Aviation Turbulence Reference
Complete reference for turbulence intensity categories, types and causes, avoidance techniques, PIREP encoding, and pilot response procedures — from light thermal bumps to severe clear air turbulence.
Turbulence Intensity Categories
Turbulence is classified into four internationally recognised intensity categories. These definitions, used in PIREPs and weather advisories, describe both the effect on aircraft and on occupants.
| Intensity | PIREP Code | Occupant Effect | Aircraft Effect |
|---|---|---|---|
| Light | LGT | Slight strain against seat belts. Unsecured items may move. Food/drink service may be difficult. | Slight erratic changes in altitude or attitude. Report as: slight/erratic variations in airspeed. |
| Moderate | MOD | Hard strain against seat belts. Unsecured items dislodged. Walking extremely difficult. | Changes in altitude/attitude, IAS variations. Aircraft remains in positive control at all times. |
| Severe | SVR | Violent movement against seat belts. Impossible to walk. Possible injuries to unsecured occupants. | Large, abrupt changes in altitude/attitude. Momentary loss of aircraft control. Large IAS variations. |
| Extreme | EXTRM | Impossible to remain seated. Significant risk of injuries. | Aircraft practically impossible to control. Risk of structural damage. Report immediately to ATC. |
Note on "Chop": PIREPs may also report LGT CHOP or MOD CHOP — this refers to rhythmic bumps without abrupt changes, typically caused by light mechanical turbulence. Less disruptive than the equivalent-intensity standard turbulence.
Types of Turbulence
Turbulence arises from several distinct atmospheric mechanisms. Understanding the cause of each type allows pilots to anticipate where and when it is likely, and how to avoid or escape it.
Convective (Thermal) Turbulence
Caused by differential surface heating creating rising columns of air (thermals). Surfaces with high heat absorption — cities, dark ploughed fields, tarmac runways, and rocky terrain — generate stronger thermals than surrounding areas. Thermals vary in strength from gentle bubbles to powerful columns reaching cloud base.
Avoidance strategies:
- Fly early morning before surface heating develops
- Fly above the convective layer / cloud tops when possible
- Increase cruise speed to Va (manoeuvring speed) in moderate or worse turbulence
- Avoid flight below TCu or Cb cloud base
- In hot conditions, plan routes over water or high terrain where thermals are weaker
Mechanical Turbulence
Created when wind flows over surface obstacles — hills, ridges, mountains, forests, buildings, and even other aircraft. The airflow separates and creates eddies and vortices downwind of the obstruction. Severity increases with wind speed and the size/sharpness of the obstacle.
Mountain Wave
At a larger scale, strong winds flowing over mountain ridges can generate standing waves extending for hundreds of miles downwind and up to the stratosphere. These waves have three components:
- Cap cloud / Banner cloud: cloud over the mountain peak; wave clouds (lenticular) are lens-shaped, stationary clouds forming in wave crests
- Rotor zone: violent turbulent area below the wave crest on the lee side — can be Extreme intensity; lenticular cloud at rotor level may appear smooth but flight within is extremely hazardous
- Wave train: series of updraught/downdraught cycles downwind; can cause significant altitude deviations
Avoidance: Cross ridges at right angles; avoid the lee side in strong wind; give wide vertical clearance above ridge height; do not fly below lenticular clouds. Increase to Va.
Clear Air Turbulence (CAT)
CAT occurs in clear air without any cloud or visual indication — making it essentially impossible to detect or avoid visually. It is primarily associated with the strong wind shear zones around the jet stream, where speed and/or direction changes rapidly over a short distance.
Location indicators:
- Edges of the jet stream — particularly the equatorward (warm) side where wind shear is greatest
- Areas where the tropopause height changes sharply (jet stream entrance and exit regions)
- Lee side of mountains at high altitude (mountain wave CAT)
- Near active thunderstorm systems — turbulence can extend many miles laterally in clear air
Detection and avoidance:
- PIREPs (pilot reports) from preceding aircraft are the most reliable current source
- Winds aloft charts and jet stream forecasts aid planning
- Some modern long-range radar systems and LIDAR can detect CAT; most aircraft cannot
- If jet stream core winds are forecast above 80 kt, expect potential CAT near the edges
Response to unexpected CAT: Slow to Va, maintain attitude (do not chase altitude), inform ATC, request level change if possible, file PIREP on exit.
Wake Turbulence
All aircraft generating lift produce two counter-rotating vortices trailing from the wingtips. These vortices are generated from the moment of rotation on takeoff until touchdown on landing. They represent one of the most dangerous hazards in the terminal area, capable of rolling a smaller aircraft beyond its ability to recover.
ICAO Wake Turbulence Categories (WTC):
| Category | MTOW | Examples |
|---|---|---|
| Super (J) | >560,000 kg | A380 |
| Heavy (H) | >136,000 kg | B747, B777, A330 |
| Upper Medium (UM) | 15,000–136,000 kg | A320, B737 |
| Medium (M) | 7,000–136,000 kg | ATR-72, Dash 8 |
| Light (L) | <7,000 kg | C172, PA-28 |
Key rules for avoiding wake turbulence:
- ATC provides mandatory separation based on WTC categories — do not waive this without understanding the risk
- On approach: stay at or above the preceding aircraft's glidepath; land beyond the preceding aircraft's touchdown point
- On departure: rotate before the preceding aircraft's rotation point; turn early to avoid the flight path
- Vortices drift with crosswind — on a right crosswind, the upwind vortex may remain over the runway; on a left crosswind, it drifts away from the runway
- In calm wind conditions, vortices may remain on the runway for up to 3 minutes — use caution with low-wind operations behind heavy aircraft
- Request additional separation from ATC if needed; it is always the PIC's responsibility to ensure safe separation
PIREP Turbulence Encoding
Turbulence encounters must be reported as Pilot Reports (PIREPs). Accurate PIREPs are the most valuable real-time weather information available to other pilots and forecasters. The standard format for a turbulence PIREP:
/TB [intensity] [type] [altitude or altitude range]
// Examples:
/TB LGT // Light turbulence
/TB MOD-SVR // Moderate to Severe
/TB LGT CHOP // Light Chop (rhythmic)
/TB NEG // Negative (no turbulence encountered)
/TB EXTRM // Extreme — report immediately to ATC
Example complete PIREP decoded:
UA = Routine PIREP
OV = Over EGCC (Manchester Airport)
TM = Time 1430Z
FL = Flight Level 095 (9,500 ft)
TP = Aircraft type C172
SK = Sky: Broken at 7,000 ft
TB = Turbulence: Moderate
RM = Remarks: PIREP (routine)
Filing a PIREP: Report to ATC as soon as practicable after encountering unexpected turbulence. Include: position (overhead or between two points), time, altitude, aircraft type, and intensity/type. In the UK, PIREPs are forwarded to the Met Office. In many jurisdictions, failing to report encountered phenomena is considered a duty of care issue.
Pilot Response to Turbulence
Unexpected Turbulence Encounter
- Reduce to manoeuvring speed (Va): Va is the maximum speed at which full deflection of a single control surface will not overstress the airframe. Do not exceed Va in turbulence. Note: Va decreases with reduced aircraft weight — check the POH tables.
- Maintain attitude, not altitude: In severe turbulence, do not chase the altimeter — control inputs to maintain altitude in severe turbulence can exceed structural limits. Maintain a level wings attitude and allow the aircraft to ride through altitude deviations within reason.
- Inform ATC: Report the turbulence as soon as workload permits. Request a level change if available and beneficial.
- File a PIREP: On exiting the turbulent area, file a PIREP to help other pilots.
When to Divert
- Sustained moderate or greater turbulence that cannot be exited by altitude or route change
- Any turbulence causing difficulty controlling the aircraft
- Passenger injury or equipment damage from turbulence — requires incident report on landing
- Structural concern — if any abnormal control response or structural sounds are noted, land at the nearest suitable aerodrome and have the aircraft inspected
Seat belts: Passengers should be instructed to keep seat belts fastened whenever seated. The majority of turbulence injuries occur to unsecured passengers and cabin crew during unexpected moderate or severe turbulence encounters in cruise flight at high altitude — often in VMC with no apparent weather.