EASA SIB No.: 2017-XX
Safety Information Bulletin
Operations
SIB No.: 2017-XX
Issued: dd March 2017
Subject: En-route Wake Turbulence Encounters
Ref. Publications:
ICAO PANS-ATM-Doc 4444.
ICAO Doc 8643 Aircraft Type Designators and Doc 9426 Air Traffic Services Planning Manual.
Commission Regulations (EU) No 965/2012 and (EU) No 923/2012.
Report ‘An Improved Understanding of En-route wake vortex encounters’, by EUROCONTROL and
TU Delft.
Applicability:
National Aviation Authorities (NAA), Air Traffic Services (ATS) providers, operators, pilots and air
traffic controllers.
Description:
With the increase of the overall volume of air traffic and enhanced navigation precision, wake
turbulence encounters in the en-route phase of flight above 10 000 feet (ft) mean sea level (MSL)
have progressively become more frequent in the last few years.
The aim of this SIB is to enhance the awareness of pilots and air traffic controllers of the risks
associated with wake turbulence encounters in the en-route phase of flight and provide
recommendations with the purpose of mitigating the associated risks.
Every flying aircraft generates turbulence in its wake. For a fixed-wing aeroplane, this wake
turbulence rolls-up into a pair of coherent, counter-rotating vortices that can persist for some
minutes in the vicinity of the generating aeroplane flight path, moving naturally downward and
laterally with the wind. This poses a potential hazard to the safe flight of another aeroplane
crossing below the trajectory of the generating aeroplane, and encountering these vortices. The
trailing vortices’ time to dissipate depends upon the weight, size and speed of the aeroplane, as
well as prevailing atmospheric conditions.
Wake turbulence encounters can occur during any phase of flight. Separation minima aim to
prevent such encounters from inducing hazardous situation, but it must be noted that these
provisions will not completely prevent wake encounters from occurring.
This is information only. Recommendations are not mandatory.
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The basic effects of wake turbulence encounter on a following aeroplane are induced roll, vertical
acceleration (can be negative) and loss or gain of altitude. The greatest danger is an induced roll
that can lead to a loss of control and possible injuries to cabin crew and passengers. The vortices
are also most hazardous to following aircraft during the take-off, initial climb, final approach and
landing.
However, en-route, the vortices evolves in altitudes at which the rate of decay leads to a typical
persistence of 2-3 minutes, with a sink rate of 2-3 metres per second. Wakes will also be
transported by wind.
Considering the high operating air speeds in cruise, wake can be encountered up to 25 nautical
miles (NM) behind the generating aeroplane, with the most significant encounters reported within
a distance of 15 NM. This is larger than in approach or departure phases of flight.
The encounters are mostly reported by pilots as sudden and unexpected events. The awareness of
hazardous traffic configuration and risk factors is therefore of particular importance to anticipate,
avoid and manage possible wake encounters.
In the en-route phase of flight, three major factors contribute to increase the likelihood of wake
turbulence:
1. Weight of the generating aeroplane: Heavier aeroplane types generate stronger wake vortexes
and are likely to induce more severe wake turbulence encounters, especially for smaller
aeroplane types.
2. Crossing traffic situation: In the case that crossing traffic is climbing or descending in proximity
(either the generating or following aeroplane), the wake generated might cross the follower’s
trajectory with minimum time for decay, so stronger wake turbulence might be encountered.
3. Thermal tropopause altitude: Wake vortex decays more slowly below the tropopause where
there is therefore an increased risk of encountering severe wake turbulence.
The typical hazardous trajectory crossing configurations are the following (shown hereafter in a
vertical plane, although the respective flight path might also be crossing on the horizontal plane):
This is information only. Recommendations are not mandatory.
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a) Follower climbing
i. Generator climbing ahead
ii. Generator flying level
iii. Generator descending ahead
This is information only. Recommendations are not mandatory.
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b) Follower flying level
i. Generator climbing ahead
ii. Generator flying level - crossing above level in opposite direction
iii. Generator descending ahead
This is information only. Recommendations are not mandatory.
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c) Follower descending
i. Generator climbing ahead
ii. Generator flying level
iii. Generator descending ahead
This is information only. Recommendations are not mandatory.
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Note: The variability in the generator aeroplane rate of climb or descent makes it quite difficult to
estimate exactly where the vortex is.
Consequently, during the en-route phase of flight, pilots should expect possible wake encounters
when other traffics in proximity appear to be on similar tracks ahead, crossing above your level,
climbing or descending ahead through your flight path, while wind direction is likely to move the
wake towards your trajectory.
At this time, the safety concern described in this SIB does not warrant the issuance of an
operational directive under Regulation (EU) 965/2012, Annex II, ARO.GEN.135(c).
Recommendation(s):
As precautionary measures, operators and pilots should be aware that:
As foreseen in Reg. 965/2012 AMC1 to CAT.OP.MPA.170, the announcement to passengers
should include an invitation to keep their seat belts fastened, even when the seat belt sign is
off, unless moving around the cabin. This minimises the risk of passenger injury in case of a
turbulence encounter en-route (wake or atmospheric).
As indicated in ICAO PANS-ATM, for aeroplanes in the heavy wake turbulence category or for
Airbus A380-800, the word “HEAVY” or “SUPER”, respectively, shall be included immediately
after the aeroplane call sign in the initial radiotelephony contact between such aeroplanes and
ATS units.
When possible, contrails should be used to visualise wakes and estimate if their flight path
brings them across or in close proximity.
When flying below the tropopause altitude, the likelihood of wake encounter increases. The
tropopause altitude varies (between days, between locations).
Upwind lateral offset should be used if the risk of a wake encounter is suspected.
Timely selecting seat belt signs to ‘ON’ and instruct cabin crew to secure themselves constitute
precautionary measures in case of likely wake encounters.
In case of a wake encounter, pilots should:
Be aware that it has been demonstrated during flight tests that if the pilot reacts at the first
roll motion, when in the core of the vortex, the roll motion could be amplified by this initial
piloting action. The result can be a final bank angle greater than if the pilot would not have
moved the controls.
Be aware that in-flight incidents have demonstrated that pilot inputs may exacerbate the
unusual attitude condition with rapid roll control reversals carried out in an “out of phase”
manner.
Be aware that if the autopilot is engaged, intentional disconnection can complicate the
scenario, and the autopilot will facilitate the recovery.
Avoid large rudder deflections that can create important lateral accelerations, which could
then generate very large forces on the vertical stabiliser that may exceed the structural
resistance. Although some recent aircraft types are protected by fly-by-wire systems, use of
the rudder does not reduce the severity of the encounter nor does it improve the ease of
recovery.
Make use of specific guidance available through AOM for their specific type(s)/fleet.
This is information only. Recommendations are not mandatory.
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ATS providers and air traffic controllers should:
Enhance their awareness about en-route wake turbulence risk, key factors and possible
mitigations, based on the information provided in this document and other relevant material.
This could be achieved through flyers, e-learning, and refresher training module.
Possible risk mitigations may consist of:
o Make use of the wake turbulence category (WTC) indication in the surveillance label
and/or the flight progress strip (whether electronic or paper), and observe closely
separated aeroplanes that are at the opposite extremes of the WTC spectrum;
o As the best practice, provide traffic information, advising “CAUTION WAKE TURBULENCE”,
when you identify that a ‘HEAVY’ or ‘SUPER HEAVY’ wake category traffic is climbing or
descending within 15 NM of another following traffic;
o Manage en-route traffic crossings such as , when possible while preserving safe tactical
management of overall traffic in the sector, avoiding to instruct climb or descend to
‘HEAVY’ or ‘SUPER HEAVY’ traffic within 15 NM distance from another following traffic;
o If at all possible, avoid vectoring an aeroplane (particularly if it is LIGHT or MEDIUM
category) through the wake of a HEAVY or SUPER HEAVY aeroplane where wake
turbulence may exist.
Contact(s):
For further information contact the EASA Safety Information Section, Certification Directorate.
E-mail:
[email protected].
This is information only. Recommendations are not mandatory.
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Dear Madam/Sir,
Please find attached hereto a preliminary draft for an EASA Safety Information Bulletin (SIB) [task
2017.077] regarding
En-route Wake Turbulence Encounters
for consultation until 06 April 2017.
We would be grateful to you if you could send any comment(s) to
[email protected] by making
reference to [task 2017.077], no later than the above-mentioned deadline.
Safety Information Section Email:
[email protected]
EASA Website
(Attachment is in Acrobat PDF format)
Postal: Postfach 10 12 53, 50452 Cologne, Germany
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