FLIGHT INSTRUMENTS
HANDBOOK
COMPILEDBY: AKSHEY SOOD
,AKSHEY SOOD
, AIR TEMPERATURE MEASUREMENT
Static Air Temperature (SAT) is the temperature of the undisturbed air through which the aircraft is
about to fly.
Total Air Temperature (TAT) is the maximum temperature attainable by the air when brought to
rest, adiabatically.
The increase of air temperature at higher speeds as a result of the adiabatic compression of the air
is known as the “Ram Rise”.
The percentage of the “Ram Rise” sensed, and recovered, by a TAT probe is termed the Recovery
Factor (Kr).
AIR TEMPERATURE THERMOMETERS
Direct Reading
1. A commonly used direct reading thermometer used in low speed aircraft uses a bimetallic strip
consisting of two metals
2. When this strip is heated, the brass, having a higher coefficient of expansion than the Invar, will
expand much more than the Invar, with the result that the strip will bend
3. How much the strip bends depends on the temperature rise to which the strip is subjected, and
is therefore a measure of the temperature
Remote Reading
1. The probe element forms one part of a resistance bridge circuit. As the temperature changes the
resistance of the sensing element changes, and the bridge becomes unbalanced causing current
to flow through the moving coil of the indicator
TOTAL AIR TEMPERATURE PROBE
1. A pure platinum wire resistance-type sensing element is used
2. A heating element is mounted integral with the probe to prevent the formation of ice
3. The bleed holes in the intake casing permit boundary layer air to be drawn off under the
influence of the higher pressure that is created within the intake and casing of the probe.
4. In order to measure air temperature on the ground an air to air ejector (aspirator) may be fitted
to the probe. Engine bleed air creates a negative differential pressure within the casing so that
outside air is drawn through it at a rate sufficient to provide a reliable indication of
temperature.
5. This device is used with turbine engine take off setting and auto throttle systems; it eliminates
temperature soaking inaccuracies caused by bright sunshine heating the probe, or hot ramp
heat radiation.
ERRORS
Aircraft thermometers used for the measurement of air temperature are subject to the following
errors:
Instrument error imperfections in manufacture.
AKSHEY SOOD
, Environmental error solar heating of the sensor
Ice accretion on the probe
Heating error -adiabatic and kinetic (friction)
heating
AKSHEY SOOD
HANDBOOK
COMPILEDBY: AKSHEY SOOD
,AKSHEY SOOD
, AIR TEMPERATURE MEASUREMENT
Static Air Temperature (SAT) is the temperature of the undisturbed air through which the aircraft is
about to fly.
Total Air Temperature (TAT) is the maximum temperature attainable by the air when brought to
rest, adiabatically.
The increase of air temperature at higher speeds as a result of the adiabatic compression of the air
is known as the “Ram Rise”.
The percentage of the “Ram Rise” sensed, and recovered, by a TAT probe is termed the Recovery
Factor (Kr).
AIR TEMPERATURE THERMOMETERS
Direct Reading
1. A commonly used direct reading thermometer used in low speed aircraft uses a bimetallic strip
consisting of two metals
2. When this strip is heated, the brass, having a higher coefficient of expansion than the Invar, will
expand much more than the Invar, with the result that the strip will bend
3. How much the strip bends depends on the temperature rise to which the strip is subjected, and
is therefore a measure of the temperature
Remote Reading
1. The probe element forms one part of a resistance bridge circuit. As the temperature changes the
resistance of the sensing element changes, and the bridge becomes unbalanced causing current
to flow through the moving coil of the indicator
TOTAL AIR TEMPERATURE PROBE
1. A pure platinum wire resistance-type sensing element is used
2. A heating element is mounted integral with the probe to prevent the formation of ice
3. The bleed holes in the intake casing permit boundary layer air to be drawn off under the
influence of the higher pressure that is created within the intake and casing of the probe.
4. In order to measure air temperature on the ground an air to air ejector (aspirator) may be fitted
to the probe. Engine bleed air creates a negative differential pressure within the casing so that
outside air is drawn through it at a rate sufficient to provide a reliable indication of
temperature.
5. This device is used with turbine engine take off setting and auto throttle systems; it eliminates
temperature soaking inaccuracies caused by bright sunshine heating the probe, or hot ramp
heat radiation.
ERRORS
Aircraft thermometers used for the measurement of air temperature are subject to the following
errors:
Instrument error imperfections in manufacture.
AKSHEY SOOD
, Environmental error solar heating of the sensor
Ice accretion on the probe
Heating error -adiabatic and kinetic (friction)
heating
AKSHEY SOOD
