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PilotSchool Knowledge Base

Using Altimeters for Aerodrome Measurements

Introduction

Understanding how to use an altimeter is crucial for pilots, especially in the context of take-off and landing performance. This section covers the fundamental principles and procedures required to determine the pressure height, elevation, and local QNH of an aerodrome using an altimeter.

Altimeters and Atmospheric Pressure

Altimeters measure altitude based on atmospheric pressure. The altimeter compares the current atmospheric pressure against a standard reference pressure. Atmospheric pressure decreases with altitude, and altimeters are calibrated to reflect this change using standard pressure references.

Key Altimeter Settings

  • QNH: Pressure setting adjusted to indicate the altitude above mean sea level (AMSL).
  • QFE: Pressure setting that measures height above a specific airfield.
  • Standard Pressure: Set at 1013.2 mb or 29.92 inHg, used above transition altitudes.

For further technical details, consult the following sources:
How Does An Altimeter Work? - Boldmethod,
Altimeter Pressure Settings - SKYbrary Aviation Safety.

Obtaining Pressure Height of an Aerodrome

Understanding Pressure Altitude

Pressure altitude is the altitude indicated when the altimeter is set to the standard pressure of 29.92 inHg. It is important for standardizing altitude measurements regardless of local atmospheric pressure conditions.

  1. Direct Altimeter Reading:

    • Set the altimeter to 29.92 inHg to read the pressure altitude directly.

  2. Pressure Altitude Formula:

Utilizing this formula allows for manual calculation of pressure altitude. Resources for practical applications include:
Pressure Altitude Explained - Pilot Institute.

Determining Aerodrome Elevation

Setting the Altimeter to QNH

The local QNH setting is essential for determining aerodrome elevation accurately:

  • Set QNH: Adjust the altimeter to reflect the local QNH. This will show the aerodrome elevation when the aircraft is on the ground.
  • Routine Checks: Checking the altimeter against known elevations ensures its accuracy.

Deviations from International Standard Atmosphere (ISA) conditions can impact the readings. Reference:
VFR altimetry - VFRG.

Obtaining Local QNH at an Aerodrome

Derivation and Application

Local QNH is derived from the sea-level pressure at or near the aerodrome and is crucial for:

  • Altitude Accuracy: Ensures that altimeter readings are accurate during approaches and departures.
  • Transition altitudes: Set the local QNH within 100 nautical miles of the aerodrome.

The QNH setting impacts flight safety, requiring regular updates from sources like ATIS and AWIS. Errors in QNH settings can lead to significant altitude misrepresentations, affecting flight safety. For extensive details, see:
QNH altimeter settings - Bob Tait’s Aviation Theory School.

Conclusion

Accurate use of altimeter settings is fundamental for determining pressure height, elevation, and local QNH at aerodromes. By adhering to these procedures and understanding the impact of atmospheric conditions, pilots can ensure safe and precise flight operations.