Pilot And Air Traffic Controller Guide To Wake Turbulence

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SECTION 22Pilot and Air Traffic ControllerGuide to Wake Turbulence

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SECTION 2Pilot and Air Traffic Controller Guide toWake TurbulenceTable of ContentsSection2Page2.02.0.12.0.22.0.3Introduction . 2.1Preview . 2.1The Goal. 2.1Participants and Review Process . 2.12.1Objectives . 2.22.2Historical Examination of the Wake-Turbulence Hazard . 2.32.3Review of Accidents and Incidents . on/Characteristics of the Wake-Turbulence Hazard . 2.8Wake-Turbulence Formation . 2.8Velocity Flow Field . 2.9The Hazard . 2.10Vertical Motion of the Wake . 2.14Horizontal Motion of the Wake . 2.15Decay Process . 2.16Gaps in Our Knowledge . 2.162.5Future Wake-Turbulence Detection Technology . 2.162.6Air Traffic Control Responsibilities for Maintaining Aircraft Separation . 2.172.6.1Wake-Turbulence Cautionary Advisories . 2.172.6.2Radar/Approach Controllers . 2.182.6.3Tower Controllers . 2.182.6.3.1Wake-Turbulence Separation for Departing Aircraft . 2.182.6.3.2Wake-Turbulence Departure Separation Criteria . 2.182.6.4Visual Separation . 2.192.6.4.1Visual Separation-Terminal Area . 2.192.6.4.2Visual Separation - En Route . 2.192.6.4.3Visual Separation - Nonapproach Control Towers . 2.192.72.7.12.7.2Pilot Responsibilities for Maintaining Wake-Turbulence Separation . 2.19Who Does What and When . 2.19Communications . 2.202.8Wake Turbulence Recommended Visual Avoidance Procedures . 2.212.8.1Specific Procedures . 2.212.8.1.1Landing Behind a Larger Aircraft - Same Runway . 2.212.8.1.2Landing Behind a Larger Aircraft - Parallel RunwayCloser Than 2500 Feet . 2.222.8.1.3Landing Behind a Larger Aircraft - Crossing Runway) . 2.222.iii

SECTION 2Table of Contents 1.82.8.1.92.8.2PageLanding Behind a Departing Larger Aircraft - Same Runway . 2.23Landing Behind a Departing Larger Aircraft - Crossing Runway . 2.23Departing Behind a Larger Aircraft . 2.24Intersection Takeoffs - Same Runway . 2.25Departing or Landing After a Heavy Aircraft Executing aLow Approach, Missed Approach or Touch-and-Go Landing . 2.26En Route Within 1000 Feet Altitude of a Large Aircraft'sAltitude . 2.26Avoiding Helicopter Outwash Vortices . 2.272.9Pilot Difficulty in Visually Maintaining Separation . 2.282.9.1Flightpaths. 2.282.9.1.1Use of ILS Glideslope . 2.292.9.1.2Visual Illusions . 2.292.9.1.3Darkness/Reduced Visibility . 2.292.9.2Instrument to Visual Situation . 2.292.10Pilot Techniques for Visually Maintaining Separation . 2.292.10.1General . 2.292.10.2Visual Cues for Estimating Leader’s Flightpath . 2.302.10.3Using ILS Glideslopes for Vertical Separation . 2.312.10.4Using ILS Localizer for Lateral Separation . 2.312.10.5Longitudinal Separation . 2.322.10.5.1Air Traffic Control Assist . 2.322.10.5.2On-board Radar . 2.322.10.5.3Time and Distance Methods . 2.322.10.6Establishing Longitudinal Separation . 2.322.10.7Radio Communications . 2.322.10.8Estimating Movement of Wake Turbulence . 2.332.iv2.11Pilot Responses Upon Encountering Wake Turbulence . 2.332.12Cooperative and Efficient Management of Capacity . 2.332.13Air Traffic Considerations When Applying Separation . 2.34

SECTIONSECTION22Pilot and Air Traffic Controller Guide toWake Turbulence2.0IntroductionThe Pilot and Air Traffic Controller Guide toWake Turbulence is one part of the WakeTurbulence Training Aid. The other partsinclude Section 1, Wake Turbulence - Overview for Training Aid Users; Section 3, Example Pilot and Air Traffic Controller WakeTurbulence Training Program; Section 4, WakeTurbulence Training Aid - Background Data,and a video.2.0.1PreviewThis Pilot and Air Traffic Controller Guide toWake Turbulence is a comprehensive document covering all the factors leading to ashared awareness and understanding of waketurbulence. A review of the history of waketurbulence studies, from the introduction ofturbo-jet aircraft to today’s environment, isthe starting point. A description of typicalaccidents and incidents allows a look at trendsand lessons learned from history. With history as a basis, a thorough description is givenof the wake-turbulence hazard. This includesthe formation, effects, and dissipation of thewake vortex phenomenon. A description isgiven of our ability to predict, detect, andmeasure the wake-turbulence hazard. Thisincludes future planned improvements inthese areas.Given our knowledge of wake turbulence, thebest solution is to avoid the wake-turbulencehazard. This document reviews the existingavoidance guidance and both air traffic control and pilot responsibilities. A discussion isoffered regarding the difficulty for pilots tovisually maintain separation and offers rec-ommended techniques. A brief discussion ofpilot responses to encountering wake turbulence precedes a section that stresses the necessary cooperation of pilots and air trafficcontrollers to safely and efficiently managethe busy airport environment and avoid waketurbulence encounters. Lastly, the importance of air traffic control considerationsassociated with assisting pilots in avoidingwake turbulence is discussed.2.0.22The GoalThe goal of the Wake Turbulence TrainingAid is to reduce the number of wake-turbulence related incidents and accidents by improving the pilot’s and air traffic controller’sdecision making and situational awarenessthrough increased and shared understandingand heightened awareness of the factors involved in wake turbulence. This can be accomplished by the application of knowledge,techniques and training applied to everydayoperations.2.0.3Participants and Review ProcessThe Wake Turbulence Training Aid is theresult of many hours of effort on the part of alarge industry team. This team consisted of:Air Line Pilots Association, Air Traffic Control Association, Airbus Industrie, Airbus Service Company, Inc., Allied Pilots Association,American Airlines, Aircraft Owners and Pilots Association, Air Transport Association,Boeing Commercial Airplane Group, DeltaAir Lines, Inc., Federal Aviation Administration, Flight Safety Foundation, General Avia-2.1

SECTION 2tion Manufacturers Association, Hydrolin Research Corporation, Independent Pilots Association, International Civil AviationOrganization, McDonnell Douglas AircraftCompany, National Aeronautics and SpaceAdministration, National Air Traffic Controllers Association, Inc., National Air Traffic Services (CAA), National Air TransportationAssociation, Inc., National Business AircraftAssociation, National Transportation SafetyBoard, Regional Airline Association, Southwest Airlines, The Communications Company, U.S. Department of Transportation, andUnited Airlines.The Pilot and Air Traffic Controller Guide toWake Turbulence will:The team worked on this project over a periodof nine months. During this period the WakeTurbulence Training Aid and associated videowas developed. In all, a total of four reviewcycles were conducted, during which the comments and recommendations of all participants were considered for inclusion in thefinal material. Three industry review meetings were held along with a final draft/finalvideo industry buy-off process. The FederalAviation Administration is responsible forthe final reproduction and distribution of theWake Turbulence Training Aid. As significant material is developed and changes arerequired to this document, a review will beconducted by the industry team and appropriate updating of the material will be developed and distributed. provide usable information to develop aground training program.2.1ObjectivesThe objectives of the Pilot and Air TrafficController Guide to Wake Turbulence are tosummarize and communicate key wake-turbulence related information relevant to pilotsand air traffic controllers. It is intended to beprovided to air traffic controllers and pilotsduring academic training and to be retainedfor future use.2.2 educate pilots and air traffic controllerson wake turbulence and avoidance of thephenomenon. increase the wake turbulence situationalawareness of pilots and air traffic controllers (situational awareness being definedas an accurate perception by pilots and airtraffic controllers of the factors and conditions currently affecting the safeoperation of the aircraft and the crew).The most important success tool availabletoday to pilots and air traffic controllers toreduce wake-turbulence accidents and incidents is awareness and education. One of theobjectives of this training aid is to educatepilots and air traffic controllers on wake turbulence and avoidance of the phenomena.This can be done by updating the basic understanding of wake turbulence to help reduceand clear up common misconceptions andgenerate respect for the hazard. This education will expand the awareness of pilots andair traffic controllers of their mutual involvement in the avoidance of wake turbulence.Additionally, education will generate baseline knowledge for instructors and thosepeople involved with developing trainingprograms.Another clear objective is to increase the waketurbulence situational awareness of pilots andair traffic controllers. This aid will providerecommendations to improve situationalawareness involving wake turbulence andtechniques for detection, avoidance and recovery. This should lead to shared awarenessand cooperation among air traffic controllersand pilots. Improved situational awareness

SECTION 2will better prepare pilots and air traffic controllers for future improvements and newtools to cope with wake turbulence.towers were also used to observe the waketurbulence generated by aircraft as they flewby. Several observations were made.Lastly, this Pilot and Air Traffic ControllersGuide to Wake Turbulence aims to provideusable information for the development ofground training programs. There are manysources of information about wake turbulence. This aid attempts to compile thosesources to provide information for trainingdevelopers. Since simulation capability islimited, the ground training material is developed into written modules, exams, and astand-alone video. The strength of the wake turbulence isgoverned by the weight, speed and wingspan of the generating aircraft.2.2Historical Examination of the WakeTurbulence HazardWake turbulence is a natural by-product ofpowered flight, but was not generally regardedas a serious flight hazard until the late 1960s.Upsets or turbulence encounters associatedwith other aircraft were usually accredited to“propwash” and later on, with “jet wash.”Interest in this phenomenon greatly increasedwith the introduction of large, wide-body turbojet aircraft during the late 1960s and a concern about the impact of greater waketurbulence. This was the impetus to conductresearch to gain additional information anddetermine what safety considerations werenecessary as more and more large aircraftentered the industry fleets.An investigation of the wake-turbulence phenomenon, conducted by Boeing in mid 1969as part of the FAA test program, includedboth analysis and limited flight test and produced more detailed information on wakevortices. The flight tests provided a directcomparison between the B-747 and a representative from the then current jet fleet, a B707-320C. The smallest Boeing jet transport,the B-737-100, was used as the primary waketurbulence probing aircraft along with an F86 and the NASA CV-990. Smoke generating The greatest strength occurs when thegenerating aircraft is heavy, at slow speedwith a clean wing configuration.Initial flight tests produced sufficient information about the strength, duration and movement of wake turbulence to come toconclusions and recommendations on how toavoid it. The wake was observed to movedown initially and then level off. It was neverencountered at the same flight level as thegenerating aircraft or more than 900 feet below the generating aircraft. Therefore, a following aircraft could avoid the waketurbulence by flying above the flightpath ofthe leading aircraft. While this can be accomplished in visual conditions, an alternativewas developed for instrument meteorological conditions. Aircraft were placed into categories determined by their gross weight. Itwas noted that a division based on the wingspan of the following aircraft was a moretechnically correct way to establish categories; however, it did not appear to be an easilyworkable method. Since there is a correlationbetween aircraft gross weight and wingspan,gross weight was selected as a means of categorizing aircraft and wake-turbulencestrength. Minimum radar-controlled waketurbulence separation distances were established for following aircraft. The separationdistances depend on the weight of both theleading and following aircraft. Adjustmentsin separation distances were made as moreinformation on the wake-turbulence phenomenon was gained during the 1960s, 1980s and1990s, but the basic concept of using aircraftweights remained constant.2.3

SECTION 2Initially, the turbojets that were being produced fit cleanly into distinct categories withlogical break points. For example, heavy aircraft such as the Boeing B-747, Lockheed L1011 and the Douglas DC-10 were clearly in aclass by themselves. There were very fewregional or business support size aircraft.Today, there is almost a continuum of aircraftsizes as manufacturers developed the “aircraft family” concept and produced manynew transport and corporate aircraft. Withimproved technology, heavier aircraft are produced with better aircraft performance allowing them the use of shorter runways thatpreviously could only be used by smalleraircraft. Additionally, a hub and spoke mix ofregional aircraft with heavy jets, coupled withan already active private and recreational aircraft population, results in a range of waketurbulence strengths produced and potentiallyencountered by a large variety of aircraft, asillustrated below (Figure 2.2-1).3.00Figure 2.2-1Calculated initialvortex strength2.502.00*RelativestrengthMax landing weight1.501.000.50Empty weight0.00B747 MD11 B777 A340 767ER A300 A310 B767 B757 B727 A320 MD80 B737 F100Aircraft type* Relative strength is the strength variation between maximum landing weight and empty weightrelative to a B-737 of a weight midway between its maximum weight and its empty weight.2.4

SECTION 2The wake-turbulence separation criteria, whilenecessary, are currently a limiting factor inseveral airport capacities. The FAA is working with NASA to develop and demonstrateintegrated systems technology for addressingseparation criteria. The thrust of the work isto develop wake-turbulence prediction capability, sensors for detecting wake-turbulencehazards on final approach and an automatedsystem to maximize operating efficiency whilemaintaining safety standards.The effort to gain more information aboutwake turbulence continues.2.3Review of Accidents and IncidentsNational Transportation Safety Board datashow that between 1983 and 1993, there wereat least 51 accidents and incidents in the UnitedStates that resulted from probable encounterswith wake turbulence. In these 51 encounters,27 occupants were killed, 8 were seriouslyinjured, and 40 aircraft were substantiallydamaged or destroyed. Numerous other encounters have been documented in the NASAAviation Safety Reporting System (ASRS).Since participation in ASRS is voluntary, thestatistics probably represent a lower measureof the true number of such events which occurred. The following are accounts of realevents.1. A pilot of a medium transport (60,000 pounds) was told to expedite the takeoff behind a large transport (150,000 pounds) onrunway 32L at Chicago. He began his takeoffroll as the large transport rotated. The largetransport went straight ahead and the pilot ofthe medium transport was instructed to turnto 180 degrees. He started the turn at 300 feetAGL with 15 degrees of bank angle. The bankangle violently increased to 30 degrees fromthe apparent wake turbulence of the largetransport.The takeoff was initiated about 30 or 40 seconds after the first aircraft.2. A Cessna Citation 550 crashed while on avisual approach. The two crew members andsix passengers were killed. Witnesses reported that the aircraft suddenly and rapidlyrolled left and then contacted the groundwhile in a near-vertical dive. Recorded ATCradar data show that at the point of upset, theCitation was about 2.78 nautical miles (about74 seconds) behind a B-757. The flightpathangle of the Citation was 3 degrees and theflightpath angle of the B-757 was 4.7 degrees.Standard IFR separation (greater than 3 nautical miles) was provided to the pilot of theCitation.About 4.5 minutes prior to theaccident while following the B-757 at a distance of 4.2 nautical miles, the pilot requestedand was cleared for a visual approach behindthe B-757. After the visual approach clearance was acknowledged, the speed of theCitation increased while the speed of the B757 decreased in preparation for landing. Thecontroller informed the Citation pilot that theB-757 was slowing and advised the pilot thata right turn could be executed to increaseseparation.Although radar data indicate that, at any instant, the Citation was at least 600 feet higherthan the leading B-757 during the last 4 milesof the approach, the flightpath of the Citationwas actually at least 300 feet below that of theB-757.3. The pilot of a Cessna 182 was executing avisual flight rules approach to runway 32 atSalt Lake City International Airport, Utah.The pilot reported that he was instructed byATC to proceed “direct to the numbers” ofrunway 32 and pass behind a “Boeing” thatwas on final approach to runway 35. TheCessna pilot reported that while on final approach, the aircraft experienced a “burble,”and then the nose pitched up and the aircraftsuddenly rolled 90 degrees to the right. Thepilot immediately put in full-left deflection ofrudder and aileron and full-down elevator inan attempt to level the aircraft and to get thenose down. As the aircraft began to respondto the correct attitude, the pilot realized thathe was near the ground and pulled the yoke2.5

SECTION 2back into his lap. The aircraft crashed short ofthe threshold of runway 32, veered to thenortheast, and came to rest in the approachend of runway 35. The pilot and the twopassengers suffered minor injuries, and theaircraft was destroyed. The wind was 5 knotsfrom the south.The approach ends of runways 32 and 35 areabout 560 feet apart. Radar data show that theCessna was at an altitude of less than 100 feetabove ground level (AGL) when it crossed theflightpath of the B-757. The B-757 had passedthe crossing position about 38 seconds priorto the Cessna 182.4. A Gulfstream IV departed New Jersey on aroutine night trip to Florida with a crew of 3and 2 passengers. The weather was clear withunlimited visibility and smooth air. During aslow descent for landing at approximatelyFlight Level 250, ATC advised the pilot that hemight see traffic crossing from right to left.The Gulfsteam pilot sighted the traffic farahead. At about 15,000 feet and 300 knots, theGulfstream pilot reported that he felt like hehad “hit a 20 foot thick concrete wall at 300knots.” The flight attendant and passengerswere injured. The passengers were jettisonedto the ceiling and slammed to the floor. Theaircraft was checked for damage and landeduneventfully.5. A McDonnell Douglas MD-88 was executing a visual approach while following a B-757to the airport. The crew of the MD-88 reported that the aircraft suddenly rolled rightabout 15 degrees and the pilot rapidly deflected both the wheel and rudder pedal tocorrect the uncommanded roll. Data from thedigital flight data recorder indicate that atabout 110 feet AGL the roll angle reached 13degrees right wing down and the ailerons andrudder were deflected about one-half of fulltravel, 10 degrees and 23 degrees respectively.The crew regained control and the approachwas continued to an uneventful landing. Recorded radar data show that at the point ofupset, the MD-88 was about 2.5 nautical miles(65 seconds) behind the Boeing 757 while theflightpath of the MD-88 was slightly belowthat of the B-757. The flightpath angle of bothaircraft was 3 degrees.2.6The MD-88 flight crew had been issued avisual approach clearance when the aircraftwas 4.5 nautical miles from the leading aircraft. However, the separation quickly reduced to 2.5 nautical miles. The MD-88 flightcrew told investigators that they thought theyhad a 4 nautical mile separation at the time ofthe encounter.6. An Israel Aircraft Industries Westwindcrashed while on a visual approach. The twocrew members and three passengers werekilled. Witnesses reported that the aircraftrolled and the cockpit voice recorder (CVR)data indicate that the onset of the event wassudden. The aircraft pitch attitude was about45 degrees nose down at ground contact. Recorded radar data show that at the point ofupset, the Westwind was about 1200 feet abovemean sea level and 3.5 nautical miles from therunway. The Westwind was about 2.1 nautical miles (60 seconds) behind a B-757 and ona flightpath that was about 400 feet below theflightpath of the B-757. The flightpath angleof the Westwind was 3 degrees and theflightpath angle of the B-757 was 5.6 degrees.CVR data indicate that the Westwind pilotswere aware they were close to a Boeing aircraft and the aircraft appeared high. Theyanticipated encountering a little wake andintended to fly one dot high on the glideslope.While receiving radar vectors to the airport,the crews of both aircraft were flying generally toward the east and would have tomake right turns to land to the south. Radardata and ATC voice transcripts show thatthe Westwind was 3.8 nautical miles northeast of the B-757 when cleared for a visualapproach. The Westwind started its rightturn from a ground track of 120 degreeswhile the B-757 ground track remained atabout 90 degrees. The resultant closureangle started at 30 degrees and becamegreater as the Westwind continued its turn.About 23 seconds later, the B-757 was clearedfor the visual approach. The average groundspeeds of the Westwind and the B-757 wereabout 200 and 150 knots, respectively. TheWestwind was established on course 37 seconds ahead of the B-757. Although thecombination of the closure angle and thefaster speed of the Westwind reduced sepa-

SECTION 2ration distance from about 3.8 nautical milesto about 2.1 nautical miles in 46 seconds, theprimary factor in the decreased separationwas the converging ground tracks. Theonly way the pilot of the Westwind couldhave maintained adequate separation wasto execute significant maneuvers.Based on radar data, at the time the visualapproach clearance was issued, the separation distance was rapidly approaching the 3nautical miles required for IFR separation. Toprevent compromise of the separation require-ment, the controller would have had to takepositive action to change the Westwind’s track,or to issue the visual approach clearance andreceive confirmation that the pilot acceptedthe visual approach within 29 seconds.These cases are extreme wake-turbulence encounters. In all cases, it was possible to avoidthe encounters if the pilots and air trafficcontrollers had sufficient knowledge of waketurbulence and applied proper avoidance procedures and techniques. Hopefully, this training aid will help prevent similar occurrences.2.7

SECTION 22.42.4.1Description/Characteristics of theWake-Turbulence HazardWake-Turbulence FormationThe phenomenon that creates wake turbulence results from the forces that lift the aircraft. High pressure air from the lower surfaceof the wings flows around the wingtips to thelower pressure region above the wings. Apair of counter-rotating vortices are thus shedfrom the wings, the right wing vortex rotatescounterclockwise, and the left wing vortexrotates clockwise as shown in Figure 2.4-1.This region of rotating air behind the aircraftis where wake turbulence occurs. The strengthof the turbulence is predominantly determinedby the weight, wingspan and speed of theaircraft.Figure 2.4-1Wake-turbulenceformationThe wake turbulence associated with helicopters also results from high pressure air on thelower surface of the rotor blades flowingaround the tips to the lower pressure regionabove the rotor blades. A hovering helicoptergenerates downwash from its main rotor(s) asFigure 2.4-1AFormation ofhelicopter waketurbulence (hover)Figure 2.4-1BFormation ofhelicopter waketurbulence(forward flight)2.8shown in Figure 2.4-1A. In forward flight apair of downward spiraling vortices are shedfrom the rotor blades, as shown in Figure 2.41B. This region of rotating air below thehelicopter is where wake turbulence occurs.

SECTION 21) The vortex strength depended on the size,weight, and speed of the aircraft;2) The pair of vortices generally descendedafter generation and would separate whenthey approached the ground;3) The vortex motion was substantially affected by the ambient wind.The lack of field testing prior to 1970, especially of vortices near the ground, precludedan in-depth understanding of vortex behavior, and in particular of the decay process.Now, two decades later, the industry recognizes that there are more factors associatedwith wake turbulence.This section briefly summarizes the currentknowledge of

Turbulence Training Program; Section 4, Wake Turbulence Training Aid - Background Data, and a video. 2.0.1 Preview This Pilot and Air Traffic Controller Guide to Wake Turbulence is a comprehensive docu-ment covering all the factors leading to a shared awareness and understanding of wake turb

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