RAT vs TAT

[StudentATP]

Good day,

What is the difference between Ram Air Temp. and Total Air Temp.? and why do we need to be aware of them in flight?

by definition,

Ram Air Temp. (RAT) = OAT + some ram temp. rise

Total Air Temp. (TAT) = OAT + full ram tmep. rise

I have no problem finding either one with equations in ATPL flight planning exam, but could any professional pilots tell me why these two are so important in jet aircraft?

I thank you in advance

[Old Smokey]

You're obviously aware of the difference between RAT and TAT, so no further mention of this.

TAT (or RAT as a substitute if TAT is unavailable), is what is sensed by the forward sections of the airframe, and the engine inlet air temperature.

Thus, TAT has significant implications for -

(1) Ice formation on the aircraft, and

(2) Thrust output.

As the difference between SAT and TAT is often of the order of 30 degrees, TAT becomes an extremely important piece of information.

Regards,

Old Smokey

[Checkboard]

The term ram air temperature RAT is used with different definitions in the literature. It can designate, for example, the recovery temperature Tr (for instance, ARINC) the indicated temperature Ti, the total temperature TT, or even the adiabatic temperature rise ATh.

From IN-FLIGHT TEMPERATURE MEASUREMENTS (http://spaceagecontrol.com/AD-InFlightTemperatureMeasurement.pdf) by F.Trenkle and M.Reinhardt (NORTH ATLANTIC TREATY ORGANIZATION ADVISORY GROUP FOR AEROSPACE RESEARCH AN!D DEVELOPMENT (ORGANISATION DU TRAITE DE L'ATLANTIQUE NORD) )

... so it appears that TAT is defined, but RAT is a loose term used differently by different organisations.

[BOAC]

Just to add an 'anecdote' to OS's post - Concorde's wing leading edges and other parts facing the airflow heated to over 120deg C and represented the upper aerodynamic heating limit for an aluminium airframe.

Of course, space re-entry TAT's are somewhat higher!

[zonnair]

You're obviously aware of the difference between RAT and TAT, so no further mention of this.

Wasn't that a part of his question?

Good day,

What is the difference between Ram Air Temp. and Total Air Temp.? and why do we need to be aware of them in flight?

[StudentATP]

thank you for that, zonnair. do you know the difference btw?

[BOAC]

If all else fails - SEARCH? (http://wiki.flightgear.org/index.php/List_of_abbreviations):ugh:

[Old Smokey]

StudentATP,

Sorry if I misinterpreted your RAT Vs TAT question in your first post, you actually did answer the question within the post.

A TAT measuring system measures the Total Air Temperature after Ram Rise on the forward facing parts of the aircraft, and the engines. A temperature measuring system qualifies as a TAT gauge if 99% or more of the Ram Temperature rise is recorder.

A RAT measuring system measures the Total Air Temperature after Ram Rise on the forward facing parts of the aircraft, and the engines, but due to system inefficiences measures less that 99% of the temperature Rise.

If, for example, the temperature rise was 30 degrees and the SAT -20 degrees, the TAT gauge would show +10 degrees. (Coefficient of temperature recovery (Ct) = 1).

RAT systems have a Ct less than 1, turbo-prop aircraft typically have a Ct of 0.8, thus, for the example given for SAT -20 degrees and rise of 30 degrees :

RAT = -20 + 30 X .8 = +4 degrees (6 degrees below actual TAT).

At Turbo-Prop 200 Kt or so speeds, Rise is quite small, and there's little difference between RAT and TAT. At higher speeds with the associated higher temperature rise, it becomes much more important.

The Rosemount probe (as fitted to the DC9) had a Ct of 0.958, so RAT was very close to TAT...... For the same example :

RAT = -20 + 30 X 0.958 = +9 degrees after rounding off, very close to TAT.

There are 2 formulae for finding TAT or Temperature rise, one from Mach Number, and one using TAS :

TAT = SAT X (1 + .2 X Mach No)^2...... Where SAT and TAT are in absolute temperature,

Temp Rise = (TAS / 87.1)^2...... Where TAS is in knots.
Regards,

Old Smokey

[gigi116]

As far as I remember :

RAT = SAT + Temp Rise
TAT = SAT+Temp Rise + reading error of the gauge (see on AFM)

[blackmail]

Hello Studentpilot,

to complete above posts good replies, see following formulae :
assume r = Ram recovery factor, with r equal or less than one(1).
Temperatures (Tt & T) are absolute temperatures eg Kelvin with T=t°C + 273, so 0°C = 273K for instance.

Tt(otal) = Ts(tatic)[1+0.2x r xMsquare] with Ram recovery factor r=1or 100%:). this happens in a perfect world & so we speak about TAT(Tt) = Total Air Temperature.

In a less perfect world with eg r=0.8or 80%, we have Tr(am)=Ts(tatic)[1+0.2x 0.8 x Msquare]

now as a rule of thumb & to simplify these formulae. to find your static air temp in flight : note your TAT, eg -20°C. now take your Mach nr. eg 0.7 & multiply by 3 : 7x3=21°C(this is your temp rise due to air friction heating) & add to TAT : 20+21= 41 & the figure must be negative so sat=-41°C. Or even easier, check progress page 2 of FMC(Smith) where you read your SAT directly.
hope this helps,
bm