後方乱気流 (Wake Turbulence)

後方乱気流とは？

Wake turbulence is the complex system of disturbed air — most notably a pair of counter-rotating vortices — generated behind a flying aircraft as a byproduct of lift production. Unlike atmospheric turbulence, wake turbulence is created by the aircraft itself and trails behind it for miles, persisting for minutes after the generating aircraft has passed. It represents one of the most serious hazards in the approach and departure environment, capable of inducing sudden and violent roll upsets in following aircraft.

仕組み

Wake turbulence forms because lift generation creates a pressure differential between the wing's upper and lower surfaces. At the wingtip, high-pressure air from below escapes around the tip to the low-pressure region above, creating a rotating vortex. These vortices trail behind each wingtip in a counter-rotating pair:

• The vortices initially descend at approximately 400–500 ft/min and level off at 500–900 ft below the flight path.
• In calm conditions, they spread laterally at 3–5 knots and can persist for 2–3 minutes.
• A crosswind can hold one vortex stationary directly on the runway centerline while drifting the other aside.
• Vortex strength is proportional to aircraft weight and inversely proportional to airspeed and wingspan.

Winglets partially disrupt vortex formation, reducing induced drag and slightly diminishing wake turbulence intensity, though not eliminating it.

航空における重要性

Regulatory wake turbulence separation standards are among the most significant constraints on airport capacity. ICAO classifies aircraft into wake turbulence categories (Super, Heavy, Medium, Light) based on maximum certificated takeoff weight. Minimum separation ranges from 2.5 nm (same-category Medium) to 6 nm (Light following Heavy). ATC enforces these separations meticulously, and a reduction of even half a mile could dramatically increase airport throughput.

The introduction of the A380 (categorized "Super") required new separation standards. Airports initially required 10 nm separation behind the A380 for some following aircraft — substantially reducing parallel runway throughput at hubs like Heathrow and Dubai.

実際の影響

In 2001, American Airlines Flight 587 broke apart over New York after the first officer made aggressive rudder inputs in response to wake turbulence from a preceding Japan Airlines Boeing 747, overstressing the composite vertical stabilizer. All 260 on board and 5 on the ground died. This remains the deadliest aviation accident on US soil not caused by terrorism. As a result, pilot training on wake turbulence response and rudder limitations was overhauled globally. Today, ICAO's RECAT (Wake Turbulence Re-Categorization) program uses advanced research to optimize separation standards, recovering significant capacity at major airports.