En-route Wake Turbulence

En-route Wake Turbulence

57TH ANNUAL CONFERENCE, Accra, Ghana, 19-23 March 2018

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 enroute 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.

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 aircraft (ICAO. (November 2016). Procedures for Air Navigation Services – ATM (Doc 4444), 16th Edition, Chapter.5, 5.8 and Chapter 8, 8.7.3.4). 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.

1.6. 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 Turbulence (EASA SIB no. 2017-10 issued on 22 June 2017). Recommendations are addressed to Operators, Pilots and ATS Providers.

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.”

 


2.3 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 tropopause (The tropopause occurs between approximately FL300 and FL600) – 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 – crosstrack 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-WAKE

2.4.1 The project (www.rwake-sesar2020.eu) is part of the “SESAR-07-2015 – Separation Management and Separation Standards” package (Founds by SESAR-JU under the European Union’s Horizon 2020 research and innovation programme) 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.

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. This includes intentional variation in route to avoid the worst effects of wake vortex turbulence.

2.5.5.2 A provision in ICAO Annex 2 “Rules of the Air” (ICAO. (November 2005). Annex 2 – Rules of the Air, 10th Edition, Chapter 3, paragraph 3.6.2.1.1 a)) requires that aircraft operating controlled flights shall, when on an established ATS route, operate along the defined centre line unless SLOPs are authorised on that route by the appropriate ATS authority, or directed by the appropriate air traffic control unit.

2.5.5.3 SLOPs are mitigating the risk of collision and wake turbulence encounters between aircraft with high precision navigation capabilities. SLOPs are meant as special procedures for oceanic and remote continental airspace. The arrangements for the application of SLOPs are detailed in ICAO Doc 4444 PANS-ATM (§16.5).

2.5.5.4 The introduction of the free-route concept is reducing the effectiveness of SLOP and, if at the same time reducing the chances of WVE, makes predicting the path of other aircraft more difficult to pilots.

2.5.5.5 IFATCA performed studies on SLOP presented at Annual Conferences in Arusha 2008 and Dubrovnik 2009. IFATCA provisional policy is:

ATS 3.16 (ADVANCED) STRATEGIC LATERAL OFFSET PROCEDURE

IFATCA endorses Strategic Lateral Offset Procedure (SLOP) in oceanic or remote continental airspace where there is no ATS surveillance service provided.

IFATCA only supports an advanced strategic offset concept provided that:

  • Studies conclude that the concept enhances safety;
  • The concept is globally harmonised;
  • The concept is taken into account in airspace and procedures design;
  • ATC surveillance systems accommodate the concept; and
  • The concept is transparent to ATC, requiring no controller intervention at all.

 

2.5.6 ICAO defined a minimum separation distance between successive arriving or departing fixed-wing aircraft according to their Maximum Take Off Weight (MTOW) classification.


2.6 Aircraft Categorization

2.6.1 The aircraft categorization presently adopted is the one described in ICAO PANS-ATM Doc 4444 chapter 4 paragraph 4.9.1 “Wake Turbulence Categories of Aircraft”:

4.9.1.1 Wake turbulence separation minima shall be based on a grouping of aircraft types into three categories according to the maximum certificated take-off mass as follows:

a) HEAVY (H) – all aircraft types of 136 000 kg or more;

b) MEDIUM (M) – all aircraft types less than 136 000 kg but more than 7 000 kg; and

c) LIGHT (L) – aircraft types of 7 000 kg or less.

4.9.1.2 Helicopters should be kept well clear of light aircraft when hovering or while air taxiing.

Note 1. – Helicopters produce vortices when in flight and there is some evidence that, per kilogram of gross mass, their vortices are more intense than those of fixedwing aircraft.

Note 2. – The provision governing wake turbulence separation minima are set forth in Chapter 5, Section 5.8, and Chapter 8, Section 8.7.3.

 

2.6.1.1 The United Kingdom, already in 1982, introduced some modifications to the weight and separation relationship to improve the airport capacity. Weight threshold between Light and Medium were modified and new aircraft groups were introduced. Because the safe operations through the years, in 2010, it was decided to extend the new classification to all the airports in the UK. Other Countries adopted similar solutions, preceding in time all the recent RECAT studies.

2.6.2 Airbus A380-800 Aircraft Specifications

2.6.3 Following the Airbus A380 introduction, ICAO published a state letter (and successive updates)(November 2005: T13/3-05-0661.SLG, October 2006: TEC/OPS/SEP/T-11/72-06-320.SLG and July 2008 TEC/OPS/SEP/08-0294.SLG) on the topic: “Wake Turbulence aspects of A380-800 aircraft”. An ad hoc group of experts under the auspice of the US-FAA, Eurocontrol, JAA and Airbus, studied the wake vortex of this new aircraft. The State Letter recommends the implementation of its guidance, pending an amendment to the PANS-ATM.

2.6.3.1 The aircraft is in the heavy wake turbulence category and the PANS-ATM Doc 4444 applies.

2.6.3.2 For A380-800 aircraft the letter “J” should be entered into the space allocated to wake turbulence under the Item 9 of the ICAO flight plan.

2.6.3.3 For A380-800 aircraft the expression “SUPER” should be included immediately after the aircraft call sign in the initial radiotelephony contact between such aircraft and ATS units.

2.6.3.4 Specific time or distance-based separations are recommended when the A380-800 is preceding any other aircraft in approach or departure phases.

2.6.3.5 En-route separation minima remain as prescribed in the PANS-ATM Doc 4444 5-5.8 and 8-8.7.3.


2.7 Wake Turbulence separation reduction

2.7.1 Projects as “Aircraft Re-categorization – Wake-RECAT” (a collaborative research between EUROCONTROL and US Federal Aviation Authority) and “Time-Based Separation (TBS)” were developed through the years with the aim of optimizing the use of runways.

2.7.2 Both studies have proven that, in addition to weight, and aircraft characteristics (such as speed and wingspan), that atmospheric conditions, also effect the strength of the wake generated; as well as the following aircraft’s reaction to that wake.

2.7.3 These projects were extensively analysed in the past by IFATCA (Resolution B2 – WP90, Bali 2013 and resolution B10 – WP92, Toronto 2017).

The Federation’s policy is:

ATS 3.36 RECATEGORIZATION OF AIRCRAFT FOR WAKE TURBULENCE

Any aircraft wake turbulence recategorisation, be it distance-based or time- based, for the purpose of increasing runway capacity, must:

– Conduct appropriate safety assessments, including a thorough understanding of the human factor element.

– Design clear procedures for the application of the new wake turbulence categories.

– Provide adequate tools to support the controller when applying those procedures.

– Incorporate contingency procedures for cases where support tools are unavailable and/or the new wake turbulence categories cannot be applied.

– Ensure that the new system does not have a negative effect on the efficiency of the overall ATM system

 

2.7.4 The current policy is the result of the review study of previous policy performed by TOC for IFATCA Annual Conference in 2017 and it focused on runway and approach operations. The reduction of the en-route longitudinal separation could be a way to increase the airspace capacity and an aircraft recategorisation project for wake turbulence en-route would be a first step in this process. A review of the current policy would extend its validity to all operations.


2.8 En-route Wake Vortex Encounter reports

Wake turbulence occurences in the en-route phase are 5% of the total generated reports by the phenomenon (whilst 70% happen during the approach, 13% during takeoff and 12% while manoeuvring in other phases)(source: Airbus).

While en-route, passengers and cabin crew are most likely not secured. An unexpected WVE with a consequential un-commanded roll and loss of control could cause severe injuries in the cabin. 2.8.1.1 On Jan 7, 2017, the wake turbulence caused by an A380-800 sent a business jet (Canadair Challenger 604) that was flying 1000ft below on opposite direction, in an uncontrolled descent.

The Bundestelle für Flugunfalluntersuchung (German Federal Bureau of Aircraft Accident Investigation – BFU) on its interim report released in May 2017, reported the crew first observed the aircraft above them in opposite direction on their TCAS, the captain subsequently identified an A380 and the airline. The A380 passed them slightly to the left and above. A short time later the aircraft was exposed to wake turbulence, the aircraft rolled to the left uncontrollably and the autopilot disconnected. Both pilots applied right aileron, however the aircraft continued to roll left and made several revolutions, both Inertial Reference Systems, the flight management system and the attitude indicators failed. Both pilots were wearing their lap belts and crotch belts; the first officer was also wearing his shoulder harness. The captain lost his head set; the quick reference manual lifted off in the cockpit and was distributed over the cockpit with single pages around the cockpit. Using external horizon reference the captain identified their attitude and was able to stabilize the aircraft again at FL240, 10.000 feet below their original altitude.

2.8.1.2 Encounters of turbulence generated by A380-800 aircraft are those more evident (because of the big weight difference compared to other categories) but there are several reports of wake turbulence generated from aircraft of the same category.


2.9 EASA Safety Information Bulletin 2017-10

2.9.1 After the WVE of Jan 7, 2017 EASA urged to publish a Safety Information Bulletin (SIB) about en-route wake turbulence encounters. The aim was to enhance the awareness of Aircraft Operators, pilots, ANSPs and air traffic controllers of the risks associated with wake turbulence encounters in the en-route phase of flight and provide recommendations and advisories with the purpose of mitigating the associated risks.

2.9.2 The Bulletin informs that en-route, the vortices evolve in altitudes at which the rate of decay leads to a typical persistence of 2-3 minutes, with a typical sink rate of about 400ft/min. Wakes will also be transported by wind.

2.9.3 The Bulletin highlights that considering the high operating air speeds in cruise and the standard 1000 feet vertical separation in RVSM airspace wake can be encountered up to 25 nautical miles (NM) behind the generating aeroplane. The most significant encounters are reported within a distance of 15 NM. However, no specific horizontal wake turbulence separation minima are detailed within PANS-ATM for en-route flight, with states using procedural or surveillance-based separation minima.

2.9.4 EASA identifies three major contributing factors: crossing traffic situation, thermal tropopause altitude and weight of the generating aircraft.

2.9.5 The SIB concludes with no procedures, but with recommendations as precautionary measures addressed to operators, pilots and ATS providers to mainly increase the knowledge and situational awareness about en-route wake turbulence.

Conclusions

3.1 Wake Vortex are strongest during approach and departure phases because of aircraft configuration, additionally, those are the phases where the highest amounts of accidents are recorded. Although en-route wake turbulence has been reported at higher distance than the prescribed separation minima applied by ATC, ICAO currently does not provide any provisions about it. In regards of WVE, all the separation standards are established to minimize the effects and not to prevent them.

3.2 Many countries independently adopted en-route wake turbulence separation criteria. Increased separation between heavier aircraft and others are often prescribed also according to factors like operating speed and altitude. In some cases, same approach/departure separation minima are applied regardless the phase of the flight.

3.3 ATC awareness of the persistence of wake turbulence at en-route altitudes, beyond the minimum separation, is most of the times insufficient. On the other hand, flight crews are in general less aware of the hazard at en-route altitudes resulting in slower in reaction if compared to TMA operations.

3.4 Because wake turbulence is invisible, and influenced by many variables, its presence and location cannot be determined or forecast with precision. Presently, reliable ground system support functions to inform and warn air traffic controllers of potentially hazardous wake encounters are not yet in operational use. Additionally, still today not all the ATC-system accepts the letter “J” as WT-identifier and display “H” for A388.

3.5 There are no specific rules regarding wake turbulence encounter prevention that ATCOs can apply for en-route separation. ATCOs are in this way indirectly called to apply their “Duty of Care” and this lead to personal action/reaction based on experience, workload, perception etc.

3.6 A re-categorisation of the aircraft for wake turbulence also for en-route operation is likely to happen as an action to increase airspace capacity enhancing separation. Ongoing projects, such as R-Wake, demonstrate the interest for international agencies on the topic.

Recommendations

It is recommended that:

4.1 IFATCA policy is:

When the prescribed separation is applied, ATCOs shall not be held responsible for wake vortex encounters and related accidents/incidents.

And it is included in the IFATCA Technical and Professional Manual.

4.2 IFATCA policy on page 3 2 3 41 of the IFATCA Technical and Professional Manual:

ATS 3.36 RECATEGORIZATION OF AIRCRAFT FOR WAKE TURBULENCE

Any aircraft wake turbulence recategorisation, be it distance-based or time- based, for the purpose of increasing runway capacity, must:

– Conduct appropriate safety assessments, including a thorough understanding of the human factor element.

– Design clear procedures for the application of the new wake turbulence categories.

– Provide adequate tools to support the controller when applying those procedures.

– Incorporate contingency procedures for cases where support tools are unavailable and/or the new wake turbulence categories cannot be applied.

– Ensure that the new system does not have a negative effect on the efficiency of the overall ATM system.

Be amended to read:

ATS 3.36 RECATEGORISATION OF AIRCRAFT FOR WAKE TURBULENCE

Any aircraft wake turbulence recategorisation, be it distance-based or time- based, for the purpose of increasing capacity, must:

– Conduct appropriate safety assessments, including a thorough understanding of the human factor element.

– Design clear procedures for the application of the new wake turbulence categories.

– Provide adequate tools to support the controller when applying those procedures.

– Incorporate contingency procedures for cases where support tools are unavailable and/or the new wake turbulence categories cannot be applied.

– Ensure that the new system does not have a negative effect on the efficiency of the overall ATM system.

References

ICAO (November 2016) Procedure for Air Navigation Services – ATM (Doc 4444), 16th Edition.

ICAO (November 2005) Annex 2 – Rules of the Air, 10th Edition.

ICAO (July 2008) ICAO State Letter. Guidance on A380-800 Wake Vortex Aspects.

IFATCA (2017) IFATCA Technical and Professional Manual (TPM). ATS 3.16 – (Advanced) Strategic Lateral Offset Separation, p 3 2 3 19.

Review of ATS 3.36 Recategorization of Aircraft for Wake Turbulence. B.5.12, WP 92. IFATCA Annual Conference 2017.

IFATCA (2017) IFATCA Technical and Professional Manual (TPM). ATS 3.36 – Recategorisation of Aircraft for Wake Turbulence, p 3 2 3 41.

EUROCONTROL. (2017) http://www.eurocontrol.eu

Airbus (June 2005) Flight Operations Briefing Notes – Wake Turbulence Awareness / Avoidance. FLT_OPS-OPS_ENV-SEQ 07-REV 01. 8 pages.

Hoogstraten, Visser, Hart, Trevé and Rooseleer. “An Improved Understanding of Enroute Wake Vortex Encounters”. 15 pages. Retrieved on December 2017 from http://www.skybrary.aero/bookshelf/books/2510.pdf

Rojo, Frei and Vitalle. “An Analysis of En-route Wake Turbulence Behaviour based on In-Flight Measurement”. 9 pages. Retrieved on January 2017 from https://www.atca.org/2017-technical-papers

Rossow, James. “Overview of wake vortex hazards during cruise”. Journal of Aircraft, vol. 37. (2000).

UK Civil Aviation Authority (2017). www.caa.uk.gov

EASA (2017) Safety Information Bulletin 2017-10. 7 pages.

Airbus (2017) www.airbus.com

Boeing (2017) Current Market Outlook 2017-2036. 63 pages.

BFU (2017) Interim Report BFU17-0024-2X. 24 pages. Retrieved in December 2017 from http://www.bfu-web.de

The Aviation Herald (2017) Wake Turbulence Reports. Retrieved in December 2017 from http://www.avherald.com

SKYbrary (2017) Wake Vortex Turbulence. Retrieved in December 2017 from http://www.skybrary.aero/index.php/Category:Wake_Vortex_Turbulence

SESAR http://www.sesarju.eu

R-WAKE (2017) The R-Wake project. Retrieved in December 2017 from http://www.rwake-sesar2020.eu

Last Update: October 1, 2020  

December 31, 2019   351   Jean-Francois Lepage    2018    

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