Temperature Measurement

Temperature Measurement

Outside Air Temperature Measurement

• Outside Air Temperature (OAT) affects air density which in turn affects
• Aircraft Performance
• Engine power
• Measurement of speed & altitude
• Temperature measurement is required to avoid icing conditions

Flight Safety Hazards of Aircraft Icing

• Aircraft Icing is quite hazardous to flight safety
• Loss of lift
• Increase in weight & drag
• Freezing and Loss of controls
• Reduced engine power
• Engine Flame out
• Airframe Damage

Direct Reading Thermometer

• Direct reading thermometer is fixed on the wind shield of low-speed aircrafts
• Measures temperature by unequal expansion of bi-metallic strip
• Normally Invar & Brass are used in the bi-metallic strip
• Strip bends in the direction of metal with low coefficient of expansion
• This thermometer is unsuitable at higher speeds due to adiabatic heating
• Adiabatic heating is caused due to high-speed movement of aircraft in air

Remote Indicating Thermometer

• Measures temperature by change in electrical resistance
• Whetstone’s bridge is used in this type of thermometer
• One part of the circuit is exposed to outside air while the other is inside
• Measures flow of current in circuit due to by imbalanced resistance
• Provides temperature information in electrical form
• Information can be transmitted to other instruments and systems

Design of Total Air Temperature Probe

• TAT Probe is located in the front fuselage to avoid boundary layer effect
• Uses platinum sensors for high conductivity and rapid response
• Housed in an air intake made of Nickel-plated beryllium Copper
• Makes the air intake strong and ensures good thermal conductivity

Construction of Total Air Temperature Probe

• Air is designed to reach the probe at right angle to the airflow
• Removes solid particles, water drops and high pressure air
• Heating element is provided to avoid icing
• Heating error is insignificant at less than 1 deg

Design of Aspirated Total Air Temperature Probe

• Stagnant air in the housing contains pre-heated air in a stationary aircraft
• This provides incorrect temperature information on ground
• Aspirator is used to remove the stagnant air by circulating air from engine
• Aspirated TAT Probe provides correct temperature information on ground

Instrument and Environmental Errors in TAT Probe

• Instrument errors are caused due to manufacturing imperfections
• Reduced by better manufacturing technology
• Environmental errors originate from environmental factors
• Error due to solar heating of temperature sensor
• Overcome by shielding the probe with covers
• Error due to ice accretion
• Reduced by using heating element

Heating Errors in Temperature Measurement

• Heating errors are caused due to movement of aircraft through the medium
• Indicates higher than prevailing temperature
• Heating errors increases at higher speeds

Kinetic Heating Errors in Temperature Measurement

• Kinetic heating is caused due to friction
• More number of molecules hit the probe every second
• Error is maximum in direct reading temperature probe

Adiabatic Heating Errors in Temperature Measurement

• Adiabatic heating is caused due to compression of air
• Energy of moving air being brought to rest rapidly
• Maximum error in remote indicating temperature probe

Static Air Temperature

• SAT is the actual temperature of un-disturbed air outside the aircraft
• Our aim is to measure this temperature correctly
• SAT is not obtained directly, but only by correcting for errors in TAT probe
• SAT is also called Corrected Outside Air Temperature (COAT)

Total Air Temperature

• TAT is the temperature attained by air when brought to rest by the probe
• Total Ram Rise is the real increase in temperature due to adiabatic heating
• TAT will always be higher than the SAT due to Total Ram Rise
• Ram rise measured will be lesser than the Total ram rise
• Due to leaks and inefficiencies in measurement

Ram Air Temperature

• RAT is the temperature as detected and indicated by the TAT probe
• Measured Ram Rise is the increase in temperature as measured
• RAT is the SAT affected by Measured Ram Rise
• RAT is also called Indicated Outside Air Temperature (IOAT)

Total and Measured Ram Rise

• Total Ram Rise is the difference between
• Total Air Temperature (TAT) and Static Air Temperature (SAT)
• Measured Ram Rise is the Ram Rise as measured by TAT probe
• Lesser than Total Ram Rise due to loss in measurement
• Measured Ram Rise is the difference between
• Ram Air Temperature (RAT) and Static Air Temperature (SAT)

Recovery Factor

• Recovery factor is the ratio between Measured and Total Ram Rise
• Measured Ram Rise = Total Ram Rise x Recovery Factor
• Measured temperature = Actual Temperature + Measured Ram Rise
• Ram Air Temperature = Static Air Temperature + Measured Ram Rise
• In other words
• IOAT = COAT + Measured Ram Rise

Units of Temperature Measurement

• Celsius scale is based on boiling point of water which is taken as 100 deg
• Kelvin Scale starts from absolute zero
• Absolute Zero indicates ‘No Heat’ in the system
• Kelvin = Celsius + 273
• Conversion of Centigrade to Kelvin
• 1 deg Celsius = 273 Kelvin
• 1 deg Kelvin = – 273 deg Centigrade

Formula for Calculation of Static Air Temperature

• Rapid formula for use in air
• Ram Rise = (TAS /100)2
• Accurate formula recommended in exams
• SAT = RAT / (1 + 0.2 X Recovery Factor X Mach Number Squared)
• Recovery factor given in aircraft operation manual

Calculation Methods for Static Air Temperature

• Calculation by Navigation computer (Recommended in exams)
• Look up in the Air data correction tables for the type of aircraft
• Automatic computation by Air data computer (ADC)

Calculation of Static Air Temperature (example)

• RAT (IOAT) (Indicated Temperature) = -20°C
• Mach No = M 0.73 (Normal Cruise Speed)
• Recovery Factor (Kr) = 0.98 (From Flight Manual)

Calculation of Static Air Temperature (example)

• Calculation of SAT (COAT)
• RAT of -20°C is 253 K
• SAT = TAT / (1 + 0.2 Kr M²)
• SAT = 253 + (0.2 × 0.98 × 0.73²) => SAT = 253/ (1 + 0.1044)
• SAT = 253/ 1.1044 = 229 K => 229 K is -44°C on the Celsius scale.

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