Hazards of mountain flying

Mountain flying combines several hazards covered individually in other lessons. Here is a consolidated overview focused on weather-induced hazards.

1. Mountain waves and rotor

With wind ≥ 25 kt perpendicular to the ridge, standing waves form with lenticularis clouds and a rotor in the valley downwind (see dedicated lesson Mountain Waves / Lee Waves (Standing Waves)).

Key risks:

Structural load from strong vertical winds,
Uncontrolled descent downwind of the ridge,
Altimeter oscillation.

2. Valley and slope winds (anabatic / katabatic)

Daytime — anabatic wind (up-slope)

Sun-heated slopes warm → warm air rises along the slope.
Wind in small valleys upward (from valley to ridge).
Strength: 5–15 kt; on extremely hot days more.

Nighttime — katabatic wind ("fall wind")

Slopes cool by radiation; cold air falls down the slope.
Wind in small valleys downward.
Strength: 5–20 kt; in large glacier regions (Antarctica, Greenland) up to 100 kt extreme.

Consequence for PPL:

At mountain aerodromes: align runway with the day wind — possibly different direction by day and by night.
Valley wind can strongly affect approaches — visual approaches in narrow valleys require allowance.

3. Föhn

Föhn is a warm, dry lee wind downstream of a mountain ridge — formed when moist air rises on the windward side (condensing, raining out) and descends dry-adiabatically on the lee side (see Local winds).

Dangers:

Sudden strong wind on the lee side (often 50–80 kt gusts).
Heavy turbulence and mountain waves.
Dryness (relative humidity < 20%) → higher fire risk, fatigue, airway irritation.

In the Northern Alps: typically with southerly flows (south Föhn).

4. Radiation inversion in valleys

At night strong radiation inversions form in valleys with:

Ground fog or low stratus,
Very poor visibility below,
Clear visibility above the inversion.

Consequence: morning departure from a valley may be impossible until the inversion breaks (often 10–11 local).

5. Icing in the mountains

Mountains force moist air to rise → clouds on the mountain are often cold (below 0 °C) → icing risk much higher.

Typical:

Orographic clouds along ranges with westerly winds.
Structural icing in upslope conditions severe and rapid.

PPL(A) without anti-icing: avoid mountain regions with clouds.

6. Downdrafts behind mountains

Downwind of a range: strong downdrafts caused by:

Mountain waves (see above),
Föhn wave,
Rotor zone.

Classic accidents: VFR pilot who "merely" crosses a ridge and is pressed against the lee slope.

7. CFIT risk (Controlled Flight Into Terrain)

Dramatically increased in mountains:

Minimum altitudes become inadequate with declining visibility.
Clouds hang on the mountains.
Valley approaches can lead into dead ends.

Counter-measures:

MORA / MSA from VFR chart strictly observed.
Approach a valley with reserves — be able to turn back immediately on cloud.
Maintain altitude rather than descending in the valley.
GPS chart with terrain (Garmin Pilot, SkyDemon).

8. Rapid weather change

In mountains weather can change strongly within 30 minutes:

New clouds form on the ridge.
Precipitation starts suddenly.
Visibility collapses.

Briefing immediately before the flight, not 4 h earlier.

9. Density altitude

Altitude + heat → high density altitude:

C172 at mountain aerodrome 5 000 ft AMSL, 30 °C → DA ≈ 8 000 ft.
Take-off distance +80%, climb performance halved.

Consequence: mountain aerodromes only with reduced load or density-altitude-adjusted ops.

Operational rules for mountain flying

Before flight:

Current weather briefing (max 2 h old).
GAFOR route chart (DWD) studied.
NOTAMs for military exercise areas in the mountains checked.

In flight:

Maintain minimum visual altitudes — don't "creep close to the terrain" because the cloud base is luring.
Have a plan B — emergency landing site in the valley noted.
When unsure, turn back early — easier with more altitude.

On cloud contact:

Immediate 180° standard turn back the way you came.
Maintain or carefully climb — never descend.