DaV8or Posted June 22, 2014 Report Posted June 22, 2014 The high density altitude take off discussion got me thinking about how much we can trust our on board OAT gauges either digital, or analog. I figure that the probe itself is just sticking out in the wind, so depending on whether or not it has direct sunlight on it seems like it would make a big difference in the reading of the actual ambient air temperature. I would think that a shaded probe would give a more accurate reading, but this doesn't happen much. What do you guys think? Quote
slowflyin Posted June 22, 2014 Report Posted June 22, 2014 Mine is on the underside of the wing--left wing opposite the exhaust stack. Accurate in flight but I have to wonder about a black taxi way radiating heat. The few times I was worried about being high and hot the field offered DA via the ATIS. I do use it on every flight after I'm airborne. Sent from my iPad using Tapatalk Quote
OR75 Posted June 22, 2014 Report Posted June 22, 2014 You are correct. That's why OAT installation manuals / STC usually specify where the probe should be located: Not in the engine exhaust stream, not under direct sunlight , usually under the wing . Quote
DaV8or Posted June 22, 2014 Author Report Posted June 22, 2014 You are correct. That's why OAT installation manuals / STC usually specify where the probe should be located: Not in the engine exhaust stream, not under direct sunlight , usually under the wing . Hmmm... my factory analog probe is sticking straight up out of the windshield into the sunshine. My aftermarket digital probe is mounted on the side of the fuselage just under the pilot's window. In all fairness to the most recent installers, they likely put it there because there was a previous JPI probe mounted there prior to my ownership. Quote
jetdriven Posted June 22, 2014 Report Posted June 22, 2014 Davtron explicitly says to mount the probe on the underside of the wing, outside the prop arc. Mine reads dead on all the time. 1 Quote
KSMooniac Posted June 22, 2014 Report Posted June 22, 2014 Hmmm... my factory analog probe is sticking straight up out of the windshield into the sunshine. My aftermarket digital probe is mounted on the side of the fuselage just under the pilot's window. In all fairness to the most recent installers, they likely put it there because there was a previous JPI probe mounted there prior to my ownership. More likely they put it there because they were lazy, or the owner was too cheap to pay to get the probe routed from an optimal position under the wing... Quote
DaV8or Posted June 23, 2014 Author Report Posted June 23, 2014 More likely they put it there because they were lazy, or the owner was too cheap to pay to get the probe routed from an optimal position under the wing... Very possible. I have seen others like mine too. Seems like that's pretty damn lazy. You have the inside side panel off anyhow, so how much harder is it to feed a wire into the wing?! Oh well, there was already a hole and my guys plugged it. I guess I can live with it. Worst case is it reads high and so it will force me to be conservative with my take off calculations and build in a margin of safety. Quote
N601RX Posted June 23, 2014 Report Posted June 23, 2014 Most temp sensors have a outer and inner sleeve to reduce the effects of the sun while moving through the air. I was very happy to get the big Scott thermometer out of my front window and get its replacement tucked up under the wing. Quote
PMcClure Posted June 23, 2014 Report Posted June 23, 2014 I am more worried how accurate it is on the cold side, rather than the hot side. 1 Quote
N201MKTurbo Posted June 23, 2014 Report Posted June 23, 2014 I am more worried how accurate it is on the cold side, rather than the hot side. Me too. As long as the ice melts at 0C I'm happy. Quote
N601RX Posted June 23, 2014 Report Posted June 23, 2014 It is pretty easy to do a basic calibration of thermocouples and rtd's with an ice bath. An ice bath is allowed by NIST standards. While fancier devices exist, a styrofoam cup can be filled with crushed ice and then add just enough water to cover the ice. Stir well with the thermocouple submerged in the middle part of the cup. Wait a few minutes or so for everything to settle out. This will give you reference temp that is extremely close to 32 degrees The JPI devices all have a menu that will allow you to move the temp up or down a few degrees. 1 Quote
cbarry Posted June 23, 2014 Report Posted June 23, 2014 At least for DA calculation, wouldn't we want the more conservative temperature reading (probe mounted in open air/exposed to sunlight)? Quote
KSMooniac Posted June 23, 2014 Report Posted June 23, 2014 At least for DA calculation, wouldn't we want the more conservative temperature reading (probe mounted in open air/exposed to sunlight)? For a DA calc you can always estimate high if you wish, or get an ATIS temp before you take off. When you're in the clouds, it is much more critical to know if you're flying at 33 dF or 31 dF! 1 Quote
DaV8or Posted June 23, 2014 Author Report Posted June 23, 2014 For a DA calc you can always estimate high if you wish, or get an ATIS temp before you take off. When you're in the clouds, it is much more critical to know if you're flying at 33 dF or 31 dF! I guess if you're in the clouds, the sun won't be a factor then. Quote
KSMooniac Posted June 23, 2014 Report Posted June 23, 2014 I guess if you're in the clouds, the sun won't be a factor then. True, but if you put it in the fresh air scoop like I've seen a lot of them, then the engine heat will make it read higher than actual. Quote
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