Fuel sensor contamination?

[Chris B]

We recently had a persistent incorrect fuel quantity indication in one tank, that eventually cleared-up. Apparently this was due to fuel sensor contamination. I am curious if others on DAN have had a similar experience.


Long version:

Recently at the first start of the day, our DA40 XLS (50 gal tanks) left fuel sensor suddenly read low. No red X, and no low-fuel sensor warning, but an obviously incorrect value (empty vs. 8 gal). No issues with the right tank, and both were reading 8 gal at the end of the previous flight, two days prior.

The DA40 AMM r7 troubleshooting procedure (28-40-00, page 101) references two possible causes for incorrect quantity indication for only one tank: 1) probe contamination & 2) defective wiring. The unmentioned third alternative is a (very expensive) defective fuel sensor.

We cleaned the connector pins (J2801) near the left fuel drain, and also used temporary jumpers to be certain that there were no connector issues, but this had no effect. Vigorously rocking the wings after filling the tanks and flying for 30 minutes in light turbulence also did not have any apparent effect. The left tank read only 7 gal when nearly full (>23 gal).

Diamond support was excellent, and emailed relevant test procedures and documentation that evening. But by the time we were getting started on more detailed tests the next morning, the sensor was magically working properly again. Presumably some small contamination which affects the sensor capacitance eventually worked out. Hopefully this is an isolated incident.

FWIW, we were IMC in moderate rain for about half of the previous four flight hours prior to sensor failure, but there was no indication of water in the gascolator or fuel drains. The aircraft was hangared two nights with partially-filled tanks (~16 gal total).

Chris

[gandalfim]

Summary:
Two weeks ago we drained 2.5 tubes of water from the right tank of my DA40. Today a persistent red X showed up on the right tank fuel level indicator. It goes into the maint shop tomorrow; I'll recommend they try to clean the sensor before jumping to replace it.

Details:
On the morning of Saturday, Nov 3, someone stole 3.5 gal of fuel from the right tank of my DA40 and, in the process, left the fuel cap lying on top of the wing. It had rained heavily for the most part of the previous two days and we didn't know when the cap was taken off. The pilot who found the cap off, drained the tank to check for water and found none. He rocked the wing, checked again and found none. The pilot who flew the plane the next day also checked for water during his pre-flight and found none. When he got to KLNS (a 40 minute flight from N14) he drained the tank again and got 2.5 tubes of water. His return flight was uneventful.

I'm a bit concerned that water could "flood" into the tank, not flow directly to the drain and not be found in 3 checks. My mechanic indicated there are two tanks in the wing (40 gal DA40) and that it seems the water didn't get past the connector tube until the flight.
He also told me that in the past, something similar happened to Cessnas and forced them to put drains all over the place in order to prevent it.
Has anyone seen this?

[Rich]

There are places in the fuel tank arrangement where water can hide. The corners of the outer tanks come to mind, and there are baffles inside the chambers to dampen sloshing within the chambers. The tanks could accommodate a limited amount of water without harm due to it occupying the area of the tanks where unusable fuel lies. FWIW I've never seen a drop of water in a fuel sample. And yes, the new 172s have a plethora of fuel drains in each tank and you're supposed to sample every single one before flight. I've never seen anyone do that.

[Colin]

It's the baffles, I bet.

One of the first long flights in our DA40 was after a long night of rain. I might have had 15hrs in the plane at that point. I moved it to a field further south, flying down on the left tank. In the morning sumping the fuel we got pink stuff instead of blue stuff from the right tank. Flew it back to SMO to yell at the FBO for putting jet fuel in the plane. They don't even SELL jet fuel. They sumped and said, "Uh, this is water."

Drained out almost five gallons. Turned out the seal on the filler cap was bad. The plane had been hangared until I bought it, so they never knew. It was under some crazy warranty (they don't do it anymore, it was "two years free maintenance") and Diamond paid for it including, eventually, replacing the fuel sensor. Man, it was a lot of work.

[gandalfim]

I spoke with Diamond support and he told me the sensor will short if it's submerged in water. Just in case it didn't short out, he's sending me the electrical specs so I can test the sensor. He also doesn't understand how that much water could be in the tank and not seen on a sample.

I expect a hefty bill for this one. (I might try turning it in as an insurance claim.)

Until this incident, I too have never seen water in any sample (and it has been kept outside for 12 years). The seals on the fuel caps are rock-solid.

[Steve]

I tended not to think about the fuel cap O-rings either, since my airplane lives in a hangar. Last annual, my IA had me replace them due to loss of pliability. The O-ring sets are cheap, so it might be a good idea to replace them.

Steve

[LuckyDube07]

All,

Thanks for the timely posting and discussion. I just suffered the same casualty today - RT tank not indicating on the G1000. This comes after a weekend of hard rains (approx 1.5 inches) in the local area. I sumped the RT tank several times today expecting to see water, but none was observed. I'll check again tomorrow AM to give the tank a chance to settle more.

My first real squawk since I bought the plane a few weeks ago - until then she was the IFR training champ for the local flight school. Looking at the previous posts and the repair procedures in the AMM, this is setting up to be a fairly expensive first squawk. Fingers crossed and credit card at the ready position...oh, and I'll guess I have them look at my fuel cap O-rings for good measure.

Will keep all in the loop with updates.

Jay