Why is the Backup Battery switch not connected to the battery?

Seriously considering moving my Backup Battery Switch and Starter Relay to the other side of the Main Relay, replacing the whited-out lines with new green lines. Normal start would remain normal, requiring the main relay to be engaged. But emergency starts would be possible even if the main switch or relay are disconnected or stuck open.

The answer in this thread indicates that not following Rotax's wiring diagram might be preferred, but there has to be a good reason to not do this. Thoughts?

Under ordinary circumstances that arrangement would present serious safety implications, as it would normally allow an engine to crank without the Master Switch closed (airplane dark).  Since the Fuse Box is powered through the Battery Master and Start Power switches, then the ECU is powered by the Fuse Box, then the ECU controls the Start Relay, that can't happen with this particular engine.

However, consider two scenarios:

1. Start Relay sticks closed: in this event, the starter cannot be shut off as it is hard-wired to the battery.

2. The wire loom from the ECU to the Start Relay shorts to ground: in this event you may have uncommanded engine cranking that cannot be shut off.

In either case, you'll need to rapidly remove cables from a battery terminal with the prop spinning around.

With regard to using this arrangement to facilitate an airborne emergency engine restart, consider the likelihood of experiencing a failure of Alt/Reg A and Alt/Reg B plus the Master Switch or Battery Contactor, all in the same tank of gas.  That seems like a pretty remote possibility.

Jason Fish wrote:

"But in that list, ONLY the Master Switch or Master Battery Contactor has to fail open to create a no-restart state."

True, but in order for an in-flight emergency restart to be necessary, both Alt/Reg A and Alt/Reg B must have failed; otherwise, the engine wouldn't have quit in the first place.  The likelihood of those two items and the battery switch failing on the same flight are vanishingly small.  If some other engine-stopping failure happens (ECU power loss or dual internal failure, both fuel pumps, 2 or 3 ignition coils, etc.), you're probably not going to get a restart anyway.

Interesting thing to think through...

Jason,

As Eric indicates in his response, once the 912iS is running, all of its electrical functions (including the fuel pumps) are powered by the engines alternator(s) in a fully isolated system.  A failed master switch or turning off the master switch will not stop the engine, so had your master switch broken in flight the engine would not even have noticed.  It is true that a restart would be inhibited by a master switch failure, but that would only be a problem after a cascading failure just as explained in the previous post.