Total Air Temperature question

"Peter" wrote in message
...
Hi,

I fly a TB20 and a friend who has the same one with a Shadin air data
computer tells me that he normally gets (say 160 ktas, 10k feet, ISA)
a TAT value of about 3 degC higher than that returned by the existing
OAT sensor.

I looked up TAT and found info like

http://mtp.jpl.nasa.gov/notes/sat/sat.html

which explains TAT as being Ts (static air temp) adjusted for mach
effects.

This raises two questions:

1) Can one really get a 3 degC difference at such a low speed? I
recall a long thread here from some years ago where it was suggested
the increase is below 1 degC.

Accoring to their website Shadin outputs "True Air Temperature" and on the
MU-2 the Shadin temp is lower than the existing OAT guage which uses a, very
accurate, flush, sensor. I haven't found a definition for "True Air
Temperature" but I think that it is "Static Air Temperature". The MU-2
flight manual has a correction table to convert indicated OAT to SAT and the
temperature output by the Shadin is consistant with what SAT should be.
Keep in mind that your friend's OAT sensor is sees Total Air Temperature and
since it doesn't know its Mach number, it can't possibly produce a corrected
temperature like SAT.

The 3C difference is a function of instrument error. The error is greater
than 3C since the Shadin should be showing a lower temp.


2) What is the use of TAT in the context of icing? Presumably the temp
rise will vary over different parts of the aircraft.

If the TAT is above freezing then you won't get much ice but it can still
form. Keep in mind that it is SAT that determines the temperature of the
droplets. You could be flying in -10C (SAT) with a +1C (TAT) and the
supercooled drops would cool the leading edges below 0C and ice would form
(If the mass of the -10C dropplets was large enough to cool the surface
faster than the 1C air could heat it). It would melt off pretty quickly
once you were out of the cloud though since it would be exposed to +1C. I
heard once that if you can go 400kts IAS then airframe icing is imposible. I
forget where I saw this. Yes, the temperature rise varies with location on
the airplane with leading edges and areas of increasing cross section seeing
more temp rise.

Mike
MU-2.