Garmin G500 PFD Temperatures… which one?

[AggieMike88]

For my current contract pilot job, I am flying a mid 90’s Bonanza A36 equipped with a Garmin G500. (PS. love the airplane and it’s performance)

Today was my first flight where my enroute altitude would be at or near the freezing level. Fortunately it is a VFR day, so low risk of airframe ice.

but the situation got me thinking of what is available to me to determine my risk by monitoring outside air temperature.

The airplane does have an analog OAT probe (it showed +4° or +5°C when the photos of the PFD were taken.

The PFD can show 3 different values… TAT, SAT & ISA.

Which is the best choice for my needs?

 

[RussR]

ISA is the difference between the current temperature and the standard temperature at your altitude. It would not be useful for determining ice risk. For a Bonanza or really most other piston aircraft, I can't see any real value to knowing it.

I am surprised there was such a difference between SAT and TAT. At these speeds, they should be virtually identical I think, well within the precision of the display (i.e. less than 1 degree C difference).

TAT will always be warmer than SAT, so if you use SAT as a predictor of icing, you will have the "worst case" scenario.

 

[masloki]

Also check the OAT. My probe is a little close the exhaust and reads high. If your G500 OAT reads high, then you should mentally adjust SAT/TAT to keep your safety margin. Since your analog OAT and SAT vary by 4C, you likely have a similar issue.

 

[Plano Pilot]

RAM and TAT temperature rise are the same. You are looking for TAT for icing. Check out the below chart from https://www.pittspecials.com/articles/RamAirTempRise.htm.



"100 knots: 100^2 / 87^2 = +1C

150 knots: 150^2 / 87^2 = +3C

200 knots: 200^2 / 87^2 = +5C

250 knots: 250^2 / 87^2 = +8C

300 knots: 300^2 / 87^2 = +12C

350 knots: 350^2 / 87^2 = +16C

400 knots: 400^2 / 87^2 = +21C

500 knots: 500^2 / 87^2 = +33C

600 knots: 600^2 / 87^2 = +48C

1000 knots: 1000^2 / 87^2 = +132C


Basically, if you keep the speed up over 350 knots, you won’t have a problem with airframe icing!"

More info: Air Temperature Explained - FLYING Magazine

 

[AKiss20]

Note that this temperature rise is true airspeed in knots/87^2 is just an empirical approximation, the number 87^2 has no real physical significance.

The rise in temperature when stagnating a moving fluid is 1/(2*cp)*V^2 where cp is the specific heat capacity at constant pressure, 1005 J/Kg*K at 25C in SI (and is basically constant at the temperatures we fly in), and V is the velocity in m/s.

The 87 comes from 0.511^2/(2*1005) = 1.299e-4 = 1/(87.7)^2 where 0.511 is the conversion factor between knots and m/s.

I had never heard this rule of thumb before so did a quick calc to check it and thought it was interesting.