Fuel flow twice what it should be.

[F1 Rocket]

Took off yesterday and the fuel flow was twice what it normally is. Everything else was OK so I continued the flight. Leaning resulted in a decrease in fuel flow as expected but still, the actual number seemed approximately twice what it should have been. After a few minutes, it went back to normal. Any thoughts on where to start to troubleshoot this?

Red cube installed between the fuel servo, and the spider. G3X Touch.

Danny

 

[Taltruda]

What were your EGTs at the time of high fuel flow? If they are normally 1300 ish, and you flood it with fuel, I would expect them to be in the low 1100s. However if it was just a faulty fuel flow indication, then you would be showing 1300ish.

 

[Ironflight]

First thing I’d do is fuel the airplane and check to make sure that you weren’t REALLY flowing that much. It’s not unheard of to have a sporadically leaking fitting…..

If the fuel usage was as expected, then you’ve got an instrumentation funny to chase.

 

[F1 Rocket]

Power was as expected on climb out. EGT/CHT were all normal and responded as expected when I leaned it. I’m pretty certain it’s an instrumentation issue but the question is what? My experience with fuel flow transducers is that they typically start to read erratic, low, or not at all when they fail. In this case the indication was double what it should have been.

 

[walkman]

First thing I’d do is fuel the airplane and check to make sure that you weren’t REALLY flowing that much. It’s not unheard of to have a sporadically leaking fitting…..

If the fuel usage was as expected, then you’ve got an instrumentation funny to chase.

idk paul, if his flow was twice expected on a full power climbout 16gph give or take is a heck of a sporadically leaking fitting

Is this an installation with a return line and a second red cube on the return line? If so, possibly a failure there.

 

[airguy]

idk paul, if his flow was twice expected on a full power climbout 16gph give or take is a heck of a sporadically leaking fitting

Is this an installation with a return line and a second red cube on the return line? If so, possibly a failure there.

With that location of the red cube, you most likely had some minor vapor bubbles in the fuel line going through the red cube from engine heat-soak prior to takeoff.

The red cube operates by means of a small plastic turbine spinning in the fuel flow, and that turbine interrupts a light beam creating a "count" for measuring the fuel flow. Bubbles in the fuel stream look just like another normal interruption of the lightbeam and gets counted the same, resulting in a higher-than-real fuel flow. The same can happen with a poor wire connection on the unit, as it makes/breaks from vibration you can get strange fuel flows resulting from that.

Given the choice between a real "excess" fuel flow versus instrumentation error from the Red Cube, and given my experience with the red cube and your installation location, my money is on the Red Cube. With the fuel pump bolted to the engine (and hot), it will heat the fuel going through it. As the fuel goes through the servo (also bolted to the engine), the pressure on the fuel in the line up to the flow divider drops A LOT allowing some of it to vaporize if the conditions are right. During taxi the flow is slow, almost stagnant, and the fuel will heat up quite a bit. After takeoff the fuel is flowing much faster through all those parts into the engine, and cools off the fuel pump and servo, reducing the temp of the fuel and stopping the vapor bubbles after a couple minutes. Been there, done that. It's more common with auto fuel, and with stop-and-go landings.

 

[dalee912]

I had this issue on my RV8. It would randomly double the fuel flow. It usually happened on takeoff or climb out. My mechanic checked a couple things and decided to replace the Red Cube FT60. It was an easy remove and replace for my mechanic, and now my fuel flow reads exactly as it should. I suppose my red cube was getting old!

 

[Dan92]

I had the opposite problem with the red cube as it showed about half of what I normally see. I called Electronics Intl. and they said the red cube could not cause that indication and to check my wiring. I installed a new red cube and that solved the problem.

Dan

 

[HFS]

IF the op is talking about a Rocket/F1, his normal takeoff flow should be around 24 1/2 gallons/hr or greater - his engine probably would "drowned" at somewhere around 50!

Just sayin'

HFS

 

[vic syracuse]

I find that these red cubes last longer if you don’t mount them to the engine and you wrap them with some heat barrier tape. The vibrations and heat of the engine seem to shorten their life.

Vic

 

[airguy]

I find that these red cubes last longer if you don’t mount them to the engine and you wrap them with some heat barrier tape. The vibrations and heat of the engine seem to shorten their life.

Vic

The installation directions also want it after the (mechanical) fuel pump, which pretty much limits where it can be installed on a standard fuel system. The FT-60 does not like the pulses in the inlet to the mechanical fuel pump, it wants to be on the more stable output side.

For those running dual electric pumps, they do not have this drawback - but the SDS/EFII systems with electric pumps also flow more fuel than needed and return the excess to the tank, which makes it problematic to measure the amount of fuel going to the injectors. Those ECU's will output a signal to our EFIS simulating the FT60 count, however, so you don't need the FT60 in that application.

Before I installed full SDS I had a bit of a hybrid setup, using their 2-pump electric fuel pump setup (no mechanical pump) with pressure relief returning to the tank, and the line going forward to the FWF carried only the injector flow, deadheading into the standard Bendix injection system. I placed my FT-60 on the cold side of the firewall on this line and it worked great for 800 hours until I removed it when I installed the full SDS.