Early last year I helped lead a dual GTN750TXi with GDU install on a french made aircraft, the TBM850. The GTN750TXi system is a NAV/COMM unit with integrated GPS, TCAS, XPDR, and audio panel and the GDU was made to replace the primary flight instruments with a large screen for the pilot and copilot.

This install was certainly my most difficult, as these aircraft are not mass produced and you could tell it was not made to be worked on aftermarket. For instance, all of the existing wire did not have any identification stamping on it. In Canada, it is mandated that any wiring added to an aircraft needs to have an identifying mark (usually just some letters and numbers) at set intervals across the entire length of the wire. This ensures that you can properly route, trace, and troubleshoot by understanding what info each wire was carrying.

Not the TBM.

The TBM had baby blue wires going everywhere, all without any markings. This made integrating into any existing wiring (for instance, splicing onto an existing harness carrying engine parameters instead of replacing entire sensors) INCREDIBLY time consuming. It was also very nerve wracking when you found a wire that was cut somewhere behind the panel; obviously as part of removals we need to cut out existing wiring to old instruments, but having a cut wire hanging out and being forced to decide whether to complete taking it out (and risk needing that wire after all) was never a fun call.

Apart from this, the TBM had DOZENS of different connection styles. What I mean by this is typically, a manufacturer will decide on one style of wire termination; cannon plugs, d-subs for units, ground stud/ring terminal, AMP grounding block, grounding rail for airframe grounds, etc.

Not the TBM.

The TBM had a little bit of everything... some ground studs, some ground blocks, even some automotive knife splice grounds. It was impossible to have all the right crimpers/terminations that you might need that day, and we ended up with all sorts of goofy borrowed crimpers from all around the airport for these bizarre decisions.

The last point I'll make about the TBM is the equipment locations and unorthodox avionics placements. Usually, a small aircraft will have avionics in one of three places; in the nose of the plane (forward of the pressure bulkhead), behind the panel of the plane directly in the cockpit, or under the floor.

Not the TBM.

It had stuff all over the place. Some in the floor, some in the nose, some in the far reaches of the tail. This made it very impossible to estimate our wire length prior to the arrival of the aircraft (when we fabricate some of the harness on the bench), so a lot of our prep work was deemed worthless because the wire lengths were so wrong. A lot of wasted time and energy. The TBM is also BLOODY SMALL... so trying to get three techs in there was damn near impossible, even if we were working on seperate systems. Very little space for tools, hands, sightlines, and appendages. A lot of Rub A535 was used on this project.