乱気流 (Turbulence)

乱気流とは？

Turbulence is irregular, unpredictable air movement — the atmospheric equivalent of rapids in a river — that subjects an aircraft to sudden changes in lift, causing altitude deviations, attitude upsets, and passenger discomfort. It ranges from barely perceptible to severe enough to injure unsecured occupants and, in extreme cases, damage the airframe. Turbulence is one of the most common causes of non-fatal aviation injuries and a significant operational challenge for airline scheduling and route planning.

仕組み

Turbulence arises from several atmospheric mechanisms:

• Convective (Thermal) Turbulence: Caused by uneven ground heating creating rising columns of warm air (thermals) and sinking cool air. Common in summer afternoons and near cumulonimbus clouds.
• Clear Air Turbulence (CAT): Found at cruise altitudes in clear sky, typically near jet streams where wind shear causes chaotic mixing. Invisible and undetectable by weather radar — the most dangerous type for in-flight injuries.
• Mechanical Turbulence: Air flowing over mountains, buildings, or terrain features breaks into chaotic eddies on the downwind side. Mountain wave turbulence can be severe for hundreds of miles.
• Wake Turbulence: Generated by preceding aircraft — see wake turbulence for details.
• Frontal Turbulence: Along the boundary between air masses of different temperature, density, or humidity.

Turbulence intensity is rated as: Light, Moderate, Severe, or Extreme. Severe turbulence can cause momentary loss of aircraft control; extreme turbulence can cause structural damage.

航空における重要性

The FAA reports that turbulence injures approximately 50–100 passengers and crew annually in the US alone, with the vast majority occurring when seatbelt signs are off. Airlines use turbulence forecasting services, PIREPs (Pilot Reports), and real-time data sharing (e.g., American Airlines' TAPS system) to route around known turbulence. Climate change is projected to increase CAT frequency by 40–170% in the North Atlantic by 2050–2080, according to University of Reading research.

実際の影響

In December 2022, a Hawaiian Airlines Airbus A330 encountered severe turbulence over the Pacific, injuring 36 people — 11 seriously — from unsecured objects and passengers. Singapore Airlines Flight SQ321 (May 2024) encountered extreme turbulence over Myanmar, killing one passenger and injuring 71 others, prompting airlines worldwide to review turbulence response procedures. The flight envelope accounts for turbulence loads through gust load factors built into structural design.