Wake Turbulence Departure Interval After a Heavy Departs
Bahrain Approach is sequencing departures from Runway 30 at Bahrain International (OBBI). A Boeing 777-300ER (heavy) departs at 0812, rotation point about 1,800 m down the runway, climb gradient shallow. Behind it at the holding point waits an Airbus A320 (medium) cleared for an intersection departure from the same runway. The tower controller plans to release the A320 as soon as the 777 is airborne. Reported surface wind is 300 degrees at 6 knots, visibility 10 km, no significant weather. The departure controller asks the tower to tighten the gap because a stream of arrivals is inbound. The A320 pilot requests an immediate rolling departure to make up time.
1. What wake turbulence separation must be applied between the heavy 777 and the following medium A320 departing the same runway, and why can the tower not simply release the A320 the moment the 777 lifts off?
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Under ICAO wake turbulence provisions, a medium aircraft departing behind a heavy on the same runway requires a minimum time interval (commonly 2 minutes) before the next aircraft may commence its take-off roll, measured from the moment the preceding aircraft becomes airborne. This is because wake vortices are generated most strongly during rotation and initial climb, when the heavy aircraft is at high angle of attack and low speed, producing strong counter-rotating vortices that sink and drift with the wind. Surface wind of 300 at 6 knots is light; crosswind component is small, so vortices will linger near the departure path rather than being rapidly dispersed. The tower cannot release the A320 on lift-off alone; the time interval must be satisfied. The controller should tell the departure controller that the interval constrains the sequence, and may offer an alternative such as a departure from a different runway or a delayed release once the interval expires. The pilot's request for a rolling departure does not waive the requirement; the tower must still ensure the interval.
2. If the wind were reported as 300 degrees at 20 knots gusting 30, how would your reasoning about vortex behaviour change, and what practical decision would you make?
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A stronger wind does not automatically permit a reduction in the prescribed time interval; the interval is a minimum standard and remains applicable unless the State has approved a specific procedure. However, stronger wind, especially with gusts, increases the rate at which vortices are transported away from the runway centreline and promotes breakdown of the vortex structure. The crosswind component at 300 degrees on Runway 30 is essentially zero, so the vortices would be carried along the runway axis rather than off it - this is the worst case for a following aircraft on the same runway. The gustiness introduces uncertainty. The practical decision is to retain the full interval, monitor the A320's rotation and initial climb for any sign of wake encounter, and warn the A320 crew of the heavy's departure and the light crosswind. If the tower wished to reduce the interval, it could only do so under an approved procedure with documented wind criteria; absent that, the safe and defensible action is to hold the A320 until the interval is met.