G5 Outside Air Temp disagreement

N56404

New G5 install. In the hangar the OAT from the G5 matches with the thermometer in the windshield and the wall thermometer. (within 2 degrees) In flight the G5 and the windshield thermometer are off 10-12 degrees. Plane is a 1973 Piper PA-28-180 I don't see how the exhaust could be impacting the probe where both are located. Installer doesn't think it could be exhaust either. He thinks the probe is accurate and the 50 year old thermometer in the window is off in flight. Anyone experience anything like this?

griff390

https://piperowner.org/talk/discussion/156966/g5-outside-air-temp-disagreement

1. Where is the probe located?
2. Are there gaps in the cowling? Hot air could be spilling out of the gap(s), and affecting the temperature reading. Most cooling air that is heated by the engine, is discharged from the bottom of the cowling. If any of the probes are under the fuselage or barely up the sides, it could cause false readings.
3. Bring a known calibrated thermometer with you and hold the sensor out of the vent window in flight. If you don't have a vent window, and the sensor is at the end of a wire, you could put the probe outside the door an inch or so, and carefully close the door so the wire is snugged in place by the door seal, and not pinched in the hinge or latching mechanism. You may want to tape down a small sheet of cork to the fuselage, then tape the temp probe to the cork. The cork will insulate the sensor from reading the fuselage temp, and will protect your paint in case the sensor begins flapping in the 140 mph breeze.

jolexa

My two cents, if the OAT probe is on the bottom of the fuselage, then the exhaust is impacting it. I had mine removed from there - poor install choice, but it was "approved" by garmin manual. The new location, under right wing is in 100% agreement with OEM temp gauge.

I find it hard to believe that the OEM gauge is inaccurate in-flight only. Regardless, it is easy to gut check. The 1000AGL temp should be within a few degrees of ground temp and not 10-12 degrees off.

rammstein1903

if the G5 OAT probe is mounted under the wing and the further away from the fuselage the better. My old windscreen OAT probe was about 20 degrees from correct value so don’t discount that possibility either.

petertvo

Had the same. I taped the wired sensor of a cheap digital thermometer to different spots and found out that underwing, about 3ft away from the fuselage the measurements are correct. Initially my Garmin probe indicated 2°C too high when mounted downstream of the cowling.

N56404

I should of mentioned. It is located sticking out the pilot side wall just above the wing. Where it is located I just can't imagine it is effected by the exhaust. I didn't specify a location and left it up to the installer who said he puts it there all the time.

griff390

https://piperowner.org/talk/discussion/comment/1445689#Comment_1445689

N56404;

I mentioned it earlier, but it's not just exhaust. You have 350 to 400 degree jugs on the engine, connected to an 800-900 degree exhaust system all generating heat inside the cowling. The hot air is discharged primarily from the bottom of the cowling where it travels below the fuselage.

Below the wing is high pressure air which will take any path it can find to lower pressure areas, including spilling out from underneath the fuselage. Some heated air will inevitably be mixed in.

A propeller driven plane also means there will be a spiral slipstream of air around the fuselage. The slipstream passes below the fuselage, picks up heated air, and wraps it around the fuselage. Put the probe in the wrong place, and the heated air could lead to errors. I can't tell you how much hotter that air will be, but it's a guarantee that it will be substantially above the surrounding ambient temp.

I agree with many of the responses above. Locating the temp probe along the wing, 3 feet or more from the fuselage on the pilot side seems to work well. The factory temp gauges were usually in the windshield above the pilot's head away from heat sources and convenient for the pilot to read. I've also seen them mounted in the fuselage on the pilot's side about dashboard height. Both are good spots to avoid engine and exhaust heat.

Back to your original question: have you taken a calibrated temp probe and held it out of the vent window during cruise? If yes, which temp probe was closest to the calibrated probe and how much of a difference was there?

N56404

Thanks for all the comments. I agree spiral slipstream seems to be the culprit where it is located. I'll be working with the installer to have it moved.

kentshaw

Look in your STC manual. I thought that it was supposed to be mounted underneath the wing. Additionally, there may be a calibration factor in the configuration-I just do not remember.

arrow76r

In my installation the G5 OAT probe is located on the bottom of the left wing about 2 feet away from the fuselage. The OAT for the G5s matches the installed Piper windshield probe to within a couple of degrees.