Rotax 914F wiring diagram - fuel pumps

Resurrecting this topic to ask - we've just had a Sling 915 forced landing in South Africa. There are questions being raised about the electrical supply of the fuel pumps, and whether or not they depend on the Voltage Regulator functioning?

My impression was that the fuel pumps are fed direct from the generator and do not require a functioning voltage regulator? Am I wrong here?

Mike

The 915 has different systems then the 914.  The 915, like the 912iS, has two internal stators (Generators A & B) which provide AC voltage to two independent voltage regulator/rectifiers. The fuel pumps are 12V DC, so the voltage regulators ARE required for the fuel pumps to be powered by the internal generators, but they can also be powered directly from the airframe battery.

This next part has been stated several times on this forum, but I guess it bears repeating. 

During normal operation Alternator A provides power for the engine systems (including the fuel pumps) and is fully isolated from the airframe electrical system.  Also, during normal operation, alternator B is charging the airframe electrical system.  If alternator A fails, the engine systems take over Alternator B, so the fuel pumps and engine systems continue to operate indefinitely.  However, at that point, the airframe is operating on battery only and will slowly discharge as the avionics continue to use power. 

If the second internal generator was to also fail (generator B), then the engine would stop immediately. The pilot would then need to close the Emergency Battery Switch and restart the engine.  Closing the Emergency Battery Switch connects the airframe electrical system to the engine electrical system and allows the fuel pumps, as well as all other engine systems, to operate directly from the battery.  Of course, depending on the reserve capacity of the battery, fight time would be limited.  With a 16 AH battery you should get 20-30 minutes if you turn off all non-essential electronic devices. Some 915's also have the optional external generator, which could be switched into the system and provide essentially unlimited flight time in the event the two internal generators fail. 

Thanks Jeff. 

I wasn’t clear I realize - I was asking about the 914 setup - what happens if the voltage regulator fails in flight to the fuel pumps on the 914?

it is always best to wire one pump to the battery bus and one to the generator side after the capacitor in the system.  Each should have a switch for checking function.  If you fail your generators and or the regulator then you can operate on one from the battery.  if you fail your electrical side, battery goes offline, then the other will power from the regulator directly after the capacitor. 

Cheers