AHARS Fault on engine start

[geoff_g]

Last weekend I experienced an issue in my Sling 4 where powering up the G3X during preflight as normal was fine, AHARS aligned correctly and was good to go, but then as soon as the engine kicked over I had Red-X on multiple instruments (A/H, HSI, Alt, Airspeed) accompanied by an 'AHARS Hardware Fault' message. After shutting down and powering off everything, I had a successful AHRS alignment again, but yet again on engine start the fault condition returned. This happened three or four times ... until I had a successful start with no AHRS dropout, and couldn't fault it again through a couple more start cycles.

My question to to the group is firstly has anyone else seen anything similar on their setups? (I have a single GDU460 and GSU25)

I have a suspicion that this may have been caused by a voltage drop during start causing the GSU to drop out and not reboot cleanly, but I would have thought the IBBS acting as the EFIS BKP would have covered that? My startup sequence has always been Master, EFIS, EFIS BKP (wait for AHRS align) then fuel pumps and engine start (per the POH).

In reading some online G3X discussions, I found a suggestion to start up with EFIS BKP (AHRS align), then Master, Pumps, Engine Start and finally EFIS - i.e. the main power applied AFTER engine start, like all other avionics. The reasoning for this was that once the EFIS master is on, the power is supplied by the aircraft battery and the IBBS reverts to charge mode and doesn't keep the system powered if the voltage drops too far during engine start. Makes sense, but differs from our POH for the Sling 4 (and TSi I believe).

Thoughts from the group??