Whenever I’ve been on a test flight it’s been either because it was an issue that we couldn’t duplicate on the ground and we needed to see what the pilots were seeing or it was written into the operators GMM. It had nothing to do with not trusting what the crew was telling us but just having either some additional equipment on board or someone with some knowledge about how the systems worked that weren’t confined to the cockpit. For example I used to take care of a G-IV owned by an elderly lady with bionic hearing and certain sensitivities. The airplane would run perfectly and she’d complain about a clunking noise just after takeoff when the gear was retracted. Eventually we figured out what was bothering her was the brakes stopping the MLG wheels before they entered the wheel wells. It had to be explained to her that there wasn’t any remedy after thoroughly investigating the brake system. As I mentioned she also had bionic hearing and would complain about a high pitched noise during cruise flight, the crew tried to find the source of the offending noise during flights and came up empty. We suspected based on the description maybe it was a pressurization leak somewhere in the cabin. We did a max delta-p ground check with folks spraying leak check fluid all over the exterior of the aircraft and found no excessive noise or leaks. We flew and took the airplane up to 45,000’ and found nothing. It was finally found sitting in a hangar with a GPU plugged in. There was an audio amplifier mounted behind the sidewall next to the VIP seat that had a cooling fan. Avionics folks came in and relocated it and the problem was solved. I don’t mind going on test flights but if I don’t trust the crew I’m just not going.
