N194SP has been having some electrical issues lately. I’m hoping we have it figured out now, but intermittent gremlins are always the worst kind. Along the way, it’s demonstrated both the strengths and a couple weaknesses of the G5s in the plane.
First, a little primer on how the G5s are powered. The top G5, called the PFD or AI, is powered off the main electrical bus. When you flip the red switch, you should expect to see it power on. The lower G5, the HSI, is powered off the avionics bus. When the white switch flips up, expect this G5 to come on. Each G5 also has its own backup battery plugged into the back. This battery, when fully charged, provides three to four hours of power to the unit. Of course, both units will also run off the plane’s battery in the event of an alternator failure as well.
That pair of backup batteries provides excellent peace of mind when flying with the units. The long backup time means you are likely to run out of fuel before you run out of battery during an electrical failure. As it turns out, this long batter run time also points out a weakness: the phenomenon of the latent fault. Failure of electrical power in the plane means the G5s will revert to the battery. A message will pop up, asking that you push any button on the unit, but after that, it will operate normally.
In the case where we had intermittent problems with the charging system, the G5s would revert to battery, depleting the backup battery. However, since the problem was difficult to find and the plane was returned to service, the battery didn’t always recharge fully. This led to a condition where the battery backup you thought you had wasn’t really there. It’s also happened that the G5s have remained on after the plane has shut down. It doesn’t take much: any touch of the controls on the G5 during the 45-second countdown to off will cause it to remain on until turned off manually.
I suggest two new checklist items for pilots flying my plane. First, during preflight with mains and avionics on, press the power button once on each G5. That will bring up the power display, which includes the battery life. If it’s not 100%, make a judgement about whether this is an impediment to your planned flight. Second, after shutdown, verify that both G5 go through their 45 second countdown and turn off. This will ensure that the next pilot is not faced with dead backup batteries.
Overall, though, the G5s are a huge step up from the vacuum-driven gyros that were there before. More reliable, longer lasting, more functional, and they even weigh less! Enjoy flying N194SP!
N194SP 