Fuel Flow Rate Conundrum

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caseyayers
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Fuel Flow Rate Conundrum

Post by caseyayers »

I've had an issue develop recently on my DA40 - beginning at full throttle during takeoff, the fuel flow rate number as shown on my G1000 MFD spikes and varies significantly, from ~19GPH up to the mid-40s. Fuel pressure is nominal like normal, and the fuel totalizer, tank gauges and actual fuel burned based on filling the tanks back up myself are all no different than normal. This issue begins to abate as I reach top of my climb, and I recognized on a recent flight that if I pull the prop RPM back from 2400-2450 to 2200 once at cruise, the issue immediately goes away entirely.

Is this likely a bad sensor, or might something else be to blame? Any thoughts on why the problem would go away immediately upon lowering the prop RPM? Thanks for your thoughts!
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Re: Fuel Flow Rate Conundrum

Post by smoss »

first thing to try would be to disconnect the sensor connection, clean the contacts, and reconnect. Intermittent or poor contacts often give wonky G1000 values, and might be exacerbated by different vibrations of the climb vs cruise, or RPM's.
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Re: Fuel Flow Rate Conundrum

Post by Boatguy »

If it's not the sensor, then it's the real flow at the sensor in which case there may be an intermittent blockage somewhere (e.g., slightly fouled fuel filter) or irregular output from the fuel pump.
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Steve
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Re: Fuel Flow Rate Conundrum

Post by Steve »

I cannot imagine a real fuel flow of 40 GPH in an IO-360 (unless the fuel line before the RSA controller is wide open. However, an intermittent connector at the flow sensor (as Steve postulated), or a bad sensor itself could account for this behavior.
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Re: Fuel Flow Rate Conundrum

Post by Boatguy »

Maybe I don't understand how these sensors work. In my experience with water flow sensors there are little paddle wheels that modify a voltage. If the connection to the sensor is interrupted it causes the readings to drop and resume, but it can't cause a higher reading than the sensor is seeing. However, sometimes the readings are damped or interpreted by software which can misinterpret a gap or fluctuation in the data. Possibly the G1000 is doing that.

In any case I agree that checking the sensor connection is an easy first step.
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Steve
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Re: Fuel Flow Rate Conundrum

Post by Steve »

This is the type of flow sensor in my airplane (VM1000):

http://www.floscan.com/html/blue/aviation.php

It sends a train of pulses to the instrument which is proportional to the flow rate. I'm not sure about the type of sensor in G1000-equipped aircraft...

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Re: Fuel Flow Rate Conundrum

Post by Boatguy »

The spec says the pulses are directly proportional to the rate of flow, more flow = more pulses. So then it seems to me that interrupting the connection would be result in less pulses being received and be interpreted as a lower flow rate.

I agree with you that the IO-360 is not going to have a sustained flow of 40gph! I just can't figure out how the sensor can generate more pulses without having more flow, at least momentarily. But maybe the problem is in the way the G1000 is interpreting the data from the sensor and an intermittent connection is messing with its algorithm. Or as you suggest maybe the G1000 airplanes have a different sensor.
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Re: Fuel Flow Rate Conundrum

Post by Steve »

A bad connection can generate electrical noise (which could mimic pulses) at the connector from engine vibration. The other possibility is that the phototransistor in the sensor is marginal/failing and that is intermittently going into oscillation. I know that the VM1000 software has a smoothing algorithm for the FF data, not sure about the G1000.

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Re: Fuel Flow Rate Conundrum

Post by caseyayers »

For posterity's sake, in case anyone searches this thread in the future with a similar problem, it was a bad transducer to blame. Everything's ship shape now. As a data point, the part cost was $466.29. I got charged a brutal 6.75 hours of labor for the change. Not sure if it's genuinely that difficult or if my shop just lacks DA40 familiarity.
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Re: Fuel Flow Rate Conundrum

Post by Steve »

caseyayers wrote: Tue Feb 18, 2020 8:43 pm For posterity's sake, in case anyone searches this thread in the future with a similar problem, it was a bad transducer to blame. Everything's ship shape now. As a data point, the part cost was $466.29. I got charged a brutal 6.75 hours of labor for the change. Not sure if it's genuinely that difficult or if my shop just lacks DA40 familiarity.
Casey, thanks for the followup. I can't imagine it taking more than 2 hours. 1) Uncowl the airplane. 2) Remove the firesleeve which surrounds the sensor (it is at the bottom front of the engine). 3) Disconnect the electrical plug. 4) Disconnect the 2 fuel lines. 5) Remove the sensor. 6) Reverse the above steps to install. 7) Leak test the unit. 8 ) Possibly enter the K-factor. 9) Document maintenance in the logbooks.
DSCN5887.jpg
The sensor is behind the firesleeve, between the two blue Ty-wraps.
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