Rube Goldberg would be proud.

There are a myriad of ways that could go bad. It doesn't take very many PSI before you blow a wing skin off for example.

Also, when a wing gets heavier, it is easier for the more empty tank to transfer since it is uphill, so it is unstable to a certain degree.

If you build some sort of leveling system by controlling pumps, then it becomes dependent on the fuel level sensing system to balance, so that introduces a dependency and new ways for things to go wrong. The timed flip flop switching is simpler, doesn't require sensors.

An issue if you have a fuel imbalance. You have to override the fuel cycling and force it on one tank, but don't do that too long or you are out of balance again the other way.

On twin jets, ejector pumps are used to transfer fuel from one side to the other in case of fuel imbalance or extended engine out operation. In normal use, they are rarely used, both engines draw equally from each tank and the two sides are kept separate. Ejector pumps are light weight and never need servicing (basically just a venturi).


You basically described the MU2 fuel system. One center tank from which both engines draw, transfer from other tanks, tips by bleed air (they are round so they can take the pressure), outers by electric pump.

Mike C.

Dang, hadn't heard that. I assumed fuel would draw from both sides simultaneously and have a transfer pump to use when necessary. Max standard fuel imbalance in the Mustang is 200 lbs. with max demonstrated of 600 lbs. I would guess it's less in the SF50. With a fuel select valve failure, you're not going to have an abundance of time to deal with that because it's likely to be a fair amount out of balance before you catch it.

TBM does the same thing every ten minutes, IIRC. I think it's a certification requirement that there be some system to prevent forgetting to switch tanks.

Another example where the SF50 is vastly more complex than the SR22.

The SR22 pilot switches tank every 20-30 minutes or so in cruise by using a manual fuel valve.

If you did the same thing, a manual valve, for the SF50, you'd be 200 lbs out of balance and you'd be way out of trim. But you don't want the SF50 pilot having to switch tanks manually every few minutes. So you have to design an automatic system, which they did. But now the automatic system can fail, so you have to provide a procedure for that and monitor for the failure. Plus you have to be able to override the automatic system, so there's a procedure for that. Plus the automatic system cycles so often, you potentially cause premature wear out.

Meanwhile, the twin jet pilot is doing... nothing, even less than the SR22 pilot.

Mike C.

I don't think there is a certification requirement (witness SR22 manual fuel valve, FAR 23 aircraft also).

I think it naturally comes about as a practical matter. You want to switch tanks often enough that roll trim is not affected, but you don't want pilots to have to do it manually on an airplane that burns several hundred pounds an hour.

Thus, you need some system to draw evenly. With fuel in low wings, you simply can't do "both", and thus the complexity starts.

Mike C.

I don't think there is a certification requirement (witness SR22 manual fuel valve, FAR 23 aircraft also).

Nah, SF50 G2 will likely go with a more common/traditional center tank fed by both wings via gravity feed using static air pressure to balance the head pressure of both tanks.

There are many ways to solve it. The above is just one solution I know works really well from the Aerostar. I am sure Cirrus will come up with their own spin, especially if they find a reliability issue with the vendor part.

Tim

I doubt they will slay that dragon with a gravity or pitot pressure solution. The TBM has been around for a long time with the motor driven selector system,,,,

Certification requirement for SE turbines. As mentioned, TBM does it the same way, Meridian and Pilatus have a different system to the same effect, I don't know about others.

Nah, you know Cirrus does not do spins.

Nah, you know Cirrus does not do spins.

If you have to have a fuel system where you need to change tanks all the time, you've designed the system wrong. Just look at what Ted Smith did and copy that.

How does the fuel system work in an Aerostar?