TAT versus SAT
Quote from Andy Rylance:
If I happened to be in an aircraft right next to the metal on the inside of the aircraft that the outside air was rushing past, I would only ever notice the temperature of the metal going down to the TAT and not the OAT - i.e. because the air is moving so fast the temperature it will bring the metal down to would be the TAT and not the OAT, unless the air was stationery? [Unquote]
I very much like your idea of putting an observer on the inside of the metal skin, but in the particular case you pose - touching the side of the fuselage - it wouldn't give you an accurate TAT.
As Mark 1 says, you've got to "stop" the oncoming ambient "moving" air completely, to convert ALL its kinetic energy into heat energy (thus causing its temperature to rise). This only happens, presumably, at forward-facing nul points, e.g., the nose, the leading edges of the wings, et cetera.
This is why it has been discussed, elsewhere on this forum, whether wing-tank fuel temperature, on a very long flight, will ultimately stabilise at the TAT or the SAT/OAT. My understanding is that, ignoring solar heating (or outgoing radiation), it will be somewhere between the two. There must be some skin friction (loss of kinetic energy into heat energy) on the side of a fuselage and the surfaces of a wing, even allowing for the boundary layer (stationary) air. But it won't be enough to raise the skin temperature to the theoretical TAT.
By the way, TAT is normally sensed nowadays by a probe (one type called a Rosemount) which effectively "catches" the air somewhere underneath the nose of the fuselage. A computer then calculates the SAT, to save the laid-back modern pilot the trouble of doing it.
[PS: Sorry, TopSwiss 737, hadn't noticed you had already covered Rosemounts and ADCs...]
Last edited by Chris Scott; 19th Feb 2008 at 12:30.