. Some FSDOs prefer an avionics shop do the work but there are many IA mechanics also doing great installations and many installations are collaborative projects. You must have the correct data, manuals and tools to do this work. Of course, the equipment list and the weight & balance must be updated. A logbook entry in the aircraft log along with a P.O.H. supplement on how to operate the new equipment will be required. Sometimes the radio manufacture will have a S.T.C. on one model of airplane that this radio was installed in; if you have the same model then you will not need a field approval. Regardless, a 337 is required.


As with all Shop Talk articles, feel free to call my aircraft repair shop at 307-789-6866 or better yet, via e-mail. Until the next Shop Talk, enjoy flying your Mooney.