Oat/tot ??

[glad rag]

OK flying as a passenger the cabin information system at one point gave an indication of OAT of -65C if the aircraft was at 33,500 ft is there anyway to corroborate the TOT apart from asking the crew??

Aircraft BA A319-100

cheers

Glad Rag.

[pkkapkka]

Hi!!
You need also the velocity (or even better the Mach number). The total temperature (Kelvin) is:
Tt = OAT*(1+0.2*M*M)
where OAT is in kelvin and M (mach number) is = v/a. v is your TAS and the speed of sound a(in m/s)=sqrt(1.4*286*OAT(in kelvin)). If you were looking at the flight map in the pax cabin probably you did not have neither the wind, neither the Mach number, so as a first approximation you could use the displayed GS. You must use the same units for v and a (i.e. m/s).

Bye,
Marco

[CAT1]

An amazingly accurate rule of thumb (works up to FL400) for working out OAT from TAT is to take 50 off the Mach number and subtract this from TAT. I think a 319 cruises at .78 so if the OAT is -65 (sounds a bit cold for FL330?) add 28 (50 less than Mach no.) and TAT would be -37 within a degree or so.

[roljoe]

33.500 ft...strange FL ??

and what's the final purpose of knowing the TAT as pax ??? :confused:

[FlightDetent]

and what's the final purpose of knowing the TAT as pax ??? :confused: Curiosity, not essentially a bad thing. ... now, if I just stuck my hand out of the window ...

OTOH, true that a metric FL is 33100 ft, not 33500. I just hope they were not climbing or descending. Scary! :ok:

FD (the un-real)

[indiscipline_girl]

A simple formula for ram rise is (TAS/89.1)^2 and this is exact, with TAS in kts.

Of course this means you need to know the TAS.

At -65C, the speed of sound is about 562 kts.

So, at M0.78, TAS is 562*0.78= 438 kts.

So (438/89.1)^2 = 24.2C.

So the TAT is -65C + 24.2C = -40.8C.

The altitude is irrelevant, only the OAT and TAS determine the ram rise.

Furthermore the TAT is the temp that is actually measured by the probe, and the OAT you see on the screen is calculated by the ADC's by the reverse of the above calculation.

[OutOfRunWay]

Mebbe it displays GPS altitude, that could be just about anything. If it were indicated alt, both the pilots and the atcos must have been comatose..

[LH2]

Perhaps the altitude displayed in the passenger cabin is taken from the GPS, and is thus not a barometric altitude?

[Rivet gun]

Indicipline girl,

It seems to me the equation you give is correct in principle, but the constant (which you give as 89.1) is wrong.

pkkapkka gives the exact equation in terms of Mach number.

Assume that that ram rise = TAT - SAT and that Mach number = TAS / sqrt(gama*R*SAT)

Then the constant in your equation should be sqrt(5*gama*R).

gama for air is 1.4
In SI units, R for air is 287.05

This gives the constant as 44.826 m/s

Converting to knots gives 87.13

The ADC cannot use the reverse of your equation to compute SAT because it cannot calculate TAS untill it has first calculated SAT. The ADC uses the Mach number equation to compute SAT because it can compute Mach number from pitot - static pressures independantly of temperature.

[indiscipline_girl]

I stand corrected. (TAS/87.1)^2 it is then. close enough for govt. work anyway.

btw this was done to death a while back, don't know if a search will find it.


The two formulae (mach and TAS) are of course just different presentations of the same thing.

[glad rag]

for the information all!

Yes, purely a diversion from stewed BA tea on my behalf I admit (and a nagging interest after BA38) and indeed the OAT display did catch my attention.

I said thanks on leaving to those waiting patiently at the front door (as I always do) but forgot to ask the first officer as he looked, well rather young, and the lady purser was VERY easy on the eye, damn distractions:ooh:

Cheers again,

Glad Rag.

[indiscipline_girl]

And because I am bored, I will add that the value of gamma for (dry)air is not exactly 1.4 but it is close. 1.4 is the theoretical value for a diatomic gas.

Air is not quite diatomic gases what with Ar, CO2 and all.

ADC's I believe use 1.4 as the number.

[Flamin_Squirrel]

The ADC cannot use the reverse of your equation to compute SAT because it cannot calculate TAS untill it has first calculated SAT.

Isn't TAS independant of temperature?