Random Low Fuel Pressure Indication.

[skippydiesel]

Sonex aircraft, Rotax 912 ULS, plumed fairly conventionally. Rotax mechanical diaphragm pump 150 hrs. Facet Boost pump with bypass circuit (includes non return valve)

 

This might test out the brains trust.

 

Usually in Cruise, sometimes on Descent I get a low fuel pressure warning. On occasion, indication as low as 1.5 psi. No change in engine note/performance. Sometimes it corrects itself over a few minutes , other times I switch on the Boost pump for immediate fix.

 

Note: I have already mounted the fuel sensor as high as I can within the engine bay, removed the silicon seal to facilitate changes in atmospheric pressure and drained the fuel pressure line.

 

On the way home from Holbrook,  the low fuel pressure warning sounded and pressure dropped to 1.5 psi, well below the Rotax 912 ULS minimum of 2 psi and the usually 4-5 psi.

 

I then noticed, the Fuel Flow had risen from 15 L/hr to 25 L/hr.

 

At this point I switched on my fuel Boost pump - immediate rise to 5 psi AND the Fuel Flow dropped back to 15L/hr - switched off the pump and pressure/flow remained where expected.

 

The same scenario happened  again a little later.

 

To me drop in pressure combined with a rise in flow is counterintuitive. This may happen with a garden hose but not a closed fuel system.

 

What might the Brains Trust think is happening.

[Red]

Is this all from readings on that Dynon you've been having problems with?

[Red]

Quote

To me drop in pressure combined with a rise in flow is counterintuitive. This may happen with a garden hose but not a closed fuel system.

Not really, it would happen in certain situations like a leak downstream

But that wouldnt explain what happened when you switched on the Boost Pump, Id be suspicious that this is all false readings

Edited Saturday at 08:50 AM by Red

[skippydiesel]

Almost certainly false readings - but how & why?

 

Yes its the Dynon Skyview ADAHRS (Air Data/Attitude/Heading Reference System) system that has been playing up. The fuel pressure is part of the EMS (Engine Management System). Mind you both systems are "read"/displayed by the SE SV/D600 screen.😈

[Moneybox]

Skippy, where is your fuel flow measurement coming from? It sounds like there is an intermittent bypass there allowing more fuel to return to the tank. What in the system would usually limit that return flow?

[skippydiesel]

2 minutes ago, Moneybox said:

Skippy, where is your fuel flow measurement coming from? It sounds like there is an intermittent bypass there allowing more fuel to return to the tank. What in the system would usually limit that return flow?

I agree with your logic but unfortunately not with its application.

 

Fuel flow data  is from two Red Cubes (fuel flow sensors). One on the main delivery line between mechanical pump & carburettors and one on the return to tank line. The return line Cube data is subtracted from the main line Cubes data to give  fuel used/fuel remain information to the pilot. There is also a fuel flow readout, that helps in engine power setting.

 

Fuel pressure is registered by a sensor, located at the end of dedicated line, coming of the fuel distributor.

 

The distributer, takes fuel from the pump, delivers fuel to each carburettor, the tank return and the deadened (no flow) pressure sensor line.

 

The return line has a fixed restrictor jet.

 

As far as I can see there is no opportunity for an increased flow, other than opening the carburettor throttles (higher consumption).😈

 

[Thruster88]

One possible cause, the engine pump has an intermittent fault in the inlet  valve. When the valve acts up the pressure naturally drops. The increase in indicated fuel flow is due to fuel pulsing back and forth in the flow meter. If the flow meter is after the engine pump then the outlet valve could do this.

 

Any time the boost pump is turned on in steady cruise flight the fuel flow INDICATION dramatically rises in my RV6a, has to be due to fuel pulsing. Fuel will pulse with a faulty valve.

[skippydiesel]

Thanks Thruster - The difference in indication between your RV & my Sonex is - Boost pump on, fuel flow reduces/goes down. The opposite of your indication.😈

[Thruster88]

3 minutes ago, skippydiesel said:

Thanks Thruster - The difference in indication between your RV & my Sonex is - Boost pump on, fuel flow reduces/goes down. The opposite of your indication.😈

In your case the boost pump is possibly restoring a smooth fuel flow, masking a fault in the engine pump if that makes sense.

[skippydiesel]

Rotax Owners Forum - suggestion it may be due to fuel vaporisation ?😈

[IBob]

I return to my original suggestion:
You have either periodic loss of fuel pressure, or periodic incorrect fuel pressure sensing.
I would find out which by temporarily fitting a steam gauge. That cuts the problem neatly in half and points you to where to start looking: instrumentation or fuel system.

[facthunter]

I go along with the Rotax forum suggestion.. Your bleed orifice could also flow too much. Your engine Pump doesn't handle it. . Nev

[3rd harmonic]

One of the Sporty's with a 912ULS at Aldinga had a partial engine failure reduced power through lack of fuel flow.

The return spring on the diaphragm pump was the issue, the spring could be broken/cracked and still work somewhat.

They do a lot of hours and the pump may not have been changed even when the engine was changed due to expired TBO.

One of the penumatic grease guns we use at work (broken spring) had similar symptoms, it would work sometimes?!

Edited 21 hours ago by 3rd harmonic
Proper noun capitalisation

[facthunter]

Fuel temp getting too high? Extra fuel flow is a relatively poor way of cooling it. Nev

[3rd harmonic]

Looks like my memory wasn't exactly accurate, found on the RAAus accident/occurrence page:

OCC0871 28/12/2016

OCCURRENCE DETAILS SUBMITTED TO RAAUS: DEFECT: Failure of the outlet valve in the fuel pump caused a loss of power on take-off. Take-off aborted and aircraft landed on remaining runway. Part of the valve is pressed into the body of the pump. The part pressed into the body came out after - the outlet valve remained open which caused a drop in the fuel pressure. Note: the standby electric pump was 'On' however also lost pressure through this failed valve. OUTCOME: This aircraft is an LSA and as such this defect was reported to the manufacturer. Technical Manager reviewed the aircraft logbooks which was provided by the maintainer. The aircraft logbook showed that it has high hours and appears the fuel pump has failed due to fair wear and tear.

[Thruster88]

One defective open valve in the engine pump cannot cause the electric boost pump to fail as well. Where did the fuel it was pumping go?

 

Pilots should be able to draw a schematic of the complete fuel system in the aircraft they are flying from memory. 

[Red]

2 hours ago, Thruster88 said:

One defective open valve in the engine pump cannot cause the electric boost pump to fail as well. Where did the fuel it was pumping go?

 

Pilots should be able to draw a schematic of the complete fuel system in the aircraft they are flying from memory. 

 

Of course It wont make the boost pump fail, but If the outlet valve remains open during the suction stroke then it will suck fuel back through the outlet thus reducing pressure overall.

[Thruster88]

9 hours ago, Red said:

 

Of course It wont make the boost pump fail, but If the outlet valve remains open during the suction stroke then it will suck fuel back through the outlet thus reducing pressure overall.

I think we can agree that any fuel pump in a carburetor aircraft will contain two check valves that only allow fuel to flow towards the carburetor. Regardless of the arrangement of the two pumps, in series or parallel, one open check valve in either pump will not cause a loss of fuel flow if the boost pump is on.