INTERNATIONAL FEDERATION OF AIR TRAFFIC CONTROLLERS’ ASSOCIATIONS 57TH ANNUAL CONFERENCE – Accra, Ghana March 19-23, 2018 Agenda Item: B.5.5 IFATCA 18 WP No. 89 En-route Wake Turbulence Presented by TOC Summary All the prescriptions about wake turbulence separation are established only for approach/ or departure phases. En-route separation (time or distance based), is not established to prevent wake vortex encounters at all, but only to reduce the possibility of them. The amount of wake turbulence encounter reports while en-route have been consistent, however a recent accident raised the general attention towards the phenomena. At the same time, the reduction of the en- route separation, due to the re-categorisation of aircraft are under preliminary study as a possible way to increase the airspace capacity in the medium-term period. 1. Introduction 1.1. Wake turbulence is generated by wake vortices that are present behind every aircraft, but is particularly severe when generated by large and wide-bodied aircraft. 1.2. Studies about wake turbulence started in the late 1960’s when wide bodied turbo-jet aircraft were introduced. Since then, the phenomenon was known but never considered as a serious hazard for the en-route phase of flight. 1.3. ICAO only prescribes separation minima (time or distance based) for wake vortex turbulence (WVT) for departing and arriving aircraft1. The prescribed en-route separation minima is the same for all aircraft types and do not necessarily prevent Wake Vortex Turbulence encounter generated from other aircraft operating in the vicinity. 1.4. En-route wake propagation is almost unpredictable. It is not only related to the weight and wingspan of the generating aircraft, but also to wind, atmospheric conditions as well as, to the aircraft configuration. Additionally, the effect on the trailing aircraft depends also on its weight, wingspan and resistance. 1.5. Wide bodies represent 24% of the worldwide commercial aircraft fleet (cargo and passenger) and the number is foreseen to increase. Precision of navigation makes aircraft fly with greater accuracy both on the vertical and the horizontal path, which potentially increases WVT encounters. 1 ICAO. (November 2016). Procedures for Air Navigation Services – ATM (Doc 4444), 16th Edition, Chapter.5, 5.8 and Chapter 8, 8.7.3.4 B.5.5 / Page 1 of 10 1.6. In history, there are numerous reports of en-route wake turbulence encounters. Some of them led to the loss of control of the aircraft, with very difficult recovery of the flight conditions. 1.7. The European Aviation Safety Agency (EASA), in June 2017, published a Safety Information Bulletin (SIB) on En-Route Wake Turbulence2. Recommendations are addressed to Operators, Pilots and ATS Providers. 2. Discussion 2.1. History 2.1.1 Boeing together with the Federal Aviation Administration (FAA) in the ‘70s conducted the first studies using smoke generating towers to observe the wake turbulence of aircraft flying by. It was noticed that: • The strength of the wake turbulence is governed by the weight, speed and wingspan of the generating aircraft; • The greatest strength occurs when the generating aircraft is heavy, at slow speed with clean wing configuration. 2.1.2 According to the result of this study, aircraft were grouped according to their maximum take-off weight. It was noted that a classification based on the wingspan of the following aircraft was more technically correct to establish categories but it did not appear to be an easily workable method. 2.1.3 Since there’s a correlation between aircraft gross weight and wingspan, the gross weight was selected as a means of categorizing aircraft and wake turbulence strength. Minimum separation values were established for the following aircraft depending on the weight of both the leading and trailing aircraft. 2.1.4 Adjustments of the separation values were made through the ‘1980’s and ‘1990’s but the basic concept of using the weight remained constant. 2.2. Definitions 2.2.1 Wake Vortex Turbulence (WVT) is defined as turbulence, which is generated by the passage of an aircraft in flight. 2.2.2 Wake Vortex Turbulence will be generated from the point when the nose gear of an aircraft leaves the ground on take-off and will cease to be generated when the nose gear touches the ground during landing. 2.2.3 When another aircraft encounters such turbulence generated by the leading aircraft, a Wake Vortex Encounter (WVE) is said to have occurred. 2.2.4 ICAO, in PANS-ATM Doc 4444 in chapter 4 paragraph 4.9 “Wake Turbulence Categories”, defines: “Wake Turbulence is the term used to describe the effect of rotating air masses generated behind the wing tips of large jet aircraft. Wake Vortex is the term that describe the nature of the air masses.” 2 EASA SIB no. 2017-10 issued on 22 June 2017 B.5.5 / Page 2 of 10 2.3 Causes and effects 2.3.1 The factors contributing to the wakes are: • Leading aircraft weight - Heavy category types, in particular with MTOW (Maximum Takeoff Weight) above 350 tonnes (incl. A340-500/600, A380-800, B747-400/800, B777-300ER) induce the strongest wake turbulence vortices; • Relative size of leading and following aircraft; • Relative track and position of proximate aircraft- the risk is greater when aircraft are in the same direction of flight and are climbing or descending behind a heavy aircraft or when an aircraft encounters a heavy aircraft climbing or descending ahead of it. • Flying below the tropopause3 - the atmospheric conditions are generally favourable for the wake vortex to remain strong for a longer period of time, and the wake vortices may potentially descend one flight level lower; • Wind velocity relative to the track being flown by the generating aircraft - cross- track wind reduces the risk to in-trail aircraft. 2.3.2 The main effects on the trailing aircraft are induced roll, loss of altitude or reduced rate of climb and possible structural stress. The impact of a WVE is stronger during a turn due to the fact that the load factor is already higher during turns. 2.3.3 In Terminal Airspace (TMA) operations, flight crews are more focused and ready to quickly react to a potential WVE, since there is more likeliness that hazardous events may occur. On the other hand, the response time in en-route may be delayed, since the crew may not expect the event and might be completely relying on the autopilot in the specific moment of a potential interaction with the WVE. 2.4 Project R-WAKE4 2.4.1 The project is part of the “SESAR-07-2015 - Separation Management and Separation Standards” package5 that has in general the target to reduce separation minima to allow an increase of airspace capacity: one of the global key objectives of SESAR. 2.4.2 The project develops a simulation framework to assess the risk and hazards of potential wake vortex encounters for the en-route phase of flight. 2.4.3 The goal of this research consists on a proposal of potential enhancements in the current separation standards to protect flights against WVE. Both pilots and controllers are involved in the expert group and the project will be concluded in March 2018. 2.5 Mitigation of Wake Vortex Encounter 2.5.1 The encounter of Wake Vortex can lead to unexpected roll and loss of control. The prescribed separation applied within controlled airspace by ATC does not necessarily prevent WVE from the preceding aircraft, but possibly reduces the risk of encounters. 2.5.2 The only direct defence for pilots is to keep high situational awareness monitoring the traffic in the vicinity and to minimize the effects of WVE, and when necessary recommend passengers to keep their seatbelt fasten when seated. 3 The tropopause occurs between approximately FL300 and FL600. 4 www.rwake-sesar2020.eu 5 Founds by SESAR-JU under the European Union’s Horizon 2020 research and innovation programme. B.5.5 / Page 3 of 10 2.5.3 When an en-route air traffic controller identifies a traffic situation with risk of a potential wake encounter, traffic information to the trailing aircraft may be provided. This procedure shouldn’t be expected by pilots: since it’s not mandatory, it is subject to ATCOs workload or personal judgement of the hazard. 2.5.4 ICAO Phraseology 2.5.4.1 ICAO only provides standard phraseology for wake turbulence warning for aerodrome and approach control. Procedure for Air Navigation Services – ATM (Doc 4444), 16th Edition, Chapter 12. 12.3.3.2. Approach instructions ... in case of successive visual approaches when the pilot of a succeeding aircraft has reported having the preceding aircraft in sight: q) CLEARED VISUAL APPROACH RUNWAY (number), MAINTAIN OWN SEPARATION FROM PRECEDING (aircraft type and wake turbulence category as appropriate) [CAUTION WAKE TURBULENCE]; 12.3.4.19 Information to aircraft …wake turbulence: e) CAUTION WAKE TURBULENCE [FROM ARRIVING (or DEPARTING) (type of aircraft)] [additional information as required]; f) CAUTION JETBLAST; g) CAUTION SLIPSTREAM; 2.5.4.2 ICAO Procedure for Air Navigation Services – ATM (Doc 4444), 16th Edition, Chapter 4, 4.9.2. “Indication of Heavy turbulence category” prescribes: For aircraft in the heavy wake turbulence category the word “Heavy” shall be included immediately after the aircraft call sign in the initial radiotelephony contact between such aircraft and ATS units. Note. – Wake turbulence categories are specified in the instructions for completing Item 9 of the flight plan in Appendix 2. 2.5.5 Strategic Lateral Offset Procedure (SLOP) and Free Route Airspace (FRA) 2.5.5.1 Route or track centrelines are now routinely flown over long distances to within a few tens of metres of lateral and vertical accuracy, and often much better than that, therefore a clearance error from any source has a reduced margin error attributable to that accuracy.