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Guest swanrunner

TAT Reading

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Just wondering if someone could explain the difference in temp between the progress screen on the MCDU and in the top right of the engine display. Both match ATIS on the ground but can have as much as 30deg difference at cruise. The progress screen temp always reflects close to the forecast temp from The Active Sky forecast but the TAT display is much warmer. Thanks John

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My (simplistic) understanding is that TAT is the ambient temperature corrected for the motion of the air against the surface of the airplane. There is a lot of energy in that airflow, which results in the higher temperature that you see on the EICAS. The temperature displayed on the progress page of the MCDU is the ambient temperature as would be measured by a thermometer at rest relative to the airflow.I suppose the TAT is the more important of the two since this is what the airplane "experiences". So you will not accumulate ice on the airplane flying in clouds as long as the TAT is above the freezing point by a safe margin, even if the ambient temperature is below freezing.Now I'm waiting for someone to correct me, since that is how I learn :)Tom

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Just wondering if someone could explain the difference in temp between the progress screen on the MCDU and in the top right of the engine display. Both match ATIS on the ground but can have as much as 30deg difference at cruise. The progress screen temp always reflects close to the forecast temp from The Active Sky forecast but the TAT display is much warmer. Thanks John
JohnHeard anything about OAT and TAT??Both belongs to basics of Aviation...You've got something to read now during the Weekend...Tell me, when you got itRegardsDietmar

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Found this at faa.govStatic air temperature (SAT) is what would be measured from a balloon, and is the temperature given in a forecast or report. It is also referred to as outside air temperature (OAT).Total air temperature (TAT) is obtained by a probe having velocity with respect to the air. Because of kinetic heating on the upstream side of the probe, TAT is warmer than SAT. SAT is computed from TAT and other flight conditions by an air data computer for dry air. There is less kinetic heating in saturated air than dry air.Indicated outside air temperature (OAT) is measured by a simple sensor in the airstream - essentially a thermometer. Typically, OAT values will be SAT or OAT plus approximately 80% of the difference from SAT to TAT.Surface temperature varies with pressure along the airfoil. At the leading edge where pressure is the highest, the surface temperature will also be higher than further aft. If the local surface temperature on the airfoil is warmer than freezing, no ice will form. Infra-red measurements of a typical airfoil in the icing tunnel at a true airspeed of 150 knots show that there can be a decrease in temperature of more than 3.5

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Right JohnThe TAT is always higher than the OAT (or SAT), except on the ground at zero speed.Because of the dynamic pressure at any airspeed the air warms up, at thefront surfaces of the aircraft. That's the reason why both Temp indicators showabout 30

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