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Old 04-08-2012, 08:48 PM
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Ironflight Ironflight is offline
VAF Moderator / Line Boy
Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,830
Default The Elusive (sensor) Short??Found!

For the past year or so, the RV-6 has had a problem with its Outside Air Temperature sensor ? the one that hooks up to the Dynon. Now in the old days, OAT was mostly a curiosity, unless some instructor or examiner was breathing down your neck to come up with a True Airspeed or Density Altitude. Usually, you just didn?t bother. But in today?s EFIS world, a lot of the things that we have come to enjoy depend on it ? you need to OAT to compute TAS, which you also need to compute current winds. This will play into fuel planning as well, and TAS is the key to figuring out how well you are leaning and how much power your engine is producing. Yes, OAT is important.

Unfortunately, ?Mikey?s? Dynon OAT has been fairly wacky. Once in awhile, it would e accurate, but more often than not, it would cycle from a reasonable value to +70 C, then back to normal ? all in about a four-second cycle. And it would keep doing this! On the ground, with everything steady, it was fine?sometimes! We talked with Dynon, we suspected the sensor, we suspected the D-180, we suspected the wiring. We finally replaced the sensor, and wiggled the connectors - no joy. At Sun n Fun, we had a Dynon fellow walk ALL the way to Homebuilt Camping (which is off the edge of the map?.literally!) to check out the configurations. He noted that it was steady until we wiggled in the seat, shaking the airplane ? then it became erratic. He did note that the +70 C reading was equivalent to a short?.so, when we got home, I sat in the seat, reached under the panel, and began wiggling wires. Sure enough ? I soon could make it misbehave on cue!

Under the panel I went, and since I had already looked up the wire colors, I knew that when I saw the purple/blue conductor, I had it nailed. Take a look at this picture:

See that ?notch? in the insulation? That wire was resting on that anodized aluminum piece on the left ? and the spot where it was rubbing had been rubbed clear of anodizing. Sure enough ? a little wire re-routing, and the yearlong mystery was solved.

Of course, if this had been a powered wire, a breaker would have blown, and we?d have solved it pretty quickly, but a strange little symptom like this ? in an airplane that gets flown as a commuter ? can go for awhile before you find the time to REALLY troubleshoot it.

Dynon has been great BTW ? always willing to take time at a big show to talk about it and give us ideas. None of them identified this exactly, but all the inputs helped us track it down.

Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
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