Fuel reading drops as altitude increases, returns with descent

[mattwood]

I've searched the archives but haven't been able to find an answer.

I've got a problem that the LH tank (capacitive fuel sensor) in my RV-7A will show the fuel level dropping considerably when ascending to altitudes say 6,500 ft or above. For example, today I was going on a long flight just building hours and sightseeing. Everything was working well for more than an hour, but shortly after refueling, the gauge for LH tank starting reading lower and lower, dropping to about 4 gal showing on the gauge. When descended later in the flight it came up some, but not all the way. When I landed and topped off the tanks, I'd only used 4 gal out of that tank.

The correlation with altitude makes me think it's pressure related, but I can't figure out how that would affect the capacitive sensor. I did have a leak in the tank and fixed that recently (It passed balloon test).

I checked that the vent line was clear some months ago when this started, and will check again this week since I can't think of anything else.

Is it maybe a ground problem?

Thanks for any suggestions you might be able to offer.

Clear Skies,
Matt Wood
Commerce, TX (2F7)

 

[nedricher]

I've searched the archives but haven't been able to find an answer.

I've got a problem that the LH tank (capacitive fuel sensor) in my RV-7A will show the fuel level dropping considerably when ascending to altitudes say 6,500 ft or above. For example, today I was going on a long flight just building hours and sightseeing. Everything was working well for more than an hour, but shortly after refueling, the gauge for LH tank starting reading lower and lower, dropping to about 4 gal showing on the gauge. When descended later in the flight it came up some, but not all the way. When I landed and topped off the tanks, I'd only used 4 gal out of that tank.

The correlation with altitude makes me think it's pressure related, but I can't figure out how that would affect the capacitive sensor. I did have a leak in the tank and fixed that recently (It passed balloon test).

I checked that the vent line was clear some months ago when this started, and will check again this week since I can't think of anything else.

Is it maybe a ground problem?

Thanks for any suggestions you might be able to offer.

Clear Skies,
Matt Wood
Commerce, TX (2F7)

Make sure you are flying coordinated with the ball in the center. The higher you climb, perhaps slower speed, high power, the more right rudder needed, the opposite on descent with low power and high airspeed.

 

[Desert Rat]

Make sure you are flying coordinated with the ball in the center. The higher you climb, perhaps slower speed, high power, the more right rudder needed, the opposite on descent with low power and high airspeed.

I don’t think there’s any way that you could be uncoordinated enough for that little 21 gallon tank to only show 4 gallons if topping it off afterward reflected that it actually had 17 gallons in there.

 

[BH1166]

Capacitive means a tube and fuel a dielectric. Since it’s about altitude and reported fuel level, something not right with the fluid flow in the tube. Partial blockage? Maybe drain tank, refill, drain again, hoping something unclogs. Thats all I got, and likely wrong, but is logical

 

[AX-O]

When you say “capacity fuel sensor” are you referring to the 2 plates inside the tank or float style?

Mine has the plates and the readings are sensitive to temperature (not changes due altitude). I can verify it on the ground too.

 

[Avanza]

I've searched the archives but haven't been able to find an answer.

I've got a problem that the LH tank (capacitive fuel sensor) in my RV-7A will show the fuel level dropping considerably when ascending to altitudes say 6,500 ft or above. For example, today I was going on a long flight just building hours and sightseeing. Everything was working well for more than an hour, but shortly after refueling, the gauge for LH tank starting reading lower and lower, dropping to about 4 gal showing on the gauge. When descended later in the flight it came up some, but not all the way. When I landed and topped off the tanks, I'd only used 4 gal out of that tank.

The correlation with altitude makes me think it's pressure related, but I can't figure out how that would affect the capacitive sensor. I did have a leak in the tank and fixed that recently (It passed balloon test).

I checked that the vent line was clear some months ago when this started, and will check again this week since I can't think of anything else.

Is it maybe a ground problem?

Thanks for any suggestions you might be able to offer.

Clear Skies,
Matt Wood
Commerce, TX (2F7)

Capacitive means a tube and fuel a dielectric. If the fuel was "warm", a climb to higher altitude may have caused air bubbles in the fuel.
The fuel is then no longer 100 % liquid. Will this affect the gauge reading ? Usually the bubbles goes away at altitude after 15 min or so.

Good luck

 

[mattwood]

Thanks all for the thoughts. These are the capacitive senders (2 plates, not the float sender). I'm going out tomorrow to see if I can figure anything out. I'm mostly worried because I had this behavior *before* I took the tank off and fixed the leak. Hoping I actually fixed the leak and don't have to go through all that again.

I'll let y'all know if I figure it out. Thanks again!

 

[mattwood]

Hi, All:
Solution found: The BNC connector at the wing root had a wire that was not making good connection. I replaced the BNC connector, and readings are now spot-on!

Thanks again to those of you who offered your thoughts above.

Cheers,
Matt

 

[bjdecker]

Hi, All:
Solution found: The BNC connector at the wing root had a wire that was not making good connection. I replaced the BNC connector, and readings are now spot-on!

Thanks again to those of you who offered your thoughts above.

Cheers,
Matt

Thank you for closing the loop on this!