One of the things that I’ve noticed on the cross-country flights I’ve made is that the right tank has been running lower than the left tank. Sometimes as much as 10 gallons.

Now in the Cessna, that type of behavior is almost always the result of a misaligned fuel vent for the tank.  But the Velocity doesn’t have fuel vents for each tank.  It has one vent for the entire fuel system.   The other thing that causes it is a bad seal or gasket on the fuel cap.  Since the top of the wing is in a low pressure area, any leaks at the fuel cap will cause the tank to depressurize slightly and draw fuel from the other tank.  The final possible cause of fuel imbalance is if the airplane flies with one wing low.

So I checked the level of the plane in flight.  Perfectly level.

Next, I taped over the fuel cap on the left tank. No change.

Then I started looking more closely at the symptom.  On the ground, the tanks are pretty equal. During the initial climb, they’re equal.  It’s not until I get to level cruise that they start to diverge.  So I graphed out the fuel tank levels with the airspeed. Sure enough, the faster I go, the greater the imbalance.  This is a huge finger pointing at a bad seal around the fuel cap. But I checked that by taping over the cap!

At this point I’m questioning my sanity.

So I started looking at the data again (this is where full flight and engine data monitoring comes in real handy).

Here’s the graph of a flight down to Sebastian and back.

Notice the fuel levels are within a couple gallons of each other on the ground but as the airspeed increases, the levels diverge (the spikes where they level out is where I was slipping the plane to see if I could balance them out).

But then I noticed something. The left tank level is a relatively smooth line while the right tank level varies about 2 gallons (the sample rate is one second). So now I’m wondering why the left tank varies .1-.2 gallons per second while the right tank is changing 10 times as much.

I called the owner of the company that makes the fuel probes and controller module (who is also one of the owners of Grand Rapids). He said that the controller module averages the readings over about 5 seconds and transmits them engine monitor.  So there’s really no way that the readings should be changing that quickly. He sent me a replacement controller… Which meant that I had to calibrate it. 🙁

After installing and calibrating, I made a test flight.  Fuel levels were never more than 2 gallons apart.  And the graph line looks just like the left tank.

So I never had a fuel imbalance problem.  I had an instrumentation problem.

Oh well.