When changing runways isn't just changing runways
Have you ever wondered what really happens when the wind changes direction, and the active runway changes? So have I...
While live-streaming a flight today, I got cleared for takeoff from one runway, and then while taxiing, the runway changed. On reviewing the current wind, it was obvious that the change of wind vector along the runway was marginal - so I reasoned that the AI tower controller should have stuck with the cleared runway - not suddenly changed their mind.
In the real world there would be have been a specific runway change message to all aircraft, and a significant delay in doing so - to allow both aircraft in the air and on the ground to be re-routed.
It got me thinking though - about what the real world procedures and concerns are - and how they typically play out - so I did some digging.
Nothing happens quickly
Air traffic services do not immediately change the active runway as soon as the wind changes. They wait for the wind to be consistently in the opposite direction and for other operational conditions to favor the change. The decision to switch runways, often called a "runway change" or "flow reversal," is a significant event that involves a complex procedure to ensure safety and efficiency.
Factors and Timings Involved
The timing and decision to change the active runway depend on several key factors:
Consistent Wind Direction and Speed: Aircraft operate most safely and efficiently when taking off and landing into a headwind. A headwind increases the lift on the wings during takeoff, allowing the aircraft to become airborne at a lower ground speed. During landing, it helps to slow the aircraft down, reducing the required landing distance. Air traffic controllers monitor wind conditions closely, using anemometers near the runways and wind data from aircraft. A runway change is typically initiated when the wind shifts to a new direction that creates a significant crosswind or a tailwind component on the current active runway. The International Civil Aviation Organization (ICAO) recommends a runway change if the tailwind component exceeds 5 knots or the crosswind component exceeds 15 knots. The wind must be consistently strong in the new direction before a change is considered.
Meteorological Forecasts: Controllers and airport management don't just react to current conditions. They also consider the Terminal Aerodrome Forecast (TAF), which predicts weather for the next few hours. If the forecast indicates the wind will shift back to the original direction soon, they might choose to ride out the temporary change to avoid a costly and time-consuming runway reversal. This is especially true at busy airports.
Operational Inertia and Traffic Flow: Changing runways is a major undertaking that disrupts the entire traffic flow. It requires re-routing all inbound and outbound flights, which can cause significant delays.
Inbound aircraft must be re-vectored for a new approach, which requires pilots to re-brief for the new runway, adjust their descent profiles, and reconfigure their aircraft. This increases pilot and controller workload.
Departing aircraft on the ground must be re-routed via taxiways to the new departure runway, often causing congestion.
Airport Configuration must be changed, including runway lighting, and instrument approach procedures.
Noise Abatement Procedures: Many large airports have noise abatement procedures that designate preferred runways to minimize noise over residential areas. A runway change due to wind may override these procedures, but controllers will try to adhere to them whenever possible.
Coordination: A runway change requires close coordination between the tower controllers (who manage local traffic), the terminal radar approach control (TRACON) facility (who handle arrivals and departures), and the air traffic control center (who manage en route traffic). This complex coordination ensures a smooth transition and is a major reason why runway changes are not instantaneous.
The Runway Change Procedure
The process is a carefully managed sequence of events, not a sudden switch.
Decision to Change: The tower supervisor, based on current wind conditions, forecasts, and traffic volume, decides a runway change is necessary.
Notification and Coordination: The tower informs all relevant ATC positions and ground services (such as fueling and baggage handlers) of the impending change. Pilots are notified via the Automatic Terminal Information Service (ATIS), which is a recorded broadcast of airport information.
Transition Period: A brief period of "split operations" may occur where some aircraft are still landing on the old runway while others are already being cleared for the new one. This phase is carefully managed to ensure separation and prevent conflicts.
Full Reversal: Once all aircraft are safely on the ground and the ground traffic is repositioned, the new runway becomes the sole active runway for arrivals and departures.
So there you go.
The more you know, the more you know :)