Measurement of Air Data

Understanding Air Data Measurement for ATPL Students

Air data measurement underpins many aircraft systems and procedures in both general aviation and airline operations. For ATPL theory, you must understand how pitot-static measurements are converted into speed, altitude, and Mach number, and how environment and instrument design affect accuracy. The pitot tube senses total pressure (static plus dynamic), while the static port measures static pressure. From these, instruments and the Air Data Computer (ADC) derive IAS/CAS/EAS/TAS, altitude, and Mach number. A servo-assisted altimeter enhances accuracy by using aneroid capsules and an electro-magnetic pick-off to detect tiny movements, while the ADC computes altitude directly from static pressure and applies corrections and filters.

Speed terminology is central. IAS is the instrument readout; CAS corrects IAS for instrument and position errors; EAS further corrects CAS for compressibility (so EAS is always less than or equal to CAS); and TAS is the speed through the air mass. At a constant CAS, climbing decreases EAS (air density drops), while descending increases EAS. Flying at a constant CAS and constant flight level from a colder to a warmer air mass raises TAS to maintain the same dynamic pressure. The Mach number is TAS divided by local speed of sound; below the tropopause in ISA conditions, a descent at constant Mach causes both IAS and TAS to increase. Although a Machmeter’s reading does not need a direct outside air temperature input, it inherently reflects temperature via the pressure ratio it senses. The Total Air Temperature (TAT) probe measures temperature rise due to kinetic heating and compression from the aircraft’s motion.

Altimetry relies on the ISA model and pressure surfaces. A Flight Level corresponds to a fixed pressure, regardless of actual temperature; for example, FL70 is approximately 782 hPa. Correct static sourcing is vital: the altimeter is supplied with static pressure, and both pitot and static systems contribute to position error (affecting Mach and airspeed). To minimize boundary-layer effects, the total pressure head is mounted on a mast that projects into free airflow. Pressure measurement devices include the Bourdon tube (for general pressure gauges) and aneroid capsules (for pitot-static instruments). From a systems perspective, reliable electrical power is essential for instrument accuracy and servo systems; a thermal current limiter in DC systems allows short transient overloads before rupturing, protecting avionics. Finally, key operating speeds are presented on the ASI: for example, VLO is the maximum landing gear operating speed defined by the manufacturer and aviation regulations.

Key Relationships and Procedures

• Mach = TAS / a, where a depends on temperature; constant Mach descent below the tropopause increases IAS and TAS.
• EAS ≤ CAS; at low speeds they are nearly equal, diverging with compressibility.
• At constant CAS and pressure altitude, warmer air (lower density) increases TAS.
• Altimeter and ADC derive altitude from static pressure; flight levels map to fixed pressures (ISA-based).
• Pitot-static installation and mast standoff reduce boundary-layer and position errors.

What This Question Bank Covers

• Pitot-static system components, position error, and pressure measurement (Bourdon tube, capsules).
• IAS, CAS, EAS, TAS, Mach number, TAT, and effects of altitude and temperature.
• Altimetry: ISA, flight levels, static pressure sourcing, and servo-assisted altimeters.
• ADC functions and data outputs used across aircraft systems and procedures.
• V-speeds on the ASI (e.g., VLO) and their operational significance under aviation regulations.
• Electrical protection (thermal current limiters) supporting instrument and avionics reliability.