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Temperature

in Basic Instruments

Determination of correct temperature is necessary for accurate computation of airspeed and altitude. Temperature, airspeed, and altitude are all closely interrelated, and the navigator must be familiar with each in order to work effectively and accurately.


Temperature Gauges

The temperature gauge that is most commonly used in aircraft employs a bimetallic element. The instrument is a single unit consisting of a stainless steel stem that projects into the airstream and a head that contains the pointer and scale. The sensitive element in the outside end of the stem is covered by a radiation shield of brightly polished metal to cut down the amount of heat that the element might absorb by direct radiation from the sun. The bimetallic element (called the sensitive element) is so named because it consists of two strips of different metal alloys welded together. When the element is heated, one alloy expands more rapidly than the other, causing this element, which is shaped like a coil spring, to turn. This, in turn, causes the indicator needle to move on the pointer dial. Temperatures between –60 °C and +50 °C can be measured on this type of gauge.

Temperature Scales

In the United States, temperature is usually expressed in terms of the Fahrenheit scale (°F). In aviation, temperature is customarily measured on the centigrade, or Celsius (°C), scale. Although aircraft thermometers are usually calibrated in °C, it is sometimes necessary to interconvert Fahrenheit and Centigrade temperatures. The following formulas may be used:

°F = (1.8 × °C) + 32
°C = (°F – 32°) ÷ 1.8

Temperature error is the total effect of scale error and heat of compression error. Scale error is simply an erroneous reading of the pointer under standard conditions. It is difficult for a crewmember to evaluate this error without sensitive testing equipment. With this in mind, the reading of the indicator is considered correct and is called indicated air temperature (IAT).

The second error, heat of compression error, causes the instrument to read too warm. Heating occurs at high speeds from friction and the compression of air on the forward edge of the temperature probe. Thus, the IAT is always corrected by a minus correction factor to produce true air temperature (TAT). Heat of compression increases with TAS. The TAT can be obtained from the aircraft flight manual.

 

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