Altimeter Principles and Limitations

Altimeter Principle and Operation

A pressure altimeter measures altitude by comparing the static pressure outside the aircraft to a sealed reference inside an aneroid capsule. As altitude increases, external pressure drops, causing the capsule to expand and move the altimeter needle. Most altimeters allow pilots to set a reference pressure (QNH, QFE, or standard 1013 hPa), which aligns the instrument to local or standard atmospheric conditions.

Types of Altimeters

• Simple Altimeter (Single Capsule): Basic design, limited range and sensitivity, suitable for light aircraft.
• Sensitive Altimeter (Multi-Capsule): Uses multiple capsules for greater accuracy and range, common in commercial aviation.
• Servo-Assisted Altimeter: Employs electromagnetic feedback to reduce mechanical friction and lag, offering improved precision at higher altitudes.

Altimeter Limitations and Errors

• Static System Error: Caused by inaccurate static pressure sampling due to airflow around the aircraft.
• Instrument Error: Stemming from internal friction, manufacturing imperfections, or capsule hysteresis.
• Barometric Error: Incorrect subscale setting leads to wrong altitude readings; can be corrected by adjusting the reference pressure.
• Temperature Error: When actual temperature deviates from ISA, the indicated altitude may over-read (cold air) or under-read (warm air). Corrections are especially crucial in cold conditions near terrain.
• Lag: Delay in the altimeter's response to rapid altitude changes, often due to capsule hysteresis or mechanical inertia.

Altimeter Setting and Altitude Definitions

• QNH: Sets altimeter to indicate elevation above mean sea level.
• QFE: Sets altimeter to read zero at a specific airfield elevation.
• Standard (1013 hPa): Used for flight levels above the transition altitude.
• Indicated Altitude: What the altimeter displays.
• True Altitude: Actual height above mean sea level.
• Pressure Altitude: Altitude above the standard datum plane (1013 hPa).
• Density Altitude: Pressure altitude corrected for non-standard temperature.

GPS Altitude as a Cross-Check

GPS provides an independent altitude reference, useful for detecting altimeter errors. However, GPS altitude is referenced to a mathematical model of the Earth (WGS-84 ellipsoid) and may not match pressure altitude precisely. GPS is not approved as a primary altitude source but is valuable for troubleshooting discrepancies.

Correction Tables

Pilots use correction tables to adjust for temperature and position errors, especially during approaches in cold weather, ensuring obstacle clearance and regulatory compliance.