Meteorology for Instrument Flight
Meteorology for Instrument Flight
Instrument meteorology questions test decoding (METAR/TAF), altimetry, icing and the weather systems you meet in West African and intertropical flying - the ITCZ, harmattan dust, squall lines and coastal fog.
Decoding and altimetry
- Read METAR groups in order: wind, visibility, weather, cloud, temperature/dewpoint, QNH. A temperature/dewpoint spread of 1–2°C with light wind signals fog or mist risk within hours.
- QNH gives altitude above mean sea level, QFE height above the aerodrome, QNE standard pressure at 1013.25 hPa. A falling QNH means the altimeter over-reads if not reset - you are lower than indicated.
- Cold air makes the altimeter over-read, so true height is less than indicated: apply a cold-temperature correction to minima and step-down fixes.
Hazards
- Harmattan dust haze (December–February, Kano, Kaduna, Abuja) can drop visibility to a few kilometres with no cloud - a visibility-limited, not ceiling-limited, problem.
- Squall lines and embedded CBs bring wind shear, microbursts and hail; avoid by 20 NM in the active quadrant.
- Airframe icing is most likely at 0 to −15°C in visible moisture; the freezing level and tropopause heights are on the significant weather chart.
Worked example
Aerodrome elevation 1,000 ft, QNH set, OAT −15°C. ISA at 1,000 ft ≈ 13°C, so the deviation is −28°C. At an indicated 3,000 ft (2,000 ft above the source), error ≈ 4 ft × 2 × 28 ≈ 224 ft low. Add roughly 220 ft to all minimum altitudes on the approach.
Traps
- Confusing QFE with QNH when computing minima.
- Assuming a high QNH protects you; only the correction protects you in cold air.