| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 6 April 2021, at about 0733 Western Standard Time,[1] an Airbus A320 aircraft began its take-off roll on runway 06 at Perth Airport, Western Australia. About 60 seconds later, the A320 was airborne and climbing out ahead, as a Fairchild Industries SA227 aircraft, with two crew and 15 passengers on board, was cleared for take-off from the same runway. The crew reported that the atmospheric conditions at the time were ‘very stable’, with clear skies and a light and variable wind.
As the SA227 climbed through 1,500 ft, the crew experienced an uncommanded ‘strong roll to the left’ that reached approximately 45° angle of bank and required full opposite control input to recover from. A lull then briefly occurred followed by another uncommanded roll, this time to the right, reaching an angle of bank of 35° and again requiring full control input from the crew to recover. The crew reported that the airspeed remained constant at about 150 knots, and the rate of climb was unaffected throughout the occurrence. Following the recovery from the second uncommanded roll, the flight continued to its destination without further incident. No passengers or crew were injured in the incident, and the aircraft did not sustain any damage.
The aircraft operator assessed that the uncommanded roll experienced by the SA227 crew was the result of wake turbulence[2] (Figure 1) generated by the A320 that had departed ahead. The very stable atmospheric conditions at the time of the incident contributed to the wingtip vortices generated by the A320 remaining in the SA227’s flight path longer than would otherwise have been anticipated
Figure 1: Wake turbulence generation
How an aircraft generates wake turbulence in flight, with associated wingtip vortices
Source: FAA (Advisory Circular: Aircraft Wake Turbulence, AC No: 90-23G)
Wake turbulence
A recent ATSB study, Analysis of wake turbulence occurrences at Sydney Airport 2021-2016, a Bayesian analysis, explains that
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.
This separation is based on an aircraft’s maximum certified take-off weight, which is used to place it into a Light, Medium, Heavy, or Super wake turbulence category (Figure 2).
Figure 2: Division of aircraft categories based on maximum take-off weight
Source: Airservices Australia (Aeronautical Information Publications)
ATC applies the required distance and time separation requirements between aircraft based on which wake turbulence category each aircraft belongs to (Figure 3).
Figure 3: Table of required wake turbulence separation between categories
Source: ATSB (Analysis of Wake Turbulence Occurrences at Sydney Airport 2012-2016)
The SA227’s maximum take-off weight was 7,484 kg and the A320’s was 77,000 kg. Due to the broad range of weights included, both the SA227 and the A320 fall into the Medium wake turbulence category, and as a result ATC is not required to apply wake turbulence separation between them. Despite this, there is a significant difference in size and weight between these two aircraft types, and as such there exists the possibility of a wake turbulence encounter such as that experienced by the SA227 crew.
The stable atmospheric conditions on the day also contributed to this occurrence. Wingtip vortices generated by an aircraft are subject to local winds which can displace them from the position in which they are generated (Figure 4). The stable atmospheric conditions reported by the SA227 crew allowed the wingtip vortices to remain stationary behind the A320, and in the SA227’s projected flightpath, longer than would otherwise have been expected.
Figure 4: Effect of wind of wingtip vortices
Source: FAA (Advisory Circular: Aircraft Wake Turbulence, AC No: 90-23G)
Safety action
The operator has advised the ATSB that a reminder of the hazards of wake turbulence will be included in a monthly safety newsletter and distributed to all flight crew.
Safety message
This incident serves as a reminder to flight crew of the hazards associated with wake turbulence, and the limitations of existing wake turbulence separation standards under some circumstances.
ATSB research has found that ‘wake turbulence separation standards will not completely eliminate the hazards associated with wake turbulence vortices’.[3] Therefore, pilots are reminded that existing wake turbulence separation standards may not provide adequate protection against wake turbulence encounters in circumstances of a large size differential between two aircraft within the same wake turbulence category.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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