thibault
Member
- Joined
- Oct 25, 2009
- Messages
- 191
I have always had the compass swing left about 12 degrees whenever the Pitot heat cycled on. Finally, I trouble shot this to root cause this week and eliminated the problem completely.
The problem occurred because my airframe is all composite and I had installed all aluminum lines between the Pitot head and the ADAHRS unit. In fact, the power return wire from the Pitot, the Pitot tube and aluminum lines are all electrically common. Since the ADAHRS fittings and the case are also common to the Skyview network ground, this provided a path for some of the Pitot heat current to return through the ADAHRS to A/C ground even though there is a dedicated power return wire to the main A/C ground bus. This flow causes a magnetic field to be generated that swings the compass.
The fix was to insert some non-metallic line in the Pitot and the AOA tubes. In fact, I did this at the Pitot tube end and at the ADAHRS end.
Mike Huff has confirmed this with the engineers and the installation manual should be getting new guidance on the topic.
My supposition is that an all metal A/C would not see this problem even with all metal lines because the grounded airframe would carry any current back to the battery that did not go through the Pitot heat power return.
I would like to hear from any composite aircraft owners that have all metal lines from Pitot to ADAHRS about there experiences.
The problem occurred because my airframe is all composite and I had installed all aluminum lines between the Pitot head and the ADAHRS unit. In fact, the power return wire from the Pitot, the Pitot tube and aluminum lines are all electrically common. Since the ADAHRS fittings and the case are also common to the Skyview network ground, this provided a path for some of the Pitot heat current to return through the ADAHRS to A/C ground even though there is a dedicated power return wire to the main A/C ground bus. This flow causes a magnetic field to be generated that swings the compass.
The fix was to insert some non-metallic line in the Pitot and the AOA tubes. In fact, I did this at the Pitot tube end and at the ADAHRS end.
Mike Huff has confirmed this with the engineers and the installation manual should be getting new guidance on the topic.
My supposition is that an all metal A/C would not see this problem even with all metal lines because the grounded airframe would carry any current back to the battery that did not go through the Pitot heat power return.
I would like to hear from any composite aircraft owners that have all metal lines from Pitot to ADAHRS about there experiences.
