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My Transponder Mode C (Altitude Reporting) function quit working. The position reporting function is working fine. I believe the failed component is the self contained Model A-30 Altitude Digitizer "box" that is bolted to the interior of the fire wall. It has a connector cable to the transponder and a rubber pitot tube connection. I am an Aerospace engineer with 32 years experience designing and testing Space Shuttle Main Engines and components. I have done my own annuals and had my local FBO mechanic inspect the work and sign it off. The replacement of the Altitude Digitizer box does not require the removal of the transponder - just bolt on the new box and re-connect the air tube and plug in the "D" connector to the transponder.

As the aircraft owner would that be approved for me to do as a non-complex assembly/maintenance? I know the calibration would have to be done by my certified avionics shop personnel, but can I do the R & R of the box? Having my avionics shop do it adds another \$200.00 to the calibration cost of $175.

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  • $\begingroup$ Before replaying the box, verify that the cable to the transponder is in good condition. The Mode C code is a Gray code that is transmitted over a parallel cable in some older model encoders / transponders. If the cable is not connected properly in the socket or has damaged internal wires you will get some wrong altitudes and big jumps in altitude reporting. $\endgroup$ – DeltaLima Aug 3 '15 at 7:02
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Per 14 CFR 43.3(d):

A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the extent necessary to ensure that it is being done properly and if the supervisor is readily available, in person, for consultation. However, this paragraph does not authorize the performance of any inspection required by Part 91 or Part 125 of this chapter or any inspection performed after a major repair or alteration.

This means if you are not a mechanic yourself, you can still perform this work as long as it is supervised by someone who is authorized to perform the work. Based on your question, I assume that you actually want to perform this work yourself. Further down in 14 CFR 43.3(g), the FARs state the type of work you can perform without supervision:

Except for holders of a sport pilot certificate, the holder of a pilot certificate issued under part 61 may perform preventive maintenance on any aircraft owned or operated by that pilot which is not used under part 121, 129, or 135 of this chapter. The holder of a sport pilot certificate may perform preventive maintenance on an aircraft owned or operated by that pilot and issued a special airworthiness certificate in the light-sport category.

Preventive maintenance is defined in Appendix A to 14 CFR 43, paragraph C. If a function does not appear in this list, it is not preventive maintenance, and as such must be performed by personnel authorized to perform repair work. The only item in this list that is even remotely relevant to your situation is item 31:

Removing and replacing self-contained, front instrument panel-mounted navigation and communication devices that employ tray-mounted connectors that connect the unit when the unit is installed into the instrument panel, (excluding automatic flight control systems, transponders, and microwave frequency distance measuring equipment (DME)). The approved unit must be designed to be readily and repeatedly removed and replaced, and pertinent instructions must be provided. Prior to the unit's intended use, and operational check must be performed in accordance with the applicable sections of part 91 of this chapter.

Note that your situation does not appear to qualify, because the item you are replacing is not part of a self-contained, tray-mounted device. Even if it were, you would still not be authorized to perform the work because this item specifically excludes work on transponders.

Of course, I am not an attorney nor do I perform aircraft repairs, so my interpretation is completely non-qualified. For the best answer, I would suggest contacting your local Flight Standards District Office for guidance.

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