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 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. The 
       lowest pressure occurs over the upper surface and the highest pressure 
       under the wing. Air will want to move towards the area of lower pressure. 
       This causes the air to move outwards under the wing and curl up and over 
       the upper surface of the wing. This starts the wake vortex. The pressure 
       differential also causes the 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 
       wingtips. Viewed from behind the left vortex rotates clockwise and the 
       right vortex rotates counter- clockwise. They spread laterally away from 
       the aircraft and descend 500 to 900 feet at distances of up to five miles 
       behind it. These vortices tend to descend 300 to 500 feet per minute 
       during the first 30 seconds. 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 causes them to 
       break up more rapidly. The intensity or strength of the vortex is 
       primarily a function of aircraft weight, wingspan and configuration (flap 
       setting, etc). 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.  
        Viewed from behind the generating 
       aircraft, the left vortex rotates clockwise and the right vortex rotates 
       counter-clockwise. They spread laterally away from the aircraft and 
       descend 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.
 Helicopters also 
       produce wake turbulence. Helicopter wakes may be of significantly greater 
       strength than those from fixed-wing aircraft of the same 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. Wake Turbulence During 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 as the nose is rotated for 
       takeoff and continue throughout flight until the nosewheel touches down 
       on the runway once again. The 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 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. Wake vortices sometimes bounce, diverge and dissipate more 
       rapidly in ground effect. Wake turbulence separation is provided by Air 
       Traffic Control (ATC) to all aircraft which may be affected by wake 
       turbulence, except in the case of IFR aircraft making a visual approach 
       or VFR aircraft arrivals. In these cases it is the pilot’s responsibility 
       to provide adequate spacing from preceding, arriving or departing 
       aircraft. Pilots should follow the guidelines below and ATC will make 
       allowance when sequencing. Wherever practicable, aerodrome 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): 
       Heavy (H)
 All aircraft types of 136,000 kg MCTOW or more. Some examples of these 
       are: Boeing B777, B767, B747, McDonnell Douglas DC–8, MD–11, and DC–10.
 
 Medium (M)
 Aircraft types of more than 7,000 kg and less than 136,000 kg MCTOW. Some 
       examples of these are: Boeing B727, B737 and B757*, Fokker Friendship, 
       Metro 4 , BAe–146, Dash 8, ATR–72, Hercules, DC–3 and Saab 340.
 
 * B757 aircraft are categorised 
       as ‘heavy’ (H) aircraft for the purpose of assessing wake turbulence 
       experienced by following aircraft.
 Light (L)Aircraft types of less than 7,000 kg MCTOW. Some of the heavier examples 
       of these are: Bandeirante, 
       *Metro 
       3 , Cessna 402 and 421, Islander, Nomad, Piper Navajo and Beech 99.
 
 *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 7,000 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.
 Departure 
        After takeoff, avoid headings which cross below 
       nd behind the path of larger aircraft.
 
       Wake Turbulence Separations 
       Radar Separations 
       
       ATC applies differing separations 
       depending on the wake turbulence category of the leading aircraft and the 
       equipment available to them to provide separation eg, radar. The tables 
       given below are issued by ICAO. The UK has slightly different values. 
       
        * The B757 is categorised as ‘heavy’ when applying following distances.
 Non-radar Separations Non-radar separation standards for arriving or departing flights for 
       aircraft using the same (or close parallel) runway are as follows: 
        * 3 mins if taking off from an intermediate position.
 These are elaborated on, and there are further standards listed, in 
       the AIP Planning Manual - such as opposite direction runway operations 
       and crossing runways. Remember wake turbulence separation is not provided to landing VFR 
       arrivals, nor to IFR on visual approach. In these cases it is up to the 
       pilot to provide adequate spacing from preceding arriving or departing 
       aircraft. 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 B727 on engine tests 
       blew in a hangar door - clear testimony to the force which can be 
       exerted.  Taking off behind larger 
       aircraft  Ensure you can rotate before the preceding aircraft's rotation point. A 
       climb above its flight path is also necessary, until you can turn clear. 
       If this is not possible, delay your takeoff.
 En route  Avoid flight below and behind larger aircraft's flight paths. If a larger 
       aircraft is observed less than 1,000 feet above you on the same track 
       (same or opposite direction) adjust your position laterally, preferably 
       upwind.
 
        When planning to take off from an intermediate point behind an aircraft 
       that has used full length, delay your takeoff.
 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. 
       Controller visual separation of aircraft should not be applied between 
       successive departures when departure routes and/or aircraft performance 
       will not allow the pilots to maintain adequate separation. In the 
       terminal area it must be day, the air traffic controller must have both 
       aircraft in sight and must be in radio contact with at least one of them. 
       The flight crew of the trailing aircraft must see the lead aircraft and 
       be informed of the lead aircraft's position, its direction of flight and 
       its crew's intentions. The pilots of the trailing aircraft must 
       acknowledge sighting the lead aircraft and be instructed to maintain 
       visual separation. The tower controller will not provide visual 
       separation between aircraft when wake turbulence separation is required. 
       In controlled airspace with ATC radar coverage, the controller must 
       inform the pilot of converging aircraft 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 are not responsible for anticipating the existence or effect of the 
       condition.  Landing behind a larger aircraft 1. Same runway 
        Stay at or above the larger aircraft's final approach flight path. Note 
       its touchdown point and land beyond it.
 2. Parallel runway or vector  Note wind for possible vortex drift on to the landing vector. Stay at or 
       above the larger aircraft's final approach flight path. Note its 
       touchdown point and land beyond a point abeam it.
 3. Crossing runway 
        Cross above the larger aircraft's flight path.
 The Warning Signs Any un-commanded 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; then must be 
       prepared for an even stronger wake vortex encounter. 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 aerodynamic warning. If the onset of wake 
       turbulence is occurring, immediate evasive action is a must! 
        How to Avoid Wake Turbulence Pilots should remember three basic warnings concerning wake 
       turbulence:
 *Do not get too close to the lead aircraft.
 
 *Do not get below the lead aircraft's flight path.
 
 *Be particularly wary when light wind conditions exist.
 
 The following avoidance procedures should be followed at all times:
 
 Takeoff. If 
       you think wake turbulence from the preceding aircraft may be a factor, 
       wait about 2 or 3 minutes before taking off. Before taking the active 
       runway, tell the tower that you want to wait. Plan to lift off prior to 
       the rotation point of the lead aircraft, and use full takeoff power or 
       thrust.
 
 Climb. If possible, climb above the lead aircraft's flight path. If 
       you can't out- climb it, fly slightly upwind and climb parallel to the 
       lead aircraft's course. Avoid headings that cause you to cross behind and 
       below the aircraft in front.
 Crossing. If you must cross behind the lead 
       aircraft, try to cross above its flight path or, terrain permitting, at 
       least 1,000 feet below. 
 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.
 
 Approach. Maintain a position on or above the lead aircraft's flight 
       path with adequate lateral separation.
 
 Landing. Ensure that your touchdown point is beyond the lead 
       aircraft's touchdown point. Land well before a departing aircraft's 
       rotation point.
 
 Crossing Approaches. When landing behind another aircraft on crossing 
       approaches, cross above the other aircraft's flight path.
 
 Crosswinds. Remember crosswinds may affect the position of wake 
       vortices. Adjust takeoff and landing points accordingly.
 
 Helicopters. Remember that their wake vortices may be of 
       significantly greater strength than those of fixed-wing aircraft of the 
       same 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 lead aeroplane may have flown a steep 
       approach (typical of cargo operations). Stay above and at least 3 miles 
       behind the normal flight path (at least 4 miles behind a B757). 
 Wake turbulence is one of the factors that pilots and air traffic 
       controllers must avoid to ensure safe flights. It takes co-operation, 
       awareness and an understanding of each other's requirements to safely 
       avoid aircraft-generated wake.
 It is your 
       responsibility 
       as 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. |