major fuel probe related issue -- need help

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Davestation
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Re: major fuel probe related issue -- need help

Post by Davestation »

I know of two aircraft that were plagued with dead probes in the same time frame, so I too suspect a bad batch situation (although these two were about a year prior to your issue). The 40s don't have nearly as hard of a time with water like the 42s simply because of a 100ll vs. Jet-A situation. But yes, I imagine if they get one tiny drop of water in them they instantly commit suicide. Very limited luck with drying them out and crossing our fingers as we plugged them back in. You can measure the Hz output on them to see if they should be functioning, but the easiest way to confirm a bad probe for us was to keep a 20ft. roll of quadruplex wire handy and plug in the probe from the other wing to see if the gauge comes alive.

It was never an intermittent problem or a bad connection problem - the probes obviously had hard failures. On one plane it was the first after around six months, the second after two weeks, the third literally overnight. Countless hours of going through wiring etc. later, probe number four reluctantly went in (for fear of the plane killing that one too since we had no solution) but the problem has not returned since. The initial probe we had replaced was probably the factory original and made it ten years before finally going out due to water.
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Re: major fuel probe related issue -- need help

Post by Lance Murray »

What is the concept or method that these work on? In my Vans RV8 they were literally just two plates electrically isolated from the structure. The resistance between the two plates gave a fuel quantity measurement.
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Re: major fuel probe related issue -- need help

Post by Karl »

chili4way wrote: Fri Jan 03, 2020 10:25 pm Karl, you got one heck of a runway @ WMKL - 12,500 feet long! Do you get much haze from the seasonal fires there? I used to train/fly out of Seletar (WSSL) in Singapore's tiny domestic airspace. Do you know the WOA folks?
Hi Paul, yes haze causes a lot of problems, sometimes you can barely see the airfield boundary. Luckily it is usually just for a few weeks a year.

I do know the guys at WOA, I have visited a number of times to assist with defects and maintenance, and we regularly use their excellent flight planning service.
I have never flown from Seletar but I was (jokingly) informed that every flight is an international flight as taking off puts you either into Indonesian or Malaysian airspace.
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Re: major fuel probe related issue -- need help

Post by Wall-e »

Dave thanks for the feedback. Multiple failures in less than six months is covered by warranty from the probe manufacturer but no one covers the labor which is more costly than the probe. You also have to be concerned that the threaded boss on the fuel tank doesn’t strip out since it is aluminum. A new tank adds another few thousand $$.

Lance, your RV probably works the same way but it is not assembled as one unit like the diamonds. The probe has a center conductor and an outer conductor. When fuel covers the whole probe, it reads full (well almost). When part of the probe is uncovered, the indicator reads less than full. I’m guessing that the material that the probes are made out of is our problem.
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Re: major fuel probe related issue -- need help

Post by Steve »

Lance Murray wrote: Mon Jan 06, 2020 5:29 am What is the concept or method that these work on? In my Vans RV8 they were literally just two plates electrically isolated from the structure. The resistance between the two plates gave a fuel quantity measurement.
Lance: Not sure about the system in your RV, but the system in the DA40 is a capacitive system. The two conductors of the sensor act as plates in a variable capacitor. The capacitance is determined by the amount of fuel. The sensor forms part of an oscillator, and the frequency is converted to a fuel level by the gauge circuitry.

Any contamination, water, or oxidation on the capacitive sensor will affect the accuracy/function of the sensor. One reason why I try to keep my tanks topped off, especially when the aircraft is parked outside of my hangar, and why I periodically replace the fuel cap O-rings.

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Re: major fuel probe related issue -- need help

Post by seresy »

agmolnar wrote: Wed May 15, 2019 4:54 pm Hello all,

I am experiencing a major issue that has more or less grounded my DA40 for the past 5-6 months, and resulted in the R/H fuel probe being replaced three times. At the outset (in late November), the plane has an intermittent R/H fuel gauge indication (occasional red-X). As you can imagine, not being able to find the root cause has been exceedingly frustrating, and I am turning to the forum to see if others have experienced the same and if so what was the eventual fix.

Here is the quick write-up from my mechanic:

The original probe was intermittent red "X". We replaced the probe with new 1/14/19, 2199.6.

The first replacement probe was in service until 1/25/19, 2201.4 (1.8 hours), where it became out of range. We found R/H static dischargers INOP based on suggestion from Neil Grant. Replaced both dischargers with new. Replaced probe with new.

The second replacement probe was in service until 3/27/19, 2215.8 (14.4 hours), where it became out of range. We tested bonding / static systems and found no fault. Probe replaced with new.

The third replacement probe was in service until 4/30/19, 2228.2 (12.4 hours). The probe was tested and found to have no output. Wiggling the wire was able to produce an output around 11 hertz, but only momentarily. We were not able to reproduce any hertz output after that. Bonding and static discharge systems normal.

Mechanic thoughts: The original probe was intermittent, but each time it was tested it was found within range. It was replaced based on experience, we encountered the same intermittent failure on another DA40 and replacing the probe solved the issue. Every probe subsequently installed has failed in a different way; two have become out of range and now the last one has no output. If the argument was made that the airframe is taking out the probes, why isn't the failure the same each time? Why was the original probe intermittent, but never out of range? More troubling, why is the current probe hard failing in a way that seems so mechanical?

Thank you all for attention to this-
Arpad
Hello,

Did you end up getting this fixed? I met the same issue recently, RH fuel gauge red x, ended up replacing the fuel probe(big job) but still showing red x after one hour flight. My mechanic checked connection and didn’t find anything. Just want to see if there is anything else we should check before removing the wing and replace probe again....
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Re: major fuel probe related issue -- need help

Post by cptndavid »

I had water in the bilge under the seats in my DA 42 the water got into a plug which caused a fuel fault , auto pilot fault and a gear prob.I moved the plug to higher ground. Check your drain hole under the belly of the airplane. I carry a chop stick for that purpose. You could also remove a seat and look.
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Re: major fuel probe related issue -- need help

Post by Wall-e »

I had a very similar problem & we finally replaced the "signal wire" to the probe. That fixed it for good. Has over 100 hours since replacement.
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