Piper Altimatic III computer

[AlanBreen]

We have been chasing an autopilot issue on our PA30 where the aircraft pitches up and down whenever the pitch or altitude modes are engaged.

We have finally found the computer is the cause of the problem by swapping out the computer with another one. We had previously bench tested the computer and it met spec, so we had chased every other thing that could cause this problem before coming back to swapping out the computer.

The Comanches are fitted with the 395 model computer. This model doesn't have any adjustment pots unlike the later 515 model computer. The way the adjustments are made on the 395 is by inserting or removing diodes in the control circuitry. From what I can determine the 515 is a better all round unit and I'd like to replace our 395 with a 515 especially since our 395 tests up OK on the bench but doesn't work in the aircraft.

The problem is the 515 isn't listed as an approved part for the Comanche. The 515 is approved for some other Piper Models using the Altimatic III just not the PA30. Our engineer will not fit a 515 since it's not listed as a Comanche part.

I understand the 395 and 515 are directly interchangeable and both have been approved on some types.

Does anyone here know of any field approvals or other process where a 515 computer has been fitted to a Comanche?

Thanks for your help.