The vortices generated at the wing tips of an airborne fixed wing aircraft, or from the rotors of a helicopter in forward flight, create transient rotational turbulence, which trails behind the aircraft in a descending and down wind direction, and may cause sudden un-commanded roll to affect a following aircraft, perhaps at a sufficient rate of roll to disconnect the autopilot if engaged. Light aircraft may also experience marked pitching moments when traversing a vortex from the side, such as when turning finals from base leg behind a larger aircraft.
Opposing the induced roll moment using normal roll control (aileron and roll spoiler) is usually effective except when the wingspan and ailerons of the affected aircraft do not extend beyond the rotational flow of the two vortices (this can occur when the following aircraft is much smaller than the leading aircraft), or where one wing is caught in a sufficiently strong vortex there may not be enough roll authority from the remaining wing.
The minimum traffic separation distances for aircraft under air traffic control (ATC) have consistently been shown to be sufficient to ensure that any loss of control caused by an encounter with wake turbulence will always be transient and easily recoverable except for some cases of spacing on approach to land or on initial climb following take off. To cover these circumstances, and various more complex permutations of runway use, minimum separation distances and times have been promulgated by the International Civil Aviation Organisation (ICAO).
The type of weather conditions in which pilots can most expect to encounter wake turbulence are:
- stable, still/light wind conditions;
- wind direction down the runway.
Conversely, low level turbulence, associated with stronger wind conditions, accelerates vortex decay and cross winds deflect vortices away from the runway centreline.
On Take Off
- Vortices begin at rotation speed (Vr), rather than the start of roll, and separation should therefore be based on the time between rotation of successive aircraft. However, for practical reasons, ATC base their timing for separation on time of roll. A heavier aircraft may have a longer/slower take-off run than the following aircraft, reducing actual separation time.
- When considered necessary, be prepared to remind departure ATC of the minimum time you require behind a preceding heavier aircraft and do not accept instructions to begin your take off roll before the prescribed time separation is achievable at the anticipated point of rotation.
- The following aircraft is unlikely to be exposed to wake turbulence if:
- there is sufficient crosswind on the departure runway as the vortices will move down wind at the speed of the cross-wind component.
- its flight path stays above that followed by the preceding heavier aircraft until their tracks cease to be the same, and,
- it complies with the recommended spacing between departures.
- Including in the pre-flight advice to passengers an invitation to keep their seat belts done up, even when the seat belt sign is off unless moving around the cabin, minimises the risk of passenger injury if there is an unexpected encounter with turbulence en route.
- In cruise, maintain situational awareness of other aircraft in the vicinity which appear to be on similar / crossing tracks ahead and at or above your own level, especially if you will climb or descend behind larger aircraft type in level flight, or if you are in level flight with a larger aircraft climbing or descending ahead. Pilots should beware flying through descending contrails from a preceding aircraft as these indicate the presence of vortices.
- Consider a track offset of 1nm upwind if following a higher aircraft at similar ground speed when allowed by airspace regulations or via specific ATC approval. Also, a change of FL to cross “HEAVY” or “SUPER” traffic flight path from above can be used when feasible and authorised by ATC.
- Check tropopause height during pre-flight briefing as, when flying below the tropopause, wake vortex decay is much slower and the risk of severe encounters is greater.
- Be prepared to select seat belt signs to ‘on’ and instruct working cabin crew to secure themselves for a precautionary interval if the above conditions are identified in due time.
In case of a wake encounter, pilots should:
- Be aware that experience has demonstrated that if the pilot reacts at the first roll motion, when in the core of the vortex, the roll motion could be potentially amplified by this initial pilot action.
- Be aware that some in-flight incidents have demonstrated that pilot inputs may exacerbate the unusual attitude condition with rapid roll control reversals carried out in an “out of phase” manner.
- Be aware that if the autopilot is engaged, intentional disconnection can complicate the scenario, and the autopilot will, in most cases, facilitate the recovery.
- Try to avoid large rudder deflections that can create important lateral accelerations, which could then generate very large forces on the vertical stabiliser that may exceed the structural resistance. Although some recent aircraft types are protected by fly-by-wire systems, typically, the use of the rudder does not reduce the severity of the encounter, nor does it improve the ease of recovery.
- Make use of specific guidance in the aircraft operating manual (AOM) for their specific type(s)/fleet, where available.
- Establish the aircraft type ahead so as to know what minimum distance behind is necessary - [Note: in a busy terminal environment, it may not be appropriate to ask ATC for details of the aircraft ahead because of the risk of clogging the frequency with unnecessary calls. However, it is often possible to determine the weight of the aircraft ahead by listening to radio traffic and through knowledge of the other operator's fleet.]
- If the Autopilot is engaged for an approach at or near to minimum recommended spacing from an aircraft ahead, ensure the pilot flying (PF) is prepared for manual recovery from a sudden roll, which might be at a high enough rate to cause simultaneous un-commanded autopilot disconnection.
- Ensure that all cabin crew are secured in their seats before final approach commences.
- If in a small aircraft and flying an approach where there is risk of a wake vortex encounter, providing operating procedures permit, consider:
- flying the same vertical profile but aiming for a touchdown point further down the runway (if the runway length allows this). In order to reduce the required wake turbulence separation, some airports operate multiple threshold ILS profiles.
- flying slightly displaced laterally upwind of the preceding aircraft’s track.
- Know the full specification of the Wake Turbulence Separation minima and any national variations which you are liable to encounter (check the State AIPs/AICs).
- TCAS information may be used for situational awareness only and pilots should not adjust their ATC cleared heading, speed, or altitude based on TCAS information UNLESS a TCAS RA is generated. See ACAS Bulletin No. 6, "Incorrect use of the TCAS traffic display", published in March 2005.