Wake Turbulence on a Parallel Runway Departure
An aerodrome operates two parallel runways, 09L and 09R, separated by 760 m. A B77W (wake category H, MTOW 351 t) departs 09L. Four minutes later, a Cessna 525 CitationJet (category L, MTOW 5.7 t) is cleared for take-off on 09R from an intersection, rolling at a point 900 m from the threshold. The wind is 090 degrees at 8 kt, visibility 8 km, no precipitation. The tower controller has a surface radar and a certified radar display, and the runways are used independently. The CitationJet pilot asks whether wake turbulence is a factor.
1. Determine the wake turbulence separation category of each aircraft and whether a time-based wake separation interval applies to the CitationJet departure.
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Under the ICAO wake turbulence categories, the B77W is category H (heavy, MTOW above 136 t) and the CitationJet is category L (light, MTOW 7 t or less). The applicable wake separation matrix for a light aircraft departing behind a heavy aircraft on the same or parallel runways is generally 3 minutes when the runways are parallel and separated by less than 760 m, and 2 minutes when the runways are separated by 760 m or more (subject to the State's AIP and the specific matrix in force).
At exactly 760 m separation, the controller must check the State's published rule: many States apply the reduced interval only when the separation is greater than 760 m, and the conservative reading at exactly 760 m is to apply the longer interval. The 4-minute gap already exceeds even the conservative 3-minute interval, so a time-based wake separation interval is satisfied.
However, time interval is not the only consideration. The CitationJet is departing from an intersection 900 m down the runway, which reduces the runway available and may place the aircraft's initial climb path closer to the heavy's departure path if the heavy rotated early. The controller should still issue a wake turbulence caution and consider an early turn or a delayed roll to keep the light aircraft above and upwind of the heavy's flight path.
2. The CitationJet pilot reports on climb-out that the aircraft rolled sharply left shortly after rotation. What is the likely cause and what should the controller do?
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The likely cause is wake vortex encounter, not an aircraft malfunction. The heavy B77W departed four minutes earlier; vortices sink and drift with the wind. With wind 090 at 8 kt, the vortices from 09L would drift toward 09R (downwind), placing them in the CitationJet's climb path, especially since the light aircraft is departing from an intersection and may be climbing through the heavy's vortex sink zone.
The controller's actions:
- Acknowledge the report and ask for the pilot's intentions and whether the aircraft is controllable. Do not assume it is a wake encounter until the pilot confirms.
- If the pilot reports loss of control or difficulty, treat it as an emergency: clear the airspace, offer a return or a diversion, and alert the tower/approach and ARFF as appropriate.
- If the aircraft is controllable, provide traffic information and allow the pilot to climb above or turn away from the vortex zone, and pass the report to the next unit.
- Record the occurrence and notify the aerodrome operator and the safety management system, because a wake encounter on a parallel runway departure is a reportable safety event and may indicate that the 760 m spacing or the intersection departure procedure needs review.