915iS Aux Fuel Pump Operation

All...

We were told by the engineers that the reason to run both pumps is that should your primary fail the engine will stop, instantly I might add.  This engine must have the pressure and without a pump there is never going to be any gravity feed that would keep it running.  In the event you only use one pump then you are faced with an inflight restart.  Given your aircraft design and flight level you then have a lot of other considerations.  As you must, and should always, check both pumps before flight then the safest is to run both in my opinion.  The pumps have to be changed at intervals regardless, I do not know of any pilot that tracks what time is on each single pump.

Cheers

I'm going to put my 2 cents in. In a Sling tsi there are 3 pumps, 2 rotax on the firewall and a boost pump in the console forward of the fuel valve. The circuits for both pumps are not the same. I used to turn pump 2 off, but quite often the pressure would drop low enough that the Caution light would turn on and sometimes the engine would stumble. I discovered that the pump 2 switch also controls the boost pump under the console. I now turn off pump 1 while in cruise and use it for a backup. I use pump 2 as the primary (which includes the boost pump) and the pressure never drops very far. I think what's happening is that if you turn pump 2 off then the fuel must be pushed through the non-operating boost pump which creates a great deal of resistance and drops the pressure too far. I don't know about other planes. 

All, regarding pump designs.  The Rotax fuel pumps are a vane pump design.  That said when they are shut off no fuel flows past the pump and that is way they have bypass check valves in the fuel circuit design.  Perhaps Sling can tell you what that 3 pumps design is.  

Cheers

This is a re-post from my aircraft owner's group:

A quick update to this topic after our longer and in-depth talk to Rotax at AERO 2025 this week:

According to them there is no real argument for switching off the AUX pump in flight (for iS engines that is!).

Here are the main arguments for having both pumps on when airborne:

1. Only with both pumps on you have redundancy. Switching one off is similar to switching one ECU Lane off. Possible, but less safe than with full redundancy.

2. In-flight AUX pump switching as part of the SOP (standard operating procedures) introduces a risk of operator error like switching the AUX pump off when the main pump is also off, for any reason.
   AFAIK there has been at least one accident caused by exactly this, which can be avoided entirely if none of the pump switches is regularly touched in-flight.

3. Having both pumps run at the same time reduces the strain and wear on each pump, since they “share the load”. Actually each pump only draws about 2/3 of the power they draw when running alone.
   So running both at the same time should actually lengthen the MTBF and reduce the risk of a pump failure.

Rotax has assured us that there is no real downside to having both pumps on:

• Since the iS engine ECUs decide how much to inject there is absolutely zero effect on fuel usage (apart from the minimally higher current draw).
• The two pumps are 100% identical and rated as “constantly running” pumps for at least 2000 hours. So, normally they should definitely outlast the 915 iS’s TBO of 1200h.
• With both pumps on there is a higher fuel flow through the feed and return fuel lines, which cycles slightly more fuel through the whole system, causing better cooling / temperature distribution to all components.
• The pumps don’t like “sucking” fuel as much as they like “pushing” it. Therefore Rotax recommends the pumps to be installed lower than the tanks to provide for some static pressure on the inlet side. This is the case in our (B23) setup.
  However, especially with high pitch angles, high fuel flow and higher temperatures (like summer take-offs and climb-outs) the low pressure on the inlet side of the main pump can sometimes cause vapor bubbles to form, which can cause intermittent low fuel pressure warnings.
  With both pumps on the risk for this to happen should be much reduced.

We asked about the argument that, with both pumps always on, they would reach a potentially unknown but fixed “max age” at exactly the same time, fail catastrophically both at once, and thus cause death and destruction.
The look on the face of the very knowledgeable and experienced gentleman we talked to said it all.
It’s not going to happen.
At least not in this universe.

So, we all (who were there) decided to change our SOPs accordingly.
No more switching off the AUX pump in flight.