Analysis of Wake Turbulence Occurrences at Sydney Airport 2012–2016

What is wake turbulence

For fixed-wing aircraft, wake turbulence is the combined effect of jet blast or propeller wash with wake vortices. Wake vortices are the primary contributor to wake turbulence. The initial strength of the vortices is primarily dependent upon the generating aircraft’s speed, weight and wingspan. These vortices decay with time and largely become non‑hazardous—depending on atmospheric conditions—within several minutes.

The wake vortices can affect following aircraft in a similar way to flying through weather-related turbulence. More specifically, aircraft encountering wake turbulence may experience an induced roll, which can increase safety risk, especially during phases of flight close to ground such as arrivals and departures. The risk of an injury resulting from a wake turbulence encounter is higher for cabin crew than passengers, who are generally secured in their seats earlier during arrivals.

To minimise the risk to safety associated with wake turbulence, air traffic control separates aircraft arriving or departing from an airport using wake turbulence separation standards. These are time and/or distance‑based measures that limit the separation of leading and following aircraft and are designed to reduce the likelihood and severity of wake turbulence occurrences. These standards are not applicable to parallel runways which are separated by more than 760 m. As a result, they do not apply during parallel runway operations at Sydney Kingsford Smith International Airport (Sydney Airport), where the parallel runways are separated by around 1,000 m.

The analyses in this investigation only includes encounters with wake turbulence, primarily reported by aircrew, that have been classified as an occurrence.[1] Generally, they were reported to have resulted in a missed approach or go‑around, control issues for the aircraft, a warning activation, and/or an injury.

Why the ATSB conducted this research

In 2016, through routine monitoring of safety occurrence data, the ATSB identified a potential safety issue regarding the high proportion of wake turbulence occurrences at Sydney Airport.

To identify the contributing factors to this higher rate of occurrences at Sydney, and the level of safety risk of wake turbulence occurrences at the airport, the ATSB initiated an Aviation Research investigation under the Transport Safety Investigation Act 2003.

What the ATSB found

In Australia, between 2012 and 2016, 179 wake turbulence occurrences were reported to the ATSB, with 78 of these occurring at Sydney Airport. In addition to its high proportion of wake turbulence occurrences, seven of the eight minor injuries reported as being a result of a wake turbulence occurrences were at Sydney. Further, when compared with the combined data from other major Australian airports (Adelaide, Brisbane, Melbourne and Perth), an aircraft was more likely to have a missed approach or go‑around, a ground proximity alert, or have control issues following a wake turbulence encounter at Sydney Airport.

Sydney Airport is the only major Australian airport currently with parallel runways. The distance between these runways is such that they are treated as individual runways and do not require the application of the wake turbulence separation standard for aircraft operating to a single runway.

The investigation found that at Sydney Airport, when the time between arriving aircraft (including those operating on parallel runways) is less than one per minute, the likelihood of encountering wake turbulence increased substantially, with Runway 34 Right (the shorter of the parallel runways) the most likely to be affected.

Despite the distance between the parallel runways at Sydney Airport exceeding the regulatory standard (for aircraft separation to treat the parallel runways the same as a single runway), evidence indicates that wake turbulence generated by aircraft arriving on one runway can affect aircraft arriving on the parallel runway, especially under certain wind conditions. Aircraft arriving on Runway 34 Left were found to be the most likely cause for more than half of the Runway 34 Right arrival wake turbulence occurrences. A leading Airbus A380 (a super heavy aircraft) probably generated more than one‑third of these occurrences.

The rate of reported wake turbulence occurrences by arriving aircraft following an Airbus A380 was more than double that of any other aircraft type arriving at Sydney. All A380 wake turbulence occurrences took place outside peak arrival periods (one or more aircraft arrivals per minute). Medium weight aircraft, such as a Bombardier DHC-8 or Boeing 737, were more likely to report an encounter with wake turbulence than larger aircraft. No light aircraft reported encountering wake turbulence at the airport.

There were no reported wake turbulence occurrences at Sydney Airport between 2012 and 2016 that occurred during a reported loss of separation (breach of the wake turbulence separation standard). In contrast to wake turbulence occurrences, the rate of other turbulence occurrences at Sydney Airport is consistent with other major Australian airports.

The investigation concluded that there was a disproportionate rate and level of consequence of reported wake turbulence occurrences for aircraft arriving at Sydney Airport compared to other major Australian airports in the years 2012 to 2016. Given the parallel runway configuration, wake turbulence occurrences at Sydney Airport were found to be associated with:

  • arrival densities of one or more aircraft per minute (including parallel runway arrivals), especially on flights that arrived on Runway 34 Right
  • wind direction from the west or north‑west for aircraft arriving on Runway 34 Right, especially when coinciding with a heavy or super heavy aircraft arriving on Runway 34 Left
  • arrivals following an Airbus A380 compared to other aircraft.

More than half of the wake turbulence occurrences during arrival at Sydney Airport were associated with one or more of the above three factors. Removing all of these factors would halve the occurrence rate and make it more comparable to other major airports, however, the rate at Sydney Airport would likely still be higher than other major Australian airports. This suggests other factors beyond the scope of this investigation are also influencing wake turbulence at Sydney.

What's been done as a result

Airservices Australia will publish an Aeronautical Information Circular (AIC) aimed at operators who operate into Sydney Airport. The AIC will advise industry of the injuries associated with wake turbulence for Runway 34 Right as identified in the ATSB Report. The AIC will also recommend that cabin crew should be seated and secured in the earlier part of the approach.

However, the ATSB did not consider that the proposed safety action would adequately reduce the risk associated with the safety issue. As such, the ATSB has issued a safety recommendation that Airservices introduce measures to reduce the frequency of wake turbulence occurrence at Sydney Airport.

Airservices has since informed the ATSB that the following measures will also be implemented:

  • provide wake turbulence caution to aircraft on approach to 34R that will operate within the wake turbulence distance of a Heavy or Super heavy aircraft making an approach to runway 34L
  • increasing separation distances for arrivals from 4 NM to 5 NM on runways 16L/R and 34L/R
  • applying the single-runway wake turbulence standard to the parallel runways when the leading aircraft is a super heavy like an A380 and the following aircraft is light (under 25,000 kg).

The ATSB agrees that these have the potential to reduce the wake turbulence risk but encourages Airservices to consider conducting their own quantitative analyses to explore other options that could further reduce the risk of wake turbulence for aircraft arriving into Sydney Airport.

Safety message

When departing or arriving at Sydney Airport, aircrews need be alert to the increased likelihood of encountering wake turbulence especially during periods of high movement density or during parallel runway operations, when operating on Runway 34 Right with wind coming from the west or north-west, and/or following an Airbus A380.

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  1. Occurrences (accidents, serious incidents or incidents) are defined, as a minimum, as an event associated with the operation of an aircraft which affects or could affect the safety of operation (International Civil Aviation Organization, Annex 13, Aircraft accident and incident investigation), and meets the definition of a ‘Transport Safety Matter’ (Transport Safety Investigations Act 2003, Section 23).

 

Safety issue

AR-2017-011-SI-01 -  

Wake turbulence occurrences at Sydney Airport

Given the parallel runway configuration, there was a disproportionate rate of reported wake turbulence occurrences for aircraft arriving at Sydney Airport compared to other major Australian airports in the years 2012 to 2016. Wake turbulence occurrences at Sydney Airport were found to be primarily associated with three factors:

  • arrival densities of one or more aircraft per minute (including parallel runway arrivals), especially on flights that arrived on Runway 34 Right
  • wind direction from the west or north‑west for aircraft arriving on Runway 34 Right, especially when coinciding with a heavy or super heavy aircraft arriving on Runway 34 Left
  • arrivals following an Airbus A380 compared to other aircraft.
Safety issue details
Issue number: AR-2017-011-SI-01
Who it affects: Aircraft arriving at Sydney Airport
Status: Safety action pending
Type: Research and Analysis Report
Investigation number: AR-2017-011
Publication date: 15 February 2019
Last update 21 February 2019
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