Pressure and altimetry
QNH gives altitude above mean sea level, QFE gives height above the airfield datum, and the standard setting 1013.25 hPa gives flight levels. Flying from high pressure to low pressure without resetting the subscale leaves you lower than indicated — high to low, look out below. Roughly 27 ft of error per hPa near sea level.
Stability, cloud and turbulence
An unstable air mass gives cumuliform cloud, showers, good visibility and gusty conditions; a stable air mass gives stratiform cloud, drizzle, poor visibility and smooth air. The dew point spread is the fastest indicator of fog risk — a narrow, closing spread on a clear night with light wind means radiation fog.
Icing
Clear ice forms in large supercooled droplets, is heavy and hard to shed; rime ice forms in small droplets and is opaque and brittle. The greatest airframe icing risk sits between 0 and −20 °C in visible moisture. Escape usually means changing altitude to warmer air or air that is genuinely below the icing envelope.
Wind and hazards
Surface wind backs and slows relative to the gradient wind because of friction. Windshear, microbursts, mountain wave and rotor are the killers: expect them near thunderstorms, near terrain in strong wind, and on hot days with a shallow inversion.
KEY TAKEAWAYS
- ▸High to low (pressure or temperature) — the altimeter over-reads.
- ▸Unstable = cumulus, showers, turbulence; stable = stratus, drizzle, haze.
- ▸Clear ice = big supercooled droplets; rime = small droplets.
Check-ride
Six questions from this module. Score 70% or better to have it signed off in your training record.