Altimeter Fundamentals: Static Pressure, Subscale Settings, and Errors

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).