RAM air temp.

[Cowboypilot]

Can anyone help me with a RAM air temp Question? In a Lear at FL350 the air temp reads -20C. I know that it is indicating warmer than the air actually is due to RAM rise. My question is: Sitting on the ramp should that same temp indicator read the current the actual air temp?

Thanks in advance for your answer.

Cowboy

 

[flewa72]

I don't know about the specific system on that aircraft, but most temp indicators are heated to prevent icing up. (Once they're iced over they will read 0 C no matter how cold it gets) So my guess is that on the ground it will read a little higher than actual ambient, but it's just a guess.

 

[banned username 2]

Can anyone help me with a RAM air temp Question? In a Lear at FL350 the air temp reads -20C. I know that it is indicating warmer than the air actually is due to RAM rise. My question is: Sitting on the ramp should that same temp indicator read the current the actual air temp?

Thanks in advance for your answer.

Cowboy

An old rule of thumb we used to use in the Lear 35 was to take the RAT (-30°C) and subtract 20 (-50°C) and subtract another degree for each .01 Mach over .70M you were doing and this would be very close to your SAT (Static Air Temp)

Example: RAT = -30°C
Mach = 0.78

-30° - 20 = -50° You are doing .08M over .70M so subtract another 8°C to get a SAT of -58°C

Disclaimer: I haven't flown a Lear 35 in nearly 7 years, I THINK I have this rule accurate.

 

[mar]

OAT not accurate on the ground

<<Sitting on the ramp should that same temp indicator read the current the actual air temp?>>

I don't think I've ever come across an OAT gauge that is accurate on the ground. On hot summer days you'll get a lot of reflective heat from the pavement or the wind direction may swirl some exhaust past the probe.

As far as I know the OAT is only to be used in flight.

Use the temp reported on the METAR for performance questions.

Good luck.

 

[Lear70]

Depends on which model Lear you're in. The early 20-series Lears had a different RAT probe than the 35's or retrofitted 24 d-f or 25's.

On the ground in the Lear with the pitot switches off, NONE of the probes should be getting ANY kind of heat! Newer aircraft like the 31a and CRJ get half heat when the aircraft has power on it to make sure they're not iced over before you turn them on as you take the active runway (USAir 737 that went off the runway into the Patomac a decade ago).

Therefore, first thing in the morning, your RAT and OAT gauge should be the same. However, after you've been flying some during the day, the probes take a LONG time to cool down and usually BOTH the RAT and OAT won't be accurate between flights (but the OAT gauge will be closer to accurate after an hour or so). That rule of thumb Falcon Capt posted is dead-on accurate!

Sounds like a great type rating oral question... Good luck!

 

[LJDRVR]

Another good rule of thumb is to take the first two digits of your indicated and add them to the RAT. (-30 degrees RAT + 280 KIAS (28) = -58 degrees SAT.)

Looking forward to trying Falcon Capt's ROT, looks like it would work pretty good.

Lear 70 is correct, the Lear's OAT gauge does not read accurately on the ground, for exactly the reasons he mentioned. Hey, I hear the CRJ flies a lot like the 55, true?

 

[Lear70]

LJ, it flies almost identically to the 55 with one exception: the 55 is still cable driven for all of its main control surfaces (elevators, ailerons), whereas the CRJ has hydraulically-actuated control surfaces with this Piece Of Sh*t speed-sensitive pressure feedback system that increases the pressure it takes to displace the control surfaces as speed increases. Makes hand-flying the airplane above Mach .70 a real pain in the *ss. But, the mach trim isn't nearly as noticeable as the Lears are, so I guess you get a little trade-off...

Blue skies!

 

[sstearns2]

I don't know about the specific system on that aircraft, but most temp indicators are heated to prevent icing up. (Once they're iced over they will read 0 C no matter how cold it gets) .

Why will it read 0 C if the TAT is less? Ice can have a temperature below freezing. It only maintains 0 C when undergoing the phase change from solid to liquid.

Scott

 

[flewa72]

Originally posted by sstearns2

Ice can have a temperature below freezing. It only maintains 0 C when undergoing the phase change from solid to liquid.

I'm not a physics whiz, I was just repeating what I had been taught in school way back in the 70's. I was taught that the temperature of ice could only be reduced below 0 C if contaminants were present. Obviously at Flight Levels the water vapor is relatively pure so I assumed the temp of the ice would be constant. You learn something new every day (at least if you keep your eyes, ears and MIND open).