Total Air Temperature question

"Peter" wrote in message
...

"Mike Rapoport" wrote

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

OK, that demolishes my argument... I will suggest he checks his OAT
gauge. I check mine, the same type, regularly against a PT100 meter
which is certified to 0.1C and it is always within the OAT display
resolution (1C).

However are you quite sure that TAT should be lower than SAT? An OAT
gauge which is that far out would be pretty obvious on the ground e.g.
when getting the ATIS.


Not exactly...IF "TAT" is Total Air Temperature then it will be higher than
SAT. But ( a couple buts actually!)

1)The OEM OAT guage reads TAT not SAT since it is not corrected for Mach
effects. Thus, there is no way that the Shadin should read higher than the
OEM guage if both are accurate. If the Shadin is displaying Total Air
Temperature, then it should read the same. If the Shadin is displaying
Static Air Temperature (which they might be calling "True Air Temperature)
then the Shadin should read lower. Either way the Shadin should not read
higher if both are accurate

2)If you to to Shadin's website, they state that the ADC puts out "True Air
Temperature" and I think that they mean acutal air temperature or SAT. My
own observation is that, on the MU-2, the OAT guage reads a higher
temperature than the ADC displays and the difference between them matches
the temperature correction table in the flight manual pretty well
http://www.shadin.com/specs/adc_200.htm

Confusing this issue further, Garmin says the TAT means "Total Air
Temperature and, on the 530 the temp is displayed as XXTAT. However, Shadin
doesn't claim that the ADC even outputs Total Air Temperature. I actually
asked Shadin this question this morining but, stupidly, I forgot to attach
my email address!

The way to test all of this is to see how the temperature spread varies
between the OEM OAT guage and the Shadin ADC at different airspeeds since
TAT will go up and SAT will not.

Keep in mind that it is SAT that determines the temperature of the
droplets.

I can see this but I would think it is the temperature of the airframe
that determines whether the droplets will freeze when they will hit
it.

Yes, but the temperature of the airframe will be different depending of
whether it is flying through +1C air or +1C air loaded with -10C droplets.
As the number and size of the droplets increases the leading edges will get
closer to -10C. This is mostly theoretical for low speed aircraft but is
relevent for jets which have large spreads between TAT and SAT.


Mike
MU-2