Rotax 914F wiring diagram - fuel pumps

Ken Ryan wrote:

The Rotax Operator Manual for the 914 advises to use both pumps for takeoffs and landings, and shut off the aux pump at safe altitude. The Operator Manual is available on this website or at the official Rotax website.

It is best to wire the main pump directly to the generator regulator output, without a switch, making it impossible to accidentally turn off the main pump.

There is no connection between the mag switches and the fuel pumps.

Yes you are correct, the operator manual does say that but how can you turn one pump off once safe altitude is reached if the pumps do not have a switch? You could by pulling the breaker but let's say that it is not a common standard practice. 

My apologies for the mag switches, agreed that there is not connection between the pumps and the mag switches.

The reason I stated " Assumed to be Switchable Breakers" was not because of an arbitrary assumption but because of the Schematic Symbol used.
46, 47, and others are technically shown as  "Isolator Switch Disconnector" Breakers.  42 is shown as a basic "Fuse".

If these were all just breakers, turning ON the Master Switch would power up Everything without any practical way to isolate anything.
It is reasonable to assume that a "Breaker" can be Reset.  It is Not reasonable to assume that all Breakers can be "Pulled". Some can Not!

Bill Hertzel wrote:

The reason I stated " Assumed to be Switchable Breakers" was not because of an arbitrary assumption but because of the Schematic Symbol used.
46, 47, and others are technically shown as  "Isolator Switch Disconnector" Breakers.  42 is shown as a basic "Fuse".

If these were all just breakers, turning ON the Master Switch would power up Everything without any practical way to isolate anything.
It is reasonable to assume that a "Breaker" can be Reset.  It is Not reasonable to assume that all Breakers can be "Pulled". Some can Not!

Hello Bill,

Thank you so much for your input and for sharing the information regarding the symbol used, that is something I learned as I was not aware of them, really appreciate it.

Now, I completely understand everything you are saying and agree to it but I am still not 100% sold on how this should accurately be done.

As you said 46 & 47 are technically shown as Isolator Switch Disconnector breakers. What are these? Regular circuit breakers we use such as the Klixon 7277 for example? If so, then we go back to having both fuel pumps connected to a regular "pull out" circuit  breaker which can be pulled as necessary in emergency and so on; this configuration will leave you with two pumps running all the time which is not in accordance with the operators manual that tells you to turn OFF the aux pump once at safe altitude; surely you could turn the aux pump OFF by pulling the breaker but I cannot believe that Rotax considered this to be a standard practice. If, on the hand these isolator switch disconnector are switch type breakers (such as W31-X2M1G-3 for example) then they will allow the fuel pump to be switched ON or OFF but according to the symbology used on the diagram they will also make all of the other breakers switchable (42,43,44,45,48,49...) which is definitely not correct I believe.

It appears that either way something is missing.

I hope I was clear with my explanation above; I would really like to have a full understand of this diagram since I think it should not be underestimated.

 Again thank you for your patience and I really appreciate your valuable inputs!

I would have no problem with all the breakers being switchable.
It is always better to have the option and not need it than need it and not have it.
42-45,50 could be located less prominently, but 46 and everything on the main bus should be readily accessible.

The Fuel Pumps, TCU, and Avionics are all items that you may need/want to disable/reset at some point.
Being able to isolate the battery and generators individually might save your bacon one day.
Unlikely, but at least the option will exist.

Rotax makes some pretty good engines but their documentation is sometimes a bit lacking.
I can see where some draftsman just picked a switchy-breaker-looking symbol and ran with it.

Hi all

First, glad that Bill is much better on electronics than me.  I do know that on the 914 we need 2 pumps for reasons of having enough pressure for full take off boost.  (should be in series) Yes can switch off one pump when at altitude if you like but for me I don't see the point.  Having one pump marked AUX is just the arbitrary name for any extra pump in the system from general aviation I think.  You do need to have at least one that you can turn on and off for checking that you have both functional before take-off in your ground run.  

Bill hits on a very good point, the diagram in the installation manuals are not really real world wiring diagrams that you might make for your particular aircraft.  Take them as suggestions and get someone who is knowledgeable in aviation wiring and the engine to make up something that works for your panel.  Most OEM already do that and for the most part they are reasonably good.  Having one pump hard wired to run in this case should be considered like your mechanical pump, always on.  A breaker is a safety device and not really a switch in my view. I would want to have access to that breaker if needed. The TCU is one that you most definitely want a switch for, it is more likely that you will use this should anything go wrong with the control of the wastegate in flight.  (hunting boost for example) the ability to switch it off locks the waste gate in position. 

Cheers