Weird Fuel Pump Problem

[N666LG]

I have a 2009 SR22 Turbo (TN) with a peculiar fuel flow issue. This issue can be replicated on the ground as well as in flight. Typical LOP cruise is at 30 inches MP, 2500 rpm, 17.5 gph. If I set this up with the fuel pump in low boost (POH calls for first 30 mins of cruise flight with pump on) when I turn the pump off the fuel flow goes UP to about 19gph. If I then reset the mixture to give 17.5 gph again and turn the pump on the fuel flow goes down about 1.5 gph. Switching tanks gives the same result, the pump makes the usual sound on the ground with the engine off and the observed fuel flow (engine off) looks as it's always done. High boost does what it should. Everything else engine wise seems normal.

Any help appreciated.

Thanks
Steve

[IO540]

I don't know the SR22 fuel system but don't Continental engines have a bypass back to the tank?

If the engine delivers the book performance and runs fine then that suggests the indicated fuel flow is wrong.

But it would suprise me to find a plane whose flow indication includes the bypassed figure. It would be slightly useless, especially if the indicator is derived from a fuel totaliser (a turbine flowmeter) whose accuracy is normally about 1%.

But then I have no idea how the totaliser makers deal with engines that have a bypass, apart from installing the transducer after the pumps.

POH calls for first 30 mins of cruise flight with pump on

I wonder why?

[stevef]

I'm not familiar with your particular aircraft type but some event in my mind from long ago suggests partial injector nozzle or nozzle line blockage. A symptom of restricted flow is high fuel flow indication and the boost pump selection may be supplying enough pressure to pass the obstruction and give 'normal' gauge readings.
Just a thought.

[rans6andrew]

the wee turbine devices that are often used to measure the fuel flow are notoriously prone to miss-reading for the oddest reasons. I have seen issues caused by a simple bend in the fuel pipe leading up to the flow sensor, apparently the fuel flow in the pipe can become turbulent after a bend and this throws the sensor off. Also, if the electric fuel pump causes fluctuations or pulses in the fuel flow this can disturb the accuracy of the sensor readings. Another thing that we have found is that if the sensor is firmly fixed to something that vibrates the readings are thrown off and if it is mounted in an unsupported length of pipe it may vibrate at some rpms and this throws the accuracy off. In all of these cases the reading may be thrown off to a higher or a lower reading than in the optimum, smooth flow, straight vibration free situation.

If the reading is taken from the fuel metering part of a fuel injection system then all of the above is irrelevant.

Do the T's and P's change to suggest that the mixture goes richer when the fuel pump is switched off? It may be some other instrument error, perhaps the voltage in the panel changes when the load of the fuel pump is removed from the bus.

Rans6....

[IO540]

I have seen issues caused by a simple bend in the fuel pipe leading up to the flow sensor, apparently the fuel flow in the pipe can become turbulent after a bend and this throws the sensor off.

Yes; very much so. I spent a lot of time sorting mine out. The Flo-Scan transducer needs a nice smooth pipe run on the way into it. If there is turbulence, things like the electric fuel pump can affect the indication considerably.

However I would be suprised if Cirrus had installed it incorrectly in the factory, which is why I did not raise this earlier.

Socata did this on maybe 100 planes, relying on the ignorance of most owners to not realise it.

[Kolossi]

Way way back when I was a power station C&I engineer, I did a course on flow measurement. I've had a look but can't find the notes just now. It was emphasised how much effect a bend or pipe diameter change could have on the accuracy of measurement.

There was a rule of thumb that there was a surprising amount of distance required from a pipe bend/corner/elbow and something like that the flow meter had to be 10x (or was it even 20x?) pipe diameter downstream from any bend BUT ALSO 5x diameter upstream from a bend (ie sensor being BEFORE a bend would also be inaccurate). More than that if you had two bends in quick succession, you need to be even further downstream from the corner (maybe that was where the 20x came in).

Basically for it to be accurate it had to smack in the middle of a long straight, constant diameter pipe. There's about a gazillion types of flow measurement devices all with different susceptibility to this issue, and I don't recall for sure where turbine flowmeters came in but I'm pretty sure they were significantly effected.

So in other words... what IO540 & rans6andrew said

[IO540]

At least 20x pipe diameter, although a long gentle bend is apparently OK. Some notes here.

[N666LG]

Thanks for the input, I've already spoken to Tornado Alley and my own maintenance co. and they've never come across this before hence the question. What I didn't mention was that this has only started happening after its annual, there were some mild changes made to the fuel flow on the mechanical pump which is the nearest thing to the problem one can imagine, yet no fathomable reason seems to fit. The flow turbulence through the flow transducer sounds like a good idea as does the voltage issue but when the flow drops as the pump is turned on the engine demonstrates the usual effects of too lean a mixture so flow IS being reduced, it isn't just a gauge issue. The electric pump is the first thing the fuel meets after the tank selector, then it's the mechanical pump, all unmetered at this point. Have considerered the pump being faulty in some way but not sure how? Perhaps I shoud just swap it and see what happens.

This one is out there so any more thoughts still welcome.

Cheers
Steve


IO540 The low boost pump is left on for 30 mins to avoid any vapour probems in case of warm fuel, some just leave it on all the time. There is also a high boost setting for use above 18000ft for the same reason.

[Mechta]

POH calls for first 30 mins of cruise flight with pump on

Given that the Cirrus is used for high altitude flying this could be to allow the fuel to de-gas (release dissolved air and other gases). 30 minutes is the sort of time it takes for Jet A1 to do so (albeit when taken to a higher altitude than the Cirrus is likely to go). I've not done it with Avgas, but I would suspect the same effect occurs but at a lower altitude.

If you pull fuel uphill with an engine driven mechanical pump, you are far more likely to get dissolved gasses drawn out of solution than if you have a pump in the tank pushing and one on the engine pulling.

If it were possible to incorporate a transparent tube after the engine driven pump for a ground test, you could see if you get bubbles when the engine-driven pump is working alone.

You can get all sorts of odd effects due to vibration. At one place I worked, we did a test with a fuel probe in a transparent tank on a vibration table. By simulating the vibration change at the top of the climb as the engines throttled back, it was possible to get what can only be described as a 'black hole' in the fuel into which bubbles were drawn. Minor adjustments of the vibration level would cause the 'black hole' to climb or descend in the fuel.

If the fuel pipes are routed through soft mounts, I would check that nothing is now touching an undamped part of the airframe or engine which might not have been before the work on the fuel system.

[N666LG]

Update

Cirrus have just come back to me and said it's perfectly normal???

[IO540]

Is this a fuel flowmeter, or is it a turbine transducer which is a part of a totaliser linked to your GPS?

If the latter, I suggest litigation may be an option to consider

[N666LG]

IO540 - There is a transducer that the fuel physically flows through, whether it is a turbine inside or not I cannot tell, but it is connected to the MFD on the Garmin Perspective and provides a total fuel situation. There are also two analogue gauges that provide fuel level but are grossly inaccurate. The engine responses, EGTs, CHTs etc all respond to indicate the changes in the fuel flow. Could you expand slightly on your post.

[IO540]

To see if it is a turbine totaliser, have a look at the pics in the link I posted and see if you recognise the transducer.

It is meant to be accurate to 1-2%.