Failed Fuel Flow Instrument

[Jaybird180]

I'm trying to learn something about fuel systems. Can I get some answers to these questions?

1. What are some of the causes of fuel flow instrumentation failure?
2. When they do fail, do they give a warning or do they just die?
3. Are there any systems that employ measuring devices other than a transducer?
4. What is the weakest link in the measurement system?
5. What is the expected MTBF for the system?
6. Have yuu ever experienced a measurement system failure? How did you detect it?

 

[Jim Carpenter]

1. Wiring faults, transducer issues, gauge malfunction. Leakage could cause bad readings, but that's not really a fuel flow instrument problem.
2. Depends on the problem, wiring can give intermittent "twitching", transducer could read improperly, gauge can act up when cold but be fine after cockpit warms up, etc.
3. Can't imagine how that would work. Most transducers (if not all) are simple paddlewheel type devices, that produce either analog or digital signal, sent through wires. (note: different than a fuel pressure instrument)
4. I'd guess wiring, rubbing, loose connectors, etc., might be top of the list.
5. no idea

 

[thomasdr72]

#1: check the wiring at the transducer, the original installations for years used connectors that easily caused intermittent/failed Operation. Either solder the wires, or used the new OLC connectors.

#2: the next step Is replace the transducer. They rely on very delicate jeweled movements at the impeller, and sometimes they just go bad. (I’ve heard of procedures to clean them, but I haven’t tried them myself). A new transducer is about $200.

V/r,

 

[Bell206]

1. What are some of the causes of fuel flow instrumentation failure?
2. When they do fail, do they give a warning or do they just die?
3. Are there any systems that employ measuring devices other than a transducer?
4. What is the weakest link in the measurement system?
5. What is the expected MTBF for the system?
6. Have yuu ever experienced a measurement system failure? How did you detect it?

Not enough info to give accurate answers. Depends on type of failure (no reading, low reading, high reading, etc), type of flow system (pressure, vane, etc), type of fuel, and in some cases aircraft/brand/OEM. But here are some general answers:
1. From broken wires and inop components to clogged flow ports. There are a lot of variables.
2. All the above
3. Depends on how you define transducers. Some employ sensors and mechanical devices.
4. Depends on the flow system type.
5. Have seen a number of failures and fixed a number of failures. The failure mode varied from outright loss of display to inaccurate readings.

 

[GRG55]

I'm trying to learn something about fuel systems. Can I get some answers to these questions

3. Are there any systems that employ measuring devices other than a transducer?

Yes. The fuel flow gauges in my Aztec use the fuel pressure at the fuel injection distributor.
This info won't be of any help to the OP, other than technically answering the one question posed.

The Piper documentation is useless for explaining how it works, but it appears the pressure at that sensing point is compared with an atmospheric vent on the gauge, which difference drives the analogue needles (the only moving part in the system).

When I bought the plane I thought it was a pretty hokey set-up and planned to send in the already installed JPI to add the fuel flow option. But numerous checks against tank refills show the gauge (plus a watch) to be remarkably accurate. And it's worked for 41 years. So I saw no reason to spend money to add complexity and additional potential failure modes to the airplane.

 

[flyingron]

In addition to the other items listed, watch to see if the hoses routed to the transducer haven't become detached or deformed. There's a definite loop needed depending on where the placement of the transducer is with relationship to the carb/servo.


JPI offers this advice as well:
https://www.jpinstruments.com/FAQ/troubleshooting-fuel-flow-issues/

 

[Bell206]

but it appears the pressure at that sensing point

FYI: the indicator/gauge in most pressure based flow systems is nothing more than a pressure gauge marked in gal/hr vs psi. There is a linear pressure drop as fuel flows through all the injectors. This is why when an injector clogs the flow changes also due to a fuel pressure change. In other type systems it's not as obvious.

 

[GRG55]

FYI: the indicator/gauge in most pressure based flow systems is nothing more than a pressure gauge marked in gal/hr vs psi. There is a linear pressure drop as fuel flows through all the injectors. This is why when an injector clogs the flow changes also due to a fuel pressure change. In other type systems it's not as obvious.

Thanks. I assumed it wasn't flow through an orifice as the gauge vents to atmosphere. And an orifice flow system would involve the complexity of differential pressure measurement.

I was quite surprised at how accurate it is in the normal cruise fuel flow range. I did a lot of checks against fuel refills on numerous flights, which convinced me it's perfectly adequate and a fancier (read $AMUs) system offered limited additional utility for me.