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PilotSchool Knowledge Base

Altimetry and Altimeter Settings

Learning Outcomes

This section provides a comprehensive guide to understanding altimetry in aviation as required for the CASA PPL pilot license theory exam. It encompasses the recall of essential terminology, the ICAO standard atmosphere parameters, altimeter subscale settings, transition layer derivation, local QNH derivation, and procedures for proper altimeter settings.

Key Terminology

Altitude

Flight Level (FL)

  • Definition: Altitude of an aircraft determined by a pressure altimeter set to the standard pressure of 1013 hPa (29.92 inHg), used in flight level assignments (e.g., FL250 = 25,000 feet).
  • Source: SKYbrary Aviation Safety, Wikipedia

Height

  • Definition: The vertical distance of an object above a specified datum, such as ground level.
  • Source: SKYbrary Aviation Safety

ICAO Standard Atmosphere Parameters

Mean Sea Level (MSL) Temperature and Pressure

  • MSL Temperature: 15°C (59°F).
  • MSL Pressure: 1013.25 hPa (29.92 inHg).
  • Pressure Lapse Rate: Approx. 6.5°C per km or 2°C per 1,000 feet in the troposphere.
  • Source: Wikipedia, SKYbrary

Altimeter Subscale Settings

Datum References

  • Standard Pressure Setting (SPS): Altimeter indicates flight levels above transition altitude.
  • Area QNH: Indicates altitude above MSL within a specified area, often reported in METARs.
  • Local QNH: Similar to area QNH but specific to a local region.
  • QFE: Altimeter reads height above the airfield or a specific reference point.

Transition Layer Derivation

  • Transition Altitude (TA): Altitude where aircraft switch from local QNH to standard pressure (1013 hPa).
  • Transition Level (TL): Lowest flight level above TA, using standard pressure for descent.
  • Transition Layer: Space between TA and TL; reference AIP for area-specific transitions.
  • Source: Recreational Flying

Deriving Local QNH

  • Method: Convert airfield elevation into pressure units and adjust QFE to determine local QNH.
  • Conversion:
    • Feet to millibar: Divide elevation by 30.
    • Inches to QNH: Divide elevation by 900.
  • Example: For 200 ft elevation and QFE 1015 mb, QNH ≈ 1022 mb.
  • Source: Aviation Stack Exchange

Calculating Height Errors

  • Height Error: Inaccurate subscale setting leads to altitude deviations.
  • Common Errors: Confusing hPa and inHg settings; significant deviations in temperature affect altimeter accuracy.
  • Error Prevention: Regular SOP checks and corrections using standard procedures.
  • Source: NASA, SKYbrary

Altimeter Adjustment Procedures

  • Climb: Change to standard pressure at transition altitude.
  • Descent: Set to local QNH at or below TL.
  • Cross-Checking: Essential to ensure accuracy; involve crew and ATC for verification.
  • Mnemonic COAL: Used for correct setting transitions.

This comprehensive guide emphasizes the critical aspects of altimetry required for pilot theory exams, encompassing detailed concepts and practical application. It highlights the importance of understanding and implementing proper altimeter settings and adjustments to ensure safe and compliant flight operations.