skippydiesel Posted Saturday at 08:07 AM Posted Saturday at 08:07 AM 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. 1
Red Posted Saturday at 08:46 AM Posted Saturday at 08:46 AM Is this all from readings on that Dynon you've been having problems with? 1
Red Posted Saturday at 08:48 AM Posted Saturday at 08:48 AM (edited) 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 Posted Saturday at 10:17 AM Author Posted Saturday at 10:17 AM 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.😈 1
Moneybox Posted Saturday at 10:18 AM Posted Saturday at 10:18 AM 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? 1
skippydiesel Posted Saturday at 10:34 AM Author Posted Saturday at 10:34 AM 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).😈 2
Thruster88 Posted Saturday at 09:36 PM Posted Saturday at 09:36 PM 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. 1
skippydiesel Posted Saturday at 10:05 PM Author Posted Saturday at 10:05 PM 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 Posted Saturday at 10:12 PM Posted Saturday at 10:12 PM 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. 1
skippydiesel Posted Saturday at 10:40 PM Author Posted Saturday at 10:40 PM Rotax Owners Forum - suggestion it may be due to fuel vaporisation ?😈
IBob Posted Sunday at 06:16 AM Posted Sunday at 06:16 AM 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. 3
facthunter Posted Sunday at 08:10 AM Posted Sunday at 08:10 AM 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 Posted Monday at 01:06 AM Posted Monday at 01:06 AM (edited) 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 Monday at 01:07 AM by 3rd harmonic Proper noun capitalisation 1
facthunter Posted Monday at 01:28 AM Posted Monday at 01:28 AM Fuel temp getting too high? Extra fuel flow is a relatively poor way of cooling it. Nev
3rd harmonic Posted Monday at 07:26 AM Posted Monday at 07:26 AM 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 Posted Monday at 08:44 AM Posted Monday at 08:44 AM 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. 1 1 1
Red Posted Monday at 10:55 AM Posted Monday at 10:55 AM 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. 1
Thruster88 Posted Monday at 08:33 PM Posted Monday at 08:33 PM 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. 2
skippydiesel Posted Tuesday at 06:08 AM Author Posted Tuesday at 06:08 AM 21 hours ago, Thruster88 said: Pilots should be able to draw a schematic of the complete fuel system in the aircraft they are flying from memory. Bit extreme Thruster. I designed & installed my three tank fuel reticulation system. Along with engine supply, it has redundancy (tank switching) and transfer. From memory, it would take me a few goes to get it all down correctly.😈
Underwood Posted Tuesday at 06:32 AM Posted Tuesday at 06:32 AM (edited) 10 hours ago, Thruster88 said: 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. But if the outlet valve fails open then when the mechanical pump does the suck part then it can suck through both the Inlet and outlet, surely this has the potential to reduce overall pressure? https://www.rotax-owner.com/en/rotax-blog/item/77-rotax-912-fuel-pumps Edited Tuesday at 06:33 AM by Underwood
skippydiesel Posted Tuesday at 06:45 AM Author Posted Tuesday at 06:45 AM 2 minutes ago, Underwood said: But if the outlet valve fails open then when the mechanical pump does the suck part then it can suck through both the Inlet and outlet, surely this has the potential to reduce overall pressure? Naa! In the unlikly event of the (Facet) Boost pump outlet valve failing open, the Mechanical will continue to draw fuel, through the pump, without impediment. Same goes for the inlet. Should the Boost pump somehow fail closed - As recommended by Rotax, I have fitted a bypass system that will allow the Mechanical pump to continue to supply fuel. Should the Mechanical outlet/inlet valves fail open, fuel pressure will drop - Boost pump will restore supply Should either of the Mechanical valves fail closed - complete interruption of fuel supply - deeeep poo!😈
Red Posted Tuesday at 07:14 AM Posted Tuesday at 07:14 AM (edited) Skippy you have misunderstood, we are talking about the outlet valve in the mechanical pump failing open. I'm gonna leave this, busy today maybe Nev can explain P.S. its highly unlikely that is your problem anyway. Edited Tuesday at 07:15 AM by Red 1
facthunter Posted Tuesday at 08:37 AM Posted Tuesday at 08:37 AM Thruster is right. You should know your fuel system like the back of your hand. There's 2 subjects with a 10O% PASS Mark Required. W&B and Fuel Management. To bypass the EDP both valves must be open to allow the electric pump to flow fuel through it. IF either valve OR the spring, malfunctions the EDP won't pump. IF the electric pump is operating it keeps the EDP Diaphragm down with the spring compressed and not stroking. It's quite hard for the lightly spring loaded valves to fail closed But if your Bypass valve didn't seal your EDP wouldn't work properly, so I wonder at the wisdom of fitting it. The FIX creates another Problem. Nev 1 1
Thruster88 Posted Tuesday at 09:00 AM Posted Tuesday at 09:00 AM Normally there is only a bypass on the electric boost pump. I think this is there because an example the facet pump as fitted to my musketeer has a filter in it that is quite fine and could become blocked so an alternative path is provided to maintain fuel flow. The fuel pressure gauge allows us to continually monitor the health of both pumps during pre start (boost) and flying, EDP. 1
facthunter Posted Tuesday at 11:09 PM Posted Tuesday at 11:09 PM Filters are a problem in many fuel systems. Nev
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