Altimeter Usage for Aerodrome Measurements

This section covers critical use of altimeters to obtain essential values for safe and effective aviation operations. Understanding how to determine local QNH, elevation, and pressure height at an aerodrome are key skills required for pilots under the Operations, Performance & Planning competency.

Local QNH at an Aerodrome

Definition and Importance of QNH

• QNH is the atmospheric pressure adjusted to mean sea level, critical for accurate altitude readings (What does “QNH” mean? - GlobeAir).
• It ensures precision for takeoff and landing operations, and is essential for effective airspace management.

Obtaining Local QNH

1. Sources of QNH:

   • QNH can be obtained from ATIS, AWIS, or directly from Air Traffic Control (ATC).
   • It’s crucial to regularly update the altimeter with the latest QNH to maintain correct altitude readings.

2. Altimeter Adjustment:

   • Manually set the altimeter to the local QNH to ensure the instrument reflects the altitude above sea level accurately.
   • This alignment provides a uniform reference for flights, aiding navigation and vertical separation safety (What does “QNH” mean? - GlobeAir).

Determining the Elevation of an Aerodrome

Setting and Reading the Altimeter

1. Ground Check:

   • Set the altimeter to the current reported airfield QNH setting.
   • Verify the altimeter reads the known aerodrome elevation. Discrepancies greater than ±75 feet should prompt reevaluation (FAA - ENR 1.7).

2. Altimeter Position:

   • By correctly setting the QNH, the altimeter reflects the elevation above mean sea level when at the aerodrome (Altimeter Pressure Settings - SKYbrary).

Calculating Pressure Height of an Aerodrome

Pressure Altitude Fundamentals

• Definition: Pressure altitude is an altitude obtained by setting the altimeter to a standard pressure setting of 29.92 inHg. It represents the height above the Standard Datum Plane (Pressure Altitude Explained - Pilot Institute).

Formula for Pressure Height Calculation

• Australian Aviation Formula:

  • Example: For a QNH of 1027 and an elevation of 1800 ft, the pressure height would be 1380 ft (Altitude Calculator - Pilot Practice Exams).

• Leveraging tools like flight computers (e.g., CX3 Flight Computer) can simplify the calculations by inputting the altimeter setting to get the pressure altitude directly (Pressure Altitude Explained - Pilot Institute).

Application of Pressure Altitude

• Flight Planning: Critical for performance calculations and operational planning, especially in assessing takeoff distances (Pressure Altitude Explained - Pilot Institute).

Understanding and correctly setting an altimeter to determine local QNH, aerodrome elevation, and pressure height are essential skills pilots must master to ensure safe and accurate aircraft operation. Mastery of these concepts underpins effective flight planning and execution, particularly in takeoff and landing scenarios.