Wake Turbulence
Wake Turbulence All aircraft produce wake turbulence, which consists of wake vortices formed any time an airfoil is producing lift. 2
Wake Turbulence Occurrences Many pilots have received a fright from encountering wake turbulence unexpectedly. Fortunately, in New Zealand accidents resulting from wake turbulence have been rare. Recorded occurrences include: In Wellington a Cessna 185 aircraft departed directly behind a Boeing 727. After getting airborne the Cessna rolled into a righthand turn and crashed on the western boundary of the airfield. A Piper Cub departing the circuit to the west from Christchurch passed under the flight path of a Boeing 747, which was joining downwind. Although some distance behind and below the Boeing, the Cub experienced a sudden 80 to 90- degree wing-down upset. One light aircraft pilot experienced severe wake turbulence from an aircraft of a similar type on takeoff close behind. Light aircraft can generate significant wake turbulence for other light aircraft. A light aircraft was downwind and accepted the controller s request that an overtaking Boeing 737 could pass overhead and descend in front of it. The 737 was slow, clean, and levelling out all factors which produce the strongest wake turbulence. The light aircraft experienced a 270-degree roll, and the startled pilot elected to continue the roll to reach level flight again. A Cherokee Six on approach behind a Boeing 737 experienced a 90-degree roll to the right. The flight path was the same profile as the Boeing. The pilot describes the experience as if someone had grabbed the propeller and stopped it but the aircraft continued rotating. 3
What is Wake Turbulence? All aircraft produce wake turbulence, which consists of wake vortices formed any time an aerofoil is producing lift. Lift is generated by the creation of a pressure differential over the wing surfaces as the aircraft moves forward. The lowest pressure occurs over the upper surface and the highest pressure under the wing. Air will always want to move towards the area of lower pressure. This causes it to move outwards under the wing and curl up and over the upper surface of the wing. This starts the wake vortex. The same pressure differential also causes air to move inwards over the wing. Small trailing edge vortices, formed by outward and inward moving streams of air meeting at the trailing edge, move outwards to the wingtip and join the large wingtip vortex. Swirling air masses trail downstream of the wing tips. Viewed from behind, the left vortex rotates clockwise and the right vortex rotates counter-clockwise (see figure 1). They spread laterally away from the aircraft and descend approximately 500 to 900 feet at distances of up to five miles behind it. These vortices tend to descend at approximately 300 to 500 feet-per-minute during the first 30 seconds (see figure 2). Light crosswinds may cause the vortices to drift, and crosswinds in excess of five knots tend to cause them to break up behind the aircraft. Atmospheric turbulence generally Figure 2 Wingtip vortices spread laterally away from the aircraft and decend 500 to 900 feet at distances of up to five miles behind it. Vortices tend to descend 300 to 500 feet-per-minute in the first 30 seconds. Figure 1 Viewed from behind the generating aircraft, the left vortex rotates clockwise and the right vortex rotates counter-clockwise. 4
causes them to break up more rapidly. The intensity or strength of the vortex is primarily a function of aircraft weight, wingspan and configuration. The strongest vortices are produced by heavy aircraft flying slowly in a clean configuration. For example, a large or heavy aircraft, which must reduce its speed to 250 knots below 10,000 feet, while flying in a clean configuration is producing very strong wake vortices while it descends. Helicopters also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from fixed-wing aircraft of similar weight. The strongest wake turbulence can occur when the helicopter is operating at lower speeds (20 to 50 knots). Some mid-size or executive class helicopters produce wake turbulence as strong as that of heavier helicopters. Two-blade main rotor systems produce stronger wake turbulence than rotor systems with more blades. Sikorsky S76 Photograph courtesy of FAA Technical Centre. Wake Turbulence Takeoff and Landing While there have been instances where wake turbulence caused structural damage, the greatest hazard is induced roll and yaw. This is especially dangerous during takeoff and landing, when there is little height for recovery. Wake turbulence-induced roll rates can be extreme. Countering roll rates may be difficult or impossible, even in high performance aircraft with excellent roll control authority. In fixed-wing aircraft, wake vortices begin to be produced as soon as there is a significant airflow across the aerofoil (ie, lift is being produced) during the takeoff roll, and continue until the aircraft comes almost to a halt at the end of its landing roll. The vortices, however, do not become pronounced until the aircraft climbs out of ground effect on takeoff. They decrease significantly as the aircraft descends back into ground effect prior to touchdown. Vortices can cause problems for aircraft crossing behind or below leading aircraft. Low approaches, touch-and-goes and go-arounds can also cause problems for taxiing or departing aircraft. During takeoff and landing, the vortices sink toward the ground and move laterally 5
aircraft. Pilots should follow the guidelines below, and ATC will make allowance when sequencing. Wherever practicable, controllers will advise pilots of the likelihood of wake turbulence by using the phrase, Caution wake turbulence. Weight Categories For the purpose of assessing wake turbulence separation, aircraft are divided into the following categories of Maximum Certificated Takeoff Weight (MCTOW): away from the runway when the wind is calm. A crosswind of 3 to 5 knots will tend to keep the upwind vortex in the runway area and may cause the downwind vortex to drift toward another runway. Care must be taken in such light crosswind conditions when a parallel runway is in use. Wake turbulence separation is provided by ATC to aircraft in the vicinity of a runway when radar separation is being applied, and when it is known that an aircraft may be affected by the phenomenon. On other occasions, particularly when on a visual or VFR approach, it is the pilot s responsibility to provide adequate spacing from preceding arriving or departing Heavy (H) All aircraft types of 136,000 kg MCTOW or more. Some examples of these are: Boeing B747, B767, B777, and the McDonnell Douglas MD 11. Photograph by Paul Harrison B757 aircraft are categorised as heavy (H) aircraft for the purposes of assessing wake turbulence experienced by following aircraft. 6
Medium (M) Aircraft types of more than 7,000 kg and less than 136,000 kg MCTOW. Some examples of these are: Boeing, B757*, B737, Dash 8, ATR 72, Saab 340, Beech 1900D, Hercules and DC 3. Light (L) Aircraft types of less than 7,000 kg MCTOW. Some of the heavier examples of these are: Metro 3, Cessna 402 and 421, Islander, Nomad, Piper Navajo and Beech 99. wake turbulence category of the leading aircraft and the equipment available to them to provide separation. Leading Following Separation aircraft aircraft distance Heavy* Heavy 4 NM Heavy Medium 5 NM Heavy Light 6 NM Medium Light 5 NM * The B757 is categorised as Heavy when applying following distances. Depending on which model of Metro, its modification status, and its operating weight on the day, it can sometimes fall into the medium category of over 7000 kg MCTOW. This would appear to make little difference to procedural separations, but all pilots should be aware that Metro wake turbulence can have a bigger bite than might be suspected from having the type listed in the light category. Wake Turbulence Separation Radar Separations Air traffic control applies differing wake turbulence separations depending on the Non-radar Separations Non-radar separation standards for arriving or departing flights, for aircraft using the same (or close parallel) runway are as follows: Leading Following Separation Separation aircraft aircraft time time arriving departing Heavy Medium 2 mins 2* mins Heavy Light 3 mins 2* mins Medium Light 3 mins 2* mins * 3 mins if taking off from an intermediate position. These are elaborated on, and there are further standards listed, in the AIP New Zealand such as opposite-direction runway operations and crossing runways. 7
Pilot Options If a pilot considers the wake turbulence separation standards inadequate, an increased separation may be requested by specifying the spacing required. Conversely, if pilots indicate that they will take responsibility for their own wake turbulence separation, then they may request exemption from these separations. This option should be treated with caution. Jet Blast Another hazard to bear in mind, particularly for light aircraft, is jet blast and propeller slipstream. Beware of passing close or landing directly behind aircraft with engines running, particularly large jets. Jet blast and propeller slipstream can produce localised wind velocities of sufficient strength to cause damage to other aircraft, vehicles, personnel and buildings. Some years ago, a Boeing 727 on engine tests blew in a hangar door clear testimony to the force that can be exerted. Issues Impacting Visual Separation Air traffic controllers may separate departing aircraft by visual means after considering aircraft performance, wake turbulence, closure rate, routes of flight, and known weather conditions. Visual separation of aircraft should not be applied by controllers between successive departures when departure routes and/or aircraft performance will not allow the pilots to maintain adequate separation. In the vicinity of an aerodrome in VMC by day, the air traffic controller must have both aircraft in sight and must be in radio contact with at least one of them. The pilot of the trailing aircraft must see the leading aircraft and be informed of the leading aircraft s position, its direction of flight, and its intentions. The pilot of the trailing aircraft must acknowledge sighting the leading aircraft and be instructed to maintain visual separation. The tower controller will not provide visual separation between aircraft when wake turbulence separation is required. 8
In controlled airspace with ATC radar coverage, the controller may inform the pilot of converging and VFR traffic. In cruise, when IFR and VFR aircraft are sometimes separated by as little as 500 feet, pilots must use appropriate avoidance procedures. Because wake turbulence is nearly always invisible, pilots need to anticipate where it might be. Air traffic controllers issue Caution wake turbulence warnings only, and they are not responsible for anticipating the existence or effect of the condition. A CX 747-200 on approach to Kai Tak airport, Hong Kong. The Warning Signs Any uncommanded aircraft movements, such as wing rocking, may be caused by wake vortices. This is why maintaining situational awareness is so critical. Atmospheric turbulence is not unusual, particularly in the approach phase. Pilots who suspect wake turbulence is affecting their aircraft should immediately move away from the wake by executing a missed approach or go-around. The onset of wake turbulence can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake, only to encounter severe wake turbulence. Pilots should not depend on any aero-dynamic warning. If the onset of wake turbulence is occurring, immediate evasive action is a must! Taken from the chequerboard at a time when there was a fire in Kowloon City, thus making the vortices very visible. Photographs, Cathay Pacific 'Crews News'. 9
10 Photograph courtesy of Boeing
How to Avoid Wake Turbulence Pilots should remember three basic warnings concerning wake turbulence: Do not get too close to the leading aircraft. Do not get below the leading aircraft s flight path. Be particularly wary when light wind conditions exist. The following avoidance procedures should be followed: Takeoff Plan to lift off well before the rotation of the leading aircraft (see figure 3). If this is not possible, apply wake turbulence separation standards before taking off (remember though, that wake turbulence separation is no guarantee wake turbulence will not be experienced). Before taking the active runway, however, tell the tower that you want to wait. If, for example, you plan Figure 3 Taking off after a large aircraft has taken off, same runway to takeoff from an intermediate runway behind an aircraft that has just landed full length, then delay your takeoff as per the wake turbulence separation standards (see figure 4A). If the landing aircraft does not use full length, line up and start your takeoff roll a suitable distance beyond its touchdown (see figure 4B). Figure 4A Taxiway Taking off after a large aircraft has landed, intermediate, same runway Touchdown When planning to take off from an intermediate behind an aircraft that has landed full length, delay your takeoff as per wake turbulence separation standards. Figure 4B Taking off after a large aircraft has landed short of intermediate, same runway Touchdown (B747) Rotation Rotation Taxiway Rotation (light aircraft) Ensure that you can take off well before the larger aircraft s rotation and are able to climb well above its flight path, or delay your takeoff as per wake turbulence separation standards. If the landing aircraft does not use full length, line up and start your takeoff roll a suitable distance beyond its touchdown. 11
Climb If possible, climb above the leading aircraft s flight path. If you can t out-climb it, fly slightly upwind and climb parallel to the leading aircraft s course. Avoid headings that cause you to cross behind and below the aircraft in front. Figure 6 2000 ft Trailing En Route Wind Figure 5 Crossing If you must cross behind the leading aircraft, try to cross above its flight path or, terrain permitting, at least 1,000 feet below (see figure 5). Crossing departure courses 3000 ft 2000 ft 3000 ft Avoid flight below and behind larger aircraft. If possible, stay well above the larger aircraft. Otherwise remain at least 1000 feet below, upwind and well behind its flight path. Head On If approaching a heavier aircraft that is less than 1,000 feet above you on a reciprocal track, alter course sufficiently (preferably upwind of the heavier aircraft) to ensure that any diverging wake turbulence from it is avoided (see figure 7). Figure 7 Head on En Route Wind After takeoff, avoid headings which cross below and behind the path of larger aircraft. Trailing Endeavour to stay either on or above the leading aircraft s flight path, or upwind, or, terrain permitting, at least 1,000 feet below and a suitable distance (refer table page 7) behind it (see figure 6). 2000 ft 3000 ft If the larger aircraft is observed less than 1000 feet above you on a reciprocal track, adjust your position laterally, preferably upwind. 12
Approach Maintain a position on or above the leading aircraft s flight path with adequate lateral separation. Landing Ensure that your touchdown is well beyond the leading aircraft s touchdown (see figure 8). Land well before a departing aircraft s rotation (see figure 9). Figure 10 Landing after a large aircraft has landed, crossing runway Touchdown s Figure 8 Landing after a large aircraft has landed, same runway Cross above the larger aircraft s flight path, and aim to touch down well beyond the runway it has just landed on, or discontinue the approach. Figure 11 Touchdown Landing after a large aircraft has taken off, crossing runway Rotation Touchdown Touchdown Approach above the larger aircraft s glideslope, note its touchdown, and land a safe distance beyond it. Figure 9 Touchdown Landing after a large aircraft has taken off, same runway Rotation Note the larger aircraft s of rotation, and aim to touch down well before it. If the larger aircraft rotates past the intersection, touch down before the intersection. If the larger aircraft rotates before the intersection, discontinue the approach (go around well above its flight path), unless the landing run can be completed before the intersection. Remain mindful of your flight path relative to the possible area of wake turbulence. Crossing Runways When landing behind another aircraft on crossing runways, cross well above the other aircraft s flight path, and aim to touchdown well beyond the it has just landed on (see figure 10). If the aircraft has rotated after the runway intersection, then be sure to touch down well before the runway intersection (see figure 11). 13
Crosswinds Remember that crosswinds may affect the position of wake vortices and can be very dangerous during parallel runway operations. Takeoff and landing s should be adjusted and wake turbulence separation standards applied accordingly (see figure 12). Figure 12 Landing after a large aircraft has landed, parallel runway Touchdown s Wind Less than 760m Note the wind direction and possible vortex drift from the larger aircraft, stay above its flight path, and aim to land beyond its touchdown. Helicopters Remember that helicopter wake vortices may be of significantly greater strength than those of fixed-wing aircraft of similar weight. Avoid flying beneath the flight paths of helicopters. When piloting a small aircraft, avoid taxiing within three wingspans of a helicopter that is hovering or hover taxiing at slow speed. Visual Approach When making a visual approach, do not assume that the aircraft you are following is on the same or lower flight path. The flight crew of the leading aeroplane may have flown a steep approach (typical of cargo operations). Stay above and at least three miles behind the normal flight path (at least four miles behind a B757 or larger). Summary Wake turbulence is one of many factors that pilots and air traffic controllers must consider to ensure flight safety. It takes co-operation, awareness and an understanding of each other s requirements to safely avoid aircraft-generated wake. It is your responsibility as the flight crew or pilot in command to anticipate the likelihood of encountering wake turbulence and to alter your flight path accordingly, or, if necessary, request an alternative clearance from ATC. Do not rely on others to provide warnings. Above all, if you suspect that you are about to encounter wake turbulence from the preceding aircraft, then take evasive action before it is too late. 14
GAPs are produced by the Civil Aviation Authority of New Zealand. Wake Turbulence was revised in July 2003