Air Temperature Measurement

1. Introduction — Why Pilots Need Air Temperature

A pilot must know the temperature of the surrounding air for the following reasons:

1.1 Avoidance of Icing Conditions

Ice formation on aircraft, particularly in cloud, can be very rapid and extremely dangerous. Icing effects include:

• Loss of lift and increase in drag — ice distorts the aerofoil shape
• Increase in mass — can be as much as ten tonnes on large aircraft with thick icing
• Freezing of control surfaces so they cannot be moved
• Loss of engine power or total engine failure due to intake or carburettor icing
• Ice shed from propellers striking the fuselage

1.2 Engine Power and Aircraft Performance

Aviation engines (jet and piston) require a correct fuel/air ratio for combustion. Dense (cold) air allows more fuel to be injected, producing more power. Less dense (warm) air reduces available power — this has significant effects on take-off performance calculations.

1.3 Measurement of Speed

Airspeed cannot be measured directly — dynamic pressure is measured instead. Dynamic pressure depends on both aircraft speed and air density. Since temperature affects density, temperature data is essential to compute True Airspeed (TAS).

1.4 Measurement of Altitude

The rate of pressure change with altitude varies with temperature. Altimeter indications contain a temperature error that can create potentially dangerous under-readings near high ground in cloud.

2. Air Temperature Thermometers

2.1 Direct Reading Thermometer

Operates on differential thermal expansion. A bimetallic strip bonds two metals with different coefficients of expansion:

• Invar — nickel-steel alloy; uniquely low coefficient of thermal expansion.
• Brass — higher coefficient of thermal expansion.

On heating, the brass expands more than the invar, causing the strip to curl. The amount of curl is proportional to temperature rise. The strip is drawn into a helix to amplify pointer movement. The probe protrudes through the windscreen or fuselage into the airstream; the dial is visible to the pilot.

2.2 Remote Reading Thermometer

Uses a sensor whose electrical resistance changes with temperature. The sensor is located anywhere suitable on the fuselage (away from boundary layer) and sends an electrical signal to a remote cockpit indicator and to other aircraft systems.

flowchart LR
    A["Outside Air\n(TAT Probe)"] --> B["Platinum\nResistance Wire\n(sensor)"]
    B --> C["Electrical\nSignal"]
    C --> D["Cockpit\nIndicator"]
    C --> E["Air Data\nComputer"]
    C --> F["Other\nSystems"]

3. Total Air Temperature (TAT) Probe

The TAT probe is a small strut and air intake made of nickel-plated beryllium copper — chosen for good thermal conductivity and mechanical strength. It is fixed to the fuselage at a point that keeps it clear of the aircraft’s boundary layer.

3.1 Key Design Features

3.2 Ground Temperature Measurement (Aspirated Probe)

Modern aircraft use reduced take-off power to protect engines from thermal stress (e.g. 93% power is a typical example, computed from runway length, weight, altitude and temperature). When stationary on the ground there is no natural airflow through the probe. An aspirator (air-to-air ejector) uses engine bleed air or APU bleed to create suction, drawing fresh outside air through the casing even when the aircraft is stationary — preventing heat-soaked stagnant air from being measured.

4. Errors in Air Temperature Measurement

5. Heating Error — Compression and Kinetic Heat

5.1 Kinetic Heating

Primary contributor in direct reading thermometers. As aircraft speed increases, more air molecules per second impact the probe surface, generating frictional heat at the surface.

5.2 Adiabatic (Compression) Heating

Primary contributor in remote reading (total head) thermometers. The high-speed airflow (potentially several hundred knots) is brought virtually to rest inside the platinum sensing chamber very rapidly. The kinetic energy of the moving air converts to temperature rise through adiabatic compression — analogous to pumping a bicycle pump (pressure energy converts to heat with no external flame).

The combined kinetic + adiabatic effect always totals a known quantity called the Total Ram Rise. Because no probe is perfectly efficient, the actually measured quantity is the Measured Ram Rise.

5.3 Temperature Definitions

6. Ram Rise and Recovery Factor — Worked Example

7. Correction of TAT/RAT to SAT

Methods to convert RAT to SAT (in order of exam relevance):

1. Rapid formula (in-flight quick approximation — not for JAA/EASA/DGCA exams)
2. CRP-5 navigation computer (blue scale — use in exams)
3. Accurate Kelvin formula (most precise)
4. Data Tables
5. Air Data Computer (automatic, used in modern aircraft)

7.1 Rapid Formula (In-Flight Only)

7.2 CRP-5 Navigation Computer

On the slide-rule face there is a blue scale: outer ring = TAS in knots; inner ring = Ram Rise in °C. Set TAS on the outer scale, read Ram Rise on the inner scale.

8. The Absolute (Kelvin) Scale & Accurate Formula

The Celsius scale changes sign at 0°C (freezing point of water), creating issues with mathematical ratios. The Kelvin (Absolute) scale starts from absolute zero — the theoretical point of no thermal energy, occurring at −273°C.

Conversion: K = °C + 273  |  One kelvin = one degree Celsius (same increment size, different baseline).

8.1 Accurate Formula

Variables: TAT = Total Air Temperature in Kelvin; K_r = Recovery Factor (dimensionless, from flight test data); M = Mach Number.

9. Calibration and the International Standard Atmosphere (ISA)

Due to the variable nature of the real atmosphere, a standard calibration model is used. All air data instruments are calibrated to the International Standard Atmosphere (ISA).

9.1 ISA Mean Sea Level (MSL) Values

9.2 ISA Atmospheric Layers

Calibration is carried out with both increasing and decreasing readings to determine lag at calibration conditions. Any residual errors within agreed tolerances are listed as instrument errors over the operating range.

Practice Questions & Detailed Answers

Master Reference Tables — Chapter 3

Numerical Values

Formula Sheet

Mnemonics

Source Answer Key