Altimeter Fundamentals: Static Pressure, Subscale Settings, and Errors

022-02-04Pablo Asensio Martínez2026-03-271 min

The altimeter is essentially a calibrated barometer that measures static pressure and displays it as altitude based on the International Standard Atmosphere (ISA).

Principle of Operation

  • Sensor: Contains essentially evacuated aneroid capsules (vacuum inside).
  • Mechanism: Static pressure is fed into the instrument case surrounding the capsules.
    • Climb: Pressure decreases $\rightarrow$ Capsules expand.
    • Descent: Pressure increases $\rightarrow$ Capsules compress.
  • Linkage: This expansion/compression is magnified by mechanical linkages to rotate the pointers.

Types of Altimeters

  • Sensitive Altimeter: Uses multiple capsules for accuracy and range (tens, hundreds, thousands of feet).
  • Servo-Altimeter: Uses an electrical motor to drive the display.
    • Advantages: Eliminates lag error, greater accuracy at high altitudes, enables digital outputs.
    • Disadvantage: Relies on electrical power (flags appear on failure).

Errors and Checks

  • ISA Deviation: Indicates true altitude only in ISA standard conditions.
  • Blockage: If the static port is blocked, the altimeter freezes at the altitude where the blockage occurred.
  • Lag Error: Mechanical altimeters have a slight delay during rapid altitude changes.
  • GPS Altitude: Geometric altitude derived from satellites. It is independent of pressure/temperature errors and useful for gross error checking of barometric altimeters, though it references a mathematical model (ellipsoid) rather than MSL.

Subscale Settings

  • QNH: Indicates Altitude above Mean Sea Level (MSL).
  • QFE: Indicates Height above a specific reference datum (like an airfield).
  • STD (1013.25 hPa): Indicates Pressure Altitude (Flight Levels).