Meteorology for Air Navigation
What is examined
Meteorology for air navigation tests altimetry, METAR/TAF interpretation, hazardous weather and performance effects. Pakistan-specific items include monsoon, dust/sand, high surface temperatures and terrain-induced effects.
Core rules
- QNH is the altimeter setting that makes the altimeter read aerodrome elevation; QFE reads height above the reference datum; standard 1013.25 hPa is used above transition altitude.
- Below standard pressure, the altimeter over-reads when 1013 is set: true altitude is lower than indicated. This is a terrain-clearance trap.
- Density altitude = pressure altitude corrected for temperature; high temperature, high elevation and low pressure reduce performance.
- METAR is observed actual weather; TAF is a forecast. Check trend, visibility, cloud base, wind shear and QNH.
- Thunderstorms, icing, low-level wind shear and jet streams are the main en-route hazards.
Worked example
Aerodrome elevation 1,000 ft, QNH 1003 hPa, OAT 35°C. Pressure altitude = 1,000 + (1013 − 1003) × 30 = 1,300 ft. ISA temperature at 1,300 ft = 15 − 2 × 1.3 = 12.4°C. ISA deviation = 35 − 12.4 = 22.6°C. Density altitude ≈ 1,300 + 120 × 22.6 = 4,012 ft. The aircraft performs as if at about 4,000 ft, so take-off distance and climb gradient must be checked.
Traps
Setting QNH above transition altitude; using QFE for en-route terrain clearance; ignoring density altitude on a hot day; reading a TAF as current weather; forgetting monsoon convective build-up and dust.