Follow the vortices, not just the aircraft

Why can a slow heavy aircraft produce a strong wake?

Relate vortex strength and movement to the generating aircraft, lift, speed and surrounding wind.

Understand the mechanism

A lifting wing produces a pressure difference that drives flow around the tip and contributes to a trailing vortex system. The rolled-up vortices contain rotational flow that can impose an abrupt rolling moment on a following aircraft. The hazard is not ordinary engine exhaust alone. Wake strength is related to the lift being produced and the spanwise loading; a heavy aircraft flying slowly in a clean configuration can generate a strong wake.

A simplified circulation relationship scales with weight divided by air density, speed and span. It illustrates why increasing weight raises wake strength while increasing speed or span can reduce the circulation needed per unit span. It is not an operational turbulence-intensity forecast. Aircraft geometry, configuration, actual lift and atmosphere affect the wake, and a smaller following aircraft may have less roll-control margin against the same disturbance.

Apply the evidence

Vortices usually descend initially and drift with the surrounding wind while gradually decaying. Near the ground, interaction with the surface changes their motion. A light crosswind can move one vortex toward a runway or hold the upwind vortex near a relevant path. Wind does not guarantee immediate disappearance. The model uses qualitative strength and drift to explain location; actual separation minima and wake categories must come from current applicable procedures.

Worked case · synthetic training operation

Two synthetic leader cases have the same density and span. Doubling weight at the same speed doubles the model’s relative circulation; increasing speed reduces it. This is deliberately a relative comparison without a dangerous/safe numerical threshold. The animation then moves the vortex pair with a supplied crosswind while the leader continues away. The follower’s risk depends on the wake’s present position, not simply the leader’s current position or the distance shown on a traffic symbol.

Why do the leader’s lift-off and touchdown points matter?

Use the leader's airborne path, lift-off or touchdown point and wind to identify the wake corridor relevant to the following aircraft.

Understand the mechanism

A departing aircraft begins generating significant lifting wake as its wing supports the aircraft; the relevant airborne path and lift-off point help locate the wake corridor. An arriving aircraft generates wake along its approach path until touchdown. Avoidance guidance therefore considers the leader’s flight path, the follower’s path and their relation to those points. Copying a statement such as “stay above” without considering a crossing path or ground movement can miss the actual geometry.

Parallel and crossing runways can create wake encounters even when the aircraft do not use the same centreline. A crosswind may transport wake into another runway’s path. A preceding aircraft’s landing or departure on an intersecting runway creates a different encounter pattern from a same-runway sequence. Surface reports, observed wind and the leader’s path provide context, while current ATC procedures and wake separation requirements remain essential.

Apply the evidence

ATC separation, a wake advisory and pilot responsibility are not interchangeable concepts. A clearance does not physically remove a vortex. If the crew cannot accept the proposed spacing or path under the applicable procedure, they need to communicate and obtain a safe resolution. A lesson should avoid publishing a single time or distance as a universal wake minimum because categories, runway relationships and local separation schemes can change the required value.

Worked case · synthetic training operation

In a synthetic crossing-runway exercise, the leader departs from the crossing runway and climbs across the follower’s future approach path. A light wind carries the wake toward that path rather than away. The follower does not assume protection merely because it is assigned a different runway. The model traces the generating segment and the later drift, then overlays the proposed follower path. Changing the wind can move the intersection, but the classroom drawing still does not authorise a reduced separation or replace the current procedure.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.