Backup & Start Power at Same Time

Hi Gary

This is a question better answered by Kitfox.  If they are using a relay via the start power it may already have addressed that.  

Cheers

I agree with RW that you should understand why Kit Fox does it this way, however:

The backup battery switch and the start power switch each energize a different pin on the X3 connector.  They each do almost the same thing, but through redundant pathways. I say almost the same thing because it is my understanding that when you use the backup battery switch, some of the normal ECU checks are bypassed so an emergency restart is faster (I think RW told me this in an earlier post). You could use these switches interchangeably, except that a restart would take a bit longer using the start power switch, and using the backup battery switch for a normal start is not recommend because some self-checks are bypassed.

I think what you are saying is that Kit Fox uses one double throw (3 position) switch instead of two individual single throw switches. I can't think of a time where you would need both on at once, but I still like the idea of two individual switches for these functions. Using a single switch for both creates a single failure point to an otherwise redundant system. And this switch is used in every flight, so it's not out of the question that it could malfunction.  And yes, typically the start button is independent of these two switches for the reason you stated.   

Thanks, that confirms my research afterwards. In the end I'm going to go with a single DPDT switch, understanding it's a single point of failure for start power & backup. I'm ok with that as I'm using a MIL spec switch that's pretty resilient, and I'd stand it up against two independent cheaper switches that many people install.

For anyone else I found a really nice switch, 2TL1-5M from Honeywell. It's a 3 position DPDT switch so you can switch ground and power to Start Power or Battery Backup. It's also MOM-OFF-ON, so Start Power can be momentary, while Battery Backup stays switched on. The on side (battery backup) has a lock out and lock on so that it can't be knocked to being on, nor can it be knocked back off if you are using it an emergency. Perfectly fits this need, other than the price and argument about single switch.

On a side note, they also make another great switch, 1TL1-2F, for Lanes and Fuel Pumps that can be switched on, without a lockout, but are then locked on, so you can't inadvertently turn them off.