I’m planning on using the “bus manager” in my setup.....What I really like about it is it’s ability to manage automatic switching of fuel pumps 1 and 2 in the event of a fuel pump failure...
Someone really needs to settle the "Cannot run two electric pumps at the same time" question. If there is no issue with two pumps running, automatic pump switching (via Bus Manager or otherwise) is an unnecessary complication. The pilot simply switches the aux pump to ON for takeoff and landing.
First off I’m incorporating two separate circuit breakers (one for each fuel pump) in the essential bus. Each fuel pump will have its own relay but they will be wired so that either relay will be able to run either fuel pump. So basically, the (NC) position (87a) on each relay will be wired to pump 1 and the (NO) position (87) will be wired to pump 2. This way, if either one of the relays were to fail for any reason the other relay would be able to handle the job of running either one of the pumps. Now, the second two relays will power the first two relays. I know this sounds crazy, but bear with me. As I mentioned above, I’ll have a C.B. for each fuel pump, so each relay will have its own separate power which will feed the two relays that are actually operating the fuel pumps as discussed above. So these second two relays will just be on/off relays with no need for the ability to switch between (87a) and (87). These will both be normally open (NO) relays and will close when the “call wire” calls for them to be closed. So, as mentioned above, I’ll also be incorporating an oil pressure switch into the system. The “call wire” to activate pump 1 will be routed through this simple automotive oil pressure switch which will cut power to that relay in the event of an accident and sudden engine stoppage which will shut the fuel pumps off immediately and automatically. All modern day cars have this safe feature built in to them. Remember, pump1/auto is the “normal” operating position. I will have the “call signal wire” for the pump 2 toggle switch position wired so that when the toggle switch is in the pump 2 position, it will be able to energize the call wire and bypass the oil pressure sender. So basically, when the toggle switch is in the pump 2 position, it will allow for the priming/pressurizing of the fuel rail and allow staring of the engine when there’s no oil pressure present yet. Once the engine had stared and there’s oil pressure present, then both fuel pumps will work as normal with the ability to switch between them both manually and automatically. Remember though, this safety feature is only valid when the toggle switch is in the pump1/auto position, which is the normal operating position and its bypassed when the switch is in the pump 2 position. I hope all of that made sense and if anyone can think of why I shouldn’t design it like that, please let me know.
Ok.
Big picture...it's a complicated system to incorporate a "safety feature" (the oil pressure switch) which is largely useless in the practical view,
and brings its own failure modes to the party,
and adds operational complication. Let's start with "useless".
In general, there are only two conditions with zero oil pressure. You're either (1) out of oil, or (2) the prop (thus the pump) has stopped turning.
Assume the out-of-oil case...a popped hose, cooler, whatever. Why would you want to surrender control of when and where the engine shuts down? If it is running, it extends the glide. It won't seize immediately.
Assume any other reason why the engine has stopped making power. As long as the prop is windmilling, you'll have oil pressure. The fuel pump will run all the way to impact, unless you shut it off manually. Given that MASTER OFF, FUEL SELECTOR OFF, and/or PUMPS OFF are all part of a reasonable forced landing checklist, why an auto system?
Crash with power? The fuel pump will be running until prop strike; the system will be pressurized...just like any other fuel delivery system.
At impact, it's worth remembering you're not in a car. Even with the pumps stopped, the unfortunate reality is that we sit with 20 gallons or so at each elbow, and the tanks are not particularly impact resistant. Impact with 40 psi (or 25, or 3, or 0) doesn't change the overall fire odds very much, if at all.