OPERATIONS Safety First #21 | January 2016 043 Wake Vortices
Where do Wake Vortices come from? All aircraft generate wake vortices, also Limited swirls exist also for the same known as wake turbulence. When an reason at the tips of the flaps. Behind aircraft is flying, there is an increase the aircraft all these small vortices mix in pressure below the wing and a together and roll up into two main depression on the top of the aerofoil. vortices turning in opposite directions, Therefore, at the tip of the wing, there clockwise behind the left wing (seen is a differential pressure that triggers from behind) and anti-clockwise behind (fig.1) the roll up of the airflow aft of the wing. the right one (fig.1). Development of wingtip vortices
------Low pressure ------+ + + + + + + + + + High pressure
Wake Vortices All aircraft generate wake vortices, also known as wake turbulence, which continue to be evident far behind the generating aircraft. Another aircraft crossing this wake may feel a sharp and brief turbulence which can be strong under some circumstances. Let’s review the specific characteristics of wake What are the characteristics of wake vortices? vortices’ and how pilots should react CLAUDE LELAIE Size: The active part of a vortex has a magnitude, in cruise, it could be 1000 ft in case of an encounter to ensure the Former Head very small radius, not more than a few below and behind the generating aircraft of Flight Test meters. However, there is a lot of energy at a range of around 15 NM. Then, safety of the flight. due to the high rotation speed of the air. when far away from the generator, the rate of descent becomes very small. In Descent rate: In calm air, a wake approach, the descent is usually limited vortex descends slowly. As an order of to around 700 ft. OPERATIONS Safety First #21 | January 2016 045 Wake Vortices
The decay However, depending on weather between aircraft needs to be so large. would have been a bit stronger behind signifi cant separations and dramatically The ICAO is much faster in conditions the descent rate may a Heavy. It is also common to have, in limit the traffi c on all airports and airways vary signifi cantly and may even be Ground effect: When the aircraft the initial approach phase, encounters at without significantly improving safety. separations ground effect. very small. One of the key factors is close to the ground, less than a distances well above the ICAO minimum It is also to be noted that statistics show have not been affecting this descent is the variation wingspan, the two vortices tend to drift separations. The ICAO separations have that the probability of injury to passengers of the temperature with the altitude. out from the centre line, each towards not been set to avoid all encounters but and crew is about fi ve times greater in set to avoid all A temperature inversion limits the rate its own side, at a speed of around 2 to prevent unsafe encounters. Avoiding turbulence due to weather, than with a encounters but of descent. to 3 kt. It is this phenomenon, when all encounters would require very wake vortex encounter. to prevent unsafe associated with a light crosswind Decay rate: One important parameter component that tends to “hold” the How does it feel to encounter a wake vortex? encounters. of a wake vortex is the decay of its “into wind” vortex roughly on the strength with time. The decay rate centreline, whilst the “downwind” In most cases the effect of the vortex is of the vortex, it will be subjected to varies slightly from one aircraft type to vortex moves away. mainly felt in roll. We will consider here the full strength of the vortex and roll another. Unfortunately, in calm air, due the case of an aircraft entering laterally in the same direction as the vortex, to to low external interference, it is rather Due to this phenomenon, the decay is in a vortex, which is the most frequent the left (fi g.2). This is the main rolling low and this is why the separation much faster in ground effect. situation. Let’s assume that a follower motion that creates the strongest roll aircraft is entering the right vortex of the acceleration. Parameters affecting the wake vortex leader aircraft from its right side. Seen from behind, this vortex is rotating anti- As a conclusion, the typical signature of Aircraft weight: Wake vortex strength It has also been demonstrated that clockwise. When the left wing of the a severe encounter is an initial small roll in increases with the weight of the aircraft. aircraft having a high inboard loading follower fi rst enters the vortex, there one direction followed by a much more This is why today the ICAO aircraft (higher defl ection of the fl aps close to is on this wing a local angle of attack signifi cant roll in the other sense. classifi cation is based on the MTOW. the fuselage as an example) have a increase and therefore the lift becomes However, such an approach is a faster decay of their vortices. higher than on the right wing. The initial When in cruise, this roll motion may be simplifi cation as other parameters also roll motion is therefore to the right. associated with signifi cant load factor affect the strength at the separation Weather conditions: The weather Then, when the aircraft is in the middle variations. distance. conditions play a major role in wake Calm weather vortex development and decay. In the ( fi g . 2 ) Wing characteristics: The wing shape case of heavy turbulence, a vortex will Aircraft behaviour in a wake vortex creates the most and the load distribution affect the wake dissipate very quickly and there is no risk encounter (The aircraft bank angle is critical situation vortex characteristics, mainly through for the “follower” aircraft. Strong winds voluntarily exagerated on the fi gure) the decay rate. are associated with turbulence and will as the strength also contribute to a rapid dissipation. decreases slowly A smaller wing span increases the decay rate. Therefore, for a given Calm weather creates the most critical and the vortex “vortex generator” or “leader” aircraft situation as the strength decreases effect may be felt weight and at the same distance, slowly and the vortex effect may be felt The typical far behind the vortex encounters are less severe far behind the vortex generating aircraft. signature of a severe behind an aircraft having a smaller Today, in order to be safe, all separations vortex generating wingspan. assume that the aircraft are flying in encounter is an initial aircraft. perfectly calm conditions. small roll in one direction followed Encountering a wake vortex by a much more signifi cant roll in the DEFINITIONS other sense. When an aircraft enters in the vortex of another aircraft, the “manoeuvre” is called an encounter. The aircraft emitting the vortex is called the generator Effect on the trajectory of the follower and the one experiencing it, the follower. To experience a severe roll encounter, When perpendicular, there will be no How likely is an encounter? it is necessary for the follower to have rotation, and any encounter will be a a trajectory with a small closing angle very brief but sharp turbulence effect. It is not possible to implement navigation in cruise, A319 vortices were identifi ed at with the vortex. However, if this angle is To experience a severe encounter, procedures such that the probability of a range of 42 NM, thanks to the contrails. too small, the aircraft will be smoothly the most critical angle between the an encounter is zero. To give an example, An encounter with such a vortex is “ejected” from the vortex (due to trajectory of the follower and the vortex during the Airbus wake vortex fl ight tests, obviously very weak but it exists and it the initial roll in the example above). is around 10 degrees. OPERATIONS Safety First #21 | January 2016 047 Wake Vortices
Severity of the encounters Use of rudder warning
The authorized separations are such when in ground effect, as explained A large defl ection of the rudder creates accident has already occurred for this that the severity of the encounters does above, the decay is much faster and a very important lateral acceleration reason. Some recent aircraft types are not create an unsafe control situation. the worldwide experience during many that may well surprise the pilot. It could protected thanks to their fly-by-wire When the aircraft is not in ground years shows that the bank angle lead to a reaction with a defl ection to systems, but anyway, any use of the effect, the order of magnitude of the achieved is much lower and does not the other side. This could then give rise rudder does not reduce the severity of bank angle for a severe encounter lead to a risk of touching the ground to very large forces on the fi n that may the encounter nor does it improve the on the approach is around 20°. But with the wingtip. exceed the structural resistance. An ease of recovery. Therefore:
Duration of an encounter DO NOT USE THE RUDDER A severe encounter, as described with the fl ight mechanics equations, it above, where the trajectories of both has been demonstrated during Airbus Lateral offset aircraft have an angle around 10 fl ight tests that the stabilization of a degrees, typically lasts around 4 to 6 large aircraft inside a vortex can only If two aircraft are fl ying exactly on the not a guarantee that an encounter will seconds. be obtained by voluntarily establishing same track, one being 1000 ft below be avoided (except if the vortices are a large sideslip angle. As airliners do the other, in the same or opposite clearly visible by contrails). It is not possible to remain for a long not and should not fl y with large sideslip direction, and if there is no cross wind, time in a severe vortex as the rotating angles, they cannot remain in a vortex. there is a risk of encounter with a vortex In case of cross wind, if the two aircraft airflow on the wing and on the fin, Therefore, a vortex cannot be the cause for the lower aircraft. In this case, it is are fl ying exactly on the same track, eject the aircraft from the vortex. In line of long duration turbulence. possible to reduce the risk by using a the wind will move the vortices out of lateral offset. the track of the lower aircraft whilst Operational procedures they are descending. In this situation, However, most of the time, it is diffi cult to if a lateral offset is decided for other General procedure increases know whether the other aircraft is fl ying reasons than wake vortex avoidance, with or without a small relative offset an offset upwind by the follower is to be Considering the way the vortex is clearly show that pilot action does not due to the lack of angular precision preferred, since a downwind one may acting on the aircraft as explained improve the situation. of the TCAS. Therefore, this offset is potentially create an encounter. previously, if the pilot reacts at the fi rst roll motion, to the right in the example In addition, in-flight incidents have Final approach given, he will correct by rolling to the demonstrated that the pilot inputs left. When in the core of the vortex, the may exacerbate the unusual attitude During the final approach, it has is already, today, the main cause main roll motion to the left will then be situation with rapid roll control reversals sometimes been suggested to of accidents and such a technique amplifi ed by this initial piloting action. carried out in an “out of phase” manner. maintain a trajectory slightly above the would only increase that risk. The result will be a fi nal bank angle glide slope. This is not a satisfactory greater than if the pilot would not have In the case of a severe encounter procedure for transport aircraft for As a conclusion, a transport aircraft moved the controls. the autopilot may disconnect auto- several reasons: should not deviate from the standard matically, but in all other cases, it will approach slope to avoid a risk This has also been demonstrated be able to counter properly the roll • When established in descent on the of encounter. However, for light during the Airbus fl ight tests. Most of and pitch motions generated by the standard approach slope, as the aircraft, with low approach speed, the encounters have been performed vortex. vortex is descending, there is little approaching on a long runway, it is stick free, but several hundred were risk of encountering the vortices of an acceptable procedure to perform carried out with the pilot trying to For these reasons, the best procedure the previous aircraft, except possibly a high approach and a long landing, minimize the bank angle. The results in case of encounter is: when reaching the area of the ground targeting a touch down point after effect. However, this possibility has that of the previous aircraft. not led to an unsafe situation (no accident in ground effect recorded It is to be noted that, when on an RELEASE THE CONTROLS on transport aircraft with standard approach, there is no risk of encounter Do not voluntarily disconnect the autopilot separations). with the vortices of an aircraft taking-off If the autopilot is disconnected, before any reaction, wait for a reasonable on the same runway as a vortex will only stabilization of the aircraft, then: • If the aircraft is fl own too high above move backward due to the wind effect. • Roll wings level. the threshold to avoid a possible Such a vortex will have a very limited • Re-establish the initial cruise level or the standard climb or descent encounter, it will lead to a long landing strength, and in the case of a strong trajectory. and therefore signifi cantly increase headwind may even be dissipated the risk of runway excursion. It is completely. However, with crossing well known that runway excursion runways, depending on their geometry, OPERATIONS Safety First #21 | January 2016 049 Wake Vortices
and with inappropriate procedures, it another runway. Pilots on the approach Other rules: The ICAO rules are used from A to F, A being the larger aircraft may be possible that, very close to the need to maintain a general vigilance worldwide except in two Countries, category. The principle is to divide ground, a landing aircraft enters the and awareness, especially with calm USA and UK. These two Countries the Heavies and the Medium each in vortex of an aircraft which took-off on wind conditions. apply a different classification with 2 categories. As an example, today, different weight limits and separations. the separations between Heavies Departure are established for the worst case RECAT (Re-categorization). that is the smaller Heavy behind the During the take-off phase, other obtain a minimum radar separation, bigger. However, if this bigger Heavy than time separation, no avoidance depending on the departure tra- Principles of the re-categorization: follows the smallest, common sense procedure is applicable as the jectory and long experience has The target of the re-categorization is indicates that a reduction of separation manoeuvre is dictated by characteristic demonstrated an acceptable level of to reduce the separations on approach is possible without any impact on speeds V1, Vr, V2, determined by the safety. and for departure between some the safety level (fi g.3). Similarly, the weight, the weather conditions and aircraft pairs, without degradation of separation may be reduced between the runway. The time separations For a light aircraft taking-off from the safety levels, in order to improve two big Heavies or two small Heavies. given for some aircraft types ensure a long runway behind a transport the landing capacity of a given runway The same principles apply to the that possible encounters after take- aircraft, it is recommended to choose or runway couple. Medium category. The target is that off remain controllable. When no time the departure point in order to achieve no situation should be worse than (fi g.3) separation is given by ICAO rules, the a trajectory well above the preceding The fi rst step is called RECAT 1. All that which exists today with ICAO Toward a reduction of aircraft separation separation is decided by the ATC to aircraft. the aircraft are placed in 6 categories separations. minima to aircraft categories Separations If this is safe... ICAO rules
Almost everywhere in the world the Approach: On approach, the sepa- separations comply with the ICAO rules. rations depend on the leader and the follower classifi cation. The table below 4 NM Classifi cations: Three categories of gives the separations for the various aircraft are defi ned according to the pairs on the same runway. They apply MTOW: also to operations on different parallel Heavy (H): above 136 tons. runways if they are separated laterally Medium (M): between 7 and 136 tons. by less than 760 m. To be noted that Light (L): below 7 tons. the A380 separations are not in the ICAO recommendations (PANS-ATM), In addition, despite being classifi ed but in a provisional State Letter pub- ...this is over conservative as Heavy, the A380 is known as lished by ICAO in 2008. Super (S), and subjected to increased separations in approach, behind. 4 NM Cruise: In cruise, the separations are identical for all aircraft types: Horizontally: 5 NM. Vertically: 1000 ft.
Follower
S H M L RECAT 1 FAA: The FAA decided to reclassify the aircraft by MTOW and S 6 NM 7 NM 8 NM wingspan. The RECAT FAA is implemented on several US airports.
H 4 NM 5 NM 6 NM RECAT 1 EU: It appeared that the RECAT FAA approach was giving few benefi ts Leader to the European airports due to the differences in the airlines fl eets on both sides M 5 NM of the Atlantic. A RECAT EU was therefore developed. It takes into consideration not only the strength of the wake vortex of the leader aircraft, but also the L resistance of the follower. The encounter tests performed by Airbus allowed validating some models used for the computations. OPERATIONS Safety First #21 | January 2016 051 Wake Vortices
The RECAT 1 EU has also 6 categories:
A - Super Heavy: Including A380 and An124. B - Upper Heavy: MTOW above 100 tons and wingspan between 52 m and 72 m. C - Lower Heavy: MTOW above 100 tons and wingspan below 52 m D - Upper Medium: MTOW between 15 and 100 tons and wingspan above 32 m. E - Lower Medium: MTOW between 15 and 100 tons and wing span below 32 m F - Light: MTOW below 15 tons.
The separations are as follows:
Follower Super Upper Lower Upper Lower Light Heavy Heavy Heavy Medium Medium
Super Heavy 3 NM 4 NM 5 NM 5 NM 6 NM 8 NM Upper Heavy 3 NM 4 NM 4 NM 5 NM 7 NM Lower Heavy 3 NM 3 NM 4 NM 6 NM Leader Upper Medium 5 NM Lower Medium 4 NM Light 3 NM
The RECAT EU was approved by EASA end 2014. The implementation is planned at Paris-Charles-de-Gaulle airport in February 2016 and it will also be implemented in some airports worldwide.
It is to be noted that this implementation is not intended to be mandatory and only the most important European airports will use it, the other ones will keep the ICAO separations.
RECAT 2 and RECAT 3: The RECAT 2 is also called “pair-wise”, with a separation that takes into consideration the leader and the follower types, possibly by groups of aircraft. It will be implemented in the coming years.
The separations are not meant to avoid all encounters but to prevent unsafe ones. In very calm air, wake vortices encounters may lead to strong turbulence with significant bank angle and possibly some load factor when at high altitude. Remember: Release the Controls and DO NOT use Rudder.