Rotax 912 fuel pressure drop on boost pump shutdown

[avl]

Hello PPRune!

I'm wondering if anyone else has experienced a problem where the fuel pressure drops to zero for a few seconds when the electrical boost pump is switched off? This is in low-wing Rotax 912 aircraft.

My flying club has this problem on both our WT-9 Dynamic, and I've heard rumors that at least one other plane in Sweden has the same problem.

Is it occurring in other parts of the world as well?

[Fostex]

Have seen the same in a Tecnam Sierra with a Rotax 912s.

Not sure if it drops all the way to zero but there is a noticeable drop when you turn off the electric pump, this last for a few seconds and then returns to normal pressure.

[tecman]

I would say that a drop to zero merits serious investigation. In my Tecnam P2002JF (Sierra) I see variations of the order of an indicator needle width on the fuel pressure gauge. Pretty similar to electric pump on/pump off that I've noted in some other GA aircraft. I assume that both your aircraft have done this since new? If so, it might just be a 'feature' but it's definitely one to understand.

[mikehallam]

Thinking about the mentioned pressure display when the electric fuel pump stops.

Whilst the electric pump runs, its pressure exceeds & backs up to the mechanical pump which therefore 'knows' it doesn't have to provide fuel.

The a/c engine is running happily & when the boost pump is off, fuel demand continues - till the line pressure drops enough to be sensed by the mechanical pump which only then begins to replenish the system.
Ergo, a noticeable sag in pressure.

It possibly depends on several dynamics, including petrol pipes diameters, lengths & stiffness - with the basic pipe line diagram probably a pressurised 'Y' configuration. i.e. Two potential feed sources and one feed to the consumer.

[Generally Rotax insist on a small bleed line back to the tank which might or not affect the pressure profile seen.]

mike hallam.

[hobbit1983]

Standard behaviour IIRC on a rotax 912 powered AT3.

[mixsfour]

Depends how the fuel lines to the pumps are connected. If in parallel then I can understand the scenario described by Mike. But if in series then I'd be concerned if that happened whilst the engine's running.

The vapour return line is connected at the highest point on the system and returns any vapour bubbles to the fuel tank via an orifice which prevents undue reduction in pressure. Many Rotax powered aircraft are running mogas which has a higher vapour pressure than avgas and therefore higher risk of vapour bubbles forming which could prevent fuel flow, hence the insistence on having a return line.

When the engine is stopped and the electric pump switched off the fuel pressure should drop to zero after about 10 seconds. If it doesn't then that's an indication that the return line might be blocked (usually at the orifice) and needs to to be checked.