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Guest venquessa

Temperature

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Guest venquessa

Let me get this straight about temps... I think I know very little about all the different temps. So please, please correct me here.OAT - Outside Air Temp (Usually variable with wearher and altitude)TAT - Total air temperature... something I don't remember from chemistry/physics, but is it the average temperature of a body? In realation to a plane, it's the temp of the leading edge or nose? Think I need to read up on this one.SAT - Standard air temp, usually +15C at sea level. How does this relate at altitude?there are others I know I have missed.Does anyone know of a good tutorial or webpage on these different figures?Also, more importantly, how do I get readings for all of these from within FS? Some aircraft show, OAT and TAT and some show SAT, but none that I know of show all temps.I think the PMDG 737 shows TAT, but not OAT, etc.Thanks.

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Paul et al,SAT - Static Air Temperature = OATOAT - Outside Air Temperature = SATTAT - Total Air Temperature = RATRAM - Ram Air Temperature = TATISA - International Standard AtmosphereSAT/OAT is 'The temperature at a point at rest relative to the ambient air' according to US DOD and NATO. The TAT/RAT is an engine related temperature as it takes the OAT/SAT in account and adds the temperature rise you get in the air intakes caused by speed and compression.The sea-level International Standard Atmosphere, ISA is definded as 101.325 kPa, 15

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TAT has got nothing to do with the engines! Rather, its simply the temperature that a body has, that is in direct contact with the air flow.The faster you fly, the more air molecules you hit. As you hit the air molecules, friction causes the body to heat up. (remember, temperature is simply the velocity of the atomic vibrations/movement).TAT can be simply measured by placing a temperature probe into the airflow.SAT is measured at some point, where there is no airflow. Usually at the side of the aircraft.The rest is pretty much explained by Mats.Regards,Mark

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Guest Valid_Andy

Do R.L. Pilots have to know how to work one out from the other using those formulas?:|

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Ha no way!Its not a memory item!Doubt its mentioned in any manual either. Maybe QRH but not sure about that.Although with time you do get a feeling for it. (If you know a value you can guesstimate the other one)Regards,Mark

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Mark,Yeah, I admit the wording was not good that the TAT is an engine related temperature. It is however a reason why the TAT is displayed on the engine displays. That's what I meant with it being engine related.And no TAT is not a temperature of a body in an airflow, even if this body will become affected by the total temperature of the air flowing around it. It is the SAT plus temperature rise due to compression/speed. 8<---. TOTAL AIR TEMPERATURE (TAT) PROBEsTotal air temperature is the static air temperature plus the rise intemperature due to the pitot effect.As Mr. Boyle pointed out quite a few years ago, increasing the pressure of air also increase its temperature. Pitot pressure is static pressure increased by a pressure factor which results from forward motion of the airplane through the air....8<---Cheers,

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"And no TAT is not a temperature of a body in an airflow"Hmmif TAT is -10

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>"And no TAT is not a temperature of a body in an airflow">>Hmm>if TAT is -10

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"TAT has got nothing to do with the engines! Rather, its simply the temperature that a body has, that is in direct contact with the air flow."It all depends on how you look at it, Mark.... A lot of the computations carried out by Boeing airplane engine EEC's use TAT probe info. E.g. on a CF6, the Air Data Computers provide the EEC's with Total Pressure (PT), Mach number, Ambient Pressure (Pamb) and Total Air Temperature (TAT) which use this data to perform trimming functions. RB211 engines even have their own P2T2 probes (Total Pressure and Temperature probes)....located in the engine inlet.Now that I think about it, every temperature indication on the aircraft is related to TAT. There are no SAT sensors ;-) Hope this helps.Cheers.Ian.

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>Now that I think about it, every temperature indication on the>aircraft is related to TAT. There are no SAT sensors ;-) Because SAT sensors may be very difficult to set up properly but you can probably very easily derive SAT from TAT - easier than having a complicated calibrated SAT probe.But I agree with the main theme of previous posts - engine power is senstive to SAT, not TAT since SAT is probably close to the actual air temperature of the sucked in air - befor it hits blades of the compressor.Michael J.http://www.reality-xp.com/community/nr/rsc/rxp-higher.jpg

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"engine power is senstive to SAT, not TAT since SAT is probably close to the actual air temperature of the sucked in air - befor it hits blades of the compressor."I'll defer to the experts on this, Michael, but nowhere in the (for example) 737NG Maintenance Manual Description/Operation (of Engine Management systems) is there any reference to SAT or OAT. Only TAT (using a word search). I don't know how the EEC's process/use TAT information, so I can't say either way what is more important to engine management. I would have thought any interaction between air and engine would have involved the air hitting something ...therefore generating a ram rise. Only the computed takeoff thrust limit, which is usually achieved before a certain ground/airspeed, seems to be derived from OAT. From then on, dynamics appear to play a part.Looking forward to more input on the subject.Cheers.Ian.

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Guest boxjockey99

Too bloody right its not a memory item! Can we stop this talk of equations and acronyms I'm getting flashbacks of my days in ATPL grounds school and its not nice! All you need to know is that engine anti icing must be ON when at cruise or descent power between +10 TAT and -40 SAT. Beyond that we don't talk of these magical incantationsKris

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Guest PittsburghII

Hi Mats, Simple question because I am feeling like a dimwit here: What do the equal signs mean in the list of the temperatures above? I am really confused! Somewhere in the back of my mind, my math teaching kicks in and tries to get me to believe that SAT = OAT etc. Also, where in this does Saturated Air Temperature fit in, if at all?Thanks,BoazEKCH

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"It all depends on how you look at it, Mark...."fair 'nough. Just wanted to make sure no one thought that TAT was influenced by the engines.Regards,Mark

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Guest venquessa

SAT = OAT etc. Also, where in this does Saturated Air Temperature fit in, if at all?Outside Air Temperature = Static Air temperature.I think what is being said is this. On the plane traveling at 290kts the airflow will create heat as it hits the plane, giving you TAT. Think of it if you stuck your head out the window the air might be -30C and your nose would almost instantly freeze, like a blast freezer. Yet the actual friction and molecular impact of the air on your face would in effect heat your face up, reletive to the -30C air.Further as the whole plane is travelling at 290kts it too will be effected by the air molecules smashing into the airframe.So... you can't really messure OAT accurately. The only thing you can try and messure is static air temperature. Using a resessed port (the Static port) you can messure the air temperature there, where there is no airflow. However, there will be a pressure drop in the static port, because the airflow will be creating a vacum effect in the port opening. Less pressure means, less temperature.So I think the distinction between OAT and SAT and why they are designated two different temps is that SAT is an inaccurate messurement of OAT. They "should" be equal, but engineers would simply refuse to label the guage OAT because technically it's not OAT, it's SAT.Of course, I'm thinking this based on a fairly average degree level knowledge of physics, most of which I am forgetting as each day goes by.As an asside, Concorde travels at a GS of very nearly 1200kts (an IAS of about 480 - 560kts (Mach 2.02). At 55,000ft and 560kts with an ISA of +5 the TAT will be about +115C, even though the OAT/SAT will be around (guessing) -50C or lower. The aluminium sheeting that makes the airframe has an operating temperature limit of around 150C, If anyone here has bought the Altitude Concorde they might notice odd things happening to cruise performance when flying to Bahrain or Singapore, as you come down from the Med into the middle east the ISA starts to rise and Concorde MAX Cruise on the A/P will start to drop speed and therefore descend slowly to keep the TAT around +115C, or the airframe might expand too much, or worse fatiqe the metal.

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Guest Pittsburgh

Thanks, Paul! Very informative...

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