Turbulence
Turbulence is the chaotic, disordered motion of the atmosphere causing aircraft stress, comfort loss, and in extreme cases structural damage.
Source: WMO; AMS Glossary; FAA AC 00-30B Atmospheric Turbulence Avoidance; ICAO Annex 3.
Turbulence classification (ICAO Annex 3)
| Class | Description | Cockpit |
|---|---|---|
| Light | Slight shaking | Drinks stay |
| Moderate | Noticeable shaking | Loose items move |
| Severe | Violent motion | Hard to control; injury risk |
| Extreme | Massive | Aircraft virtually uncontrolled |
Main turbulence types
1. Mechanical / friction turbulence
Friction turbulence is most significant in the altitude band between the surface and 3000 ft above ground:
- Caused by wind shear at obstacles, buildings, terrain.
- Effective: up to ~3000 ft AGL.
- VFR consequence: expect turbulence at low altitude with strong winds.
2. Orographic / mountain turbulence
Terrain shape and wind speed cause orographic turbulence:
- Wind over mountains → lee waves, rotors.
- Stable layering: lenticularis on lee.
- Rotor: cylindrical turbulence zone below the lee wave — severe turbulence on crossing (see local winds).
- Strong winds over mountains: detectable far downstream.
3. Thermal / convective turbulence
- Caused by updrafts over heated ground in summer.
- Typical beneath cumulus — pilot flies "in thermals".
- Moderate, predictable.
- Strongest form: in and under cumulonimbus (see thunderstorms).
4. Wake turbulence
- Caused by other aircraft (wingtip vortices).
- See Subject 070 wake turbulence lesson.
Conditions for turbulence
- Wind: strong = more turbulence.
- Stability: unstable atmosphere promotes thermal turbulence.
- Terrain: rough (mountains, forest) > smooth (water).
- Convection: cumulus activity = more turbulence.
Operational response
- Reduce speed to Va (manoeuvring speed) — structural protection.
- Smooth control inputs — no sudden movements.
- Change altitude if possible (often smoother above or below).
- Seat belts for pilot and passengers.
Va — manoeuvring speed
At or below Va the aircraft can absorb structural limit load by stalling first → no damage from strong gusts (see Subject 030 V-speeds).
Hazards outside precipitation
Dangerous weather phenomena can also occur outside of thunderstorm precipitation close to the ground. These are gusts and turbulence:
- Outflow boundary from a distant thunderstorm.
- Cold-front passage without rain.