Turbulence/windshear/microburst

Boeing 737-476, VH-TJG

Safety Action

As a result of this occurrence, the following safety actions were initiated:

The operator of Canberra International Airport requested Airservices Australia to include a caution note in Canberra aerodrome information contained in the ERSA, as follows:

`During strong westerly winds down stream of buildings, severe turbulence may be experienced in the touch down area while landing Runway 35.'

The operator also requested Airservices Australia to issue a Notice to Airmen (NOTAM) to reflect that cautionary advice until the ERSA was amended, and to give consideration to making:

`…necessary caution announcements on the ATIS during similar strong wind conditions.'

Airservices Australia issued a local instruction to air traffic controllers at Canberra Tower. The instruction contained information that when the crosswind component (including gusts) from the west equals or exceeds 12 kts and runway 35 or 17 is nominated, the following shall be included on the ATIS:

`Expect turbulence over runway south of runway intersection.'

The instruction included information that controllers were to make a directed broadcast to aircraft operating on runway 35, or departing runway 17, when this crosswind condition exists and runway 35 or 17 were not nominated on the ATIS.

Airservices reported that it did not consider the issue of a NOTAM providing cautionary advice of turbulence was warranted, as directed broadcasts would provide pilots with information of the meteorological phenomenon.

Airservices Australia also reported that it would conduct a survey of the turbulence phenomenon until 1 July 2003 to determine the extent of the condition, including:

a. occasions when the westerly crosswind component, including gusts, equals or exceeds 12 kts,

b. pilot reports of turbulence / shear at touch down or take-off, including aircraft type,

c. pilot comments, and

d. pilot reports of turbulence when the crosswind is less than 12 kts.

Analysis

At the time of the occurrence, the wind direction and speed at Canberra aerodrome was 280 degrees T at 18 kts, with gusts exceeding 20 kts at times. The wind direction was therefore 80 degrees removed from the runway direction, providing a left crosswind of about 18 knots.

It is probable that the prevailing wind conditions at the time of the occurrence resulted in turbulent downwind wake eddies from the hangar located adjacent to, and to the west of, the touchdown zone of runway 35.

The downwind convergence of those turbulent wake eddies was probably in the vicinity of the touchdown zone of runway 35, and was likely to have been the source of the turbulence encountered during the landing flare.

Summary

At 1718 ESuT On 5 November 2002, VH-TJG, a Boeing 737-476 aircraft, encountered turbulence 1718 ESuT during the landing flare on runway 35 at Canberra International Airport. The aircraft was operating a scheduled fare-paying passenger service from Melbourne, Victoria to Canberra, ACT. The pilot in command was the handling pilot for the flight.

At 1700, the wind direction and speed at Canberra was 280 degrees T at 18 kts, gusting to 23 kts. At 1730, it was 280 degrees T at 18 kts, gusting to 26 kts. Runway 35 was aligned on magnetic heading 348 degrees, which was equivalent to 360 degrees T.

The automatic terminal information service (ATIS) at Canberra airport provided information on the prevailing weather conditions. At the time of the occurrence, information "Sierra" was current. It included information that runway 35 was in use, and that the wind direction and speed was 270 degrees M, with a minimum speed of 15 kts and maximum speed of 25 kts.

The aircraft was equipped with a solid-state digital flight data recorder (SSFDR). The flight data plots revealed that the pilot in command applied left control wheel to achieve a left wing low attitude of about 3 degrees as the aircraft descended through a radio altitude of about 60 ft. At about 6 ft radio altitude, the aircraft suddenly rolled left to a left wing low attitude of about 6 degrees, and the pilot in command rapidly applied right control wheel input to arrest the roll to the left. The aircraft landed about one second later in a slightly right wing low attitude.

The landing was completed without further incident, and there were no reported injuries to any of the 34 occupants of the aircraft.

The pilot in command subsequently reported that the turbulence encountered during the landing flare appeared to have resulted from a hangar located adjacent to, and to the west of, the touchdown zone of runway 35.

Construction of the hangar was completed in April 2002. The airside (eastern) face of the hangar was located 283.5 metres from the centreline of runway 35, and the roof height on the airside face of the hangar was 21.7 metres.

Turbulent wake eddies may be generated downwind of obstacles by wind flowing over and around them. The turbulent effects will depend on the size and location of an obstacle or group of obstacles, such as a cluster of buildings, as well as the direction and speed of the wind. If obstacles are located close to runways, turbulent wake eddies from those obstacles may have the potential to affect safety of flight if they result in aircraft experiencing difficulties during takeoff or landing.

The International Civil Aviation Organization (ICAO) has published standards and recommended practices that relate to aircraft, personnel, airways and auxiliary services. Those standards and recommended practices are contained in various Annexes to the Convention on International Civil Aviation, which was signed at Chicago on 7 December 1944 (the Chicago Convention). Australia is a contracting State to the convention. It is obliged under Article 37 of the convention to conform to standards and to endeavour to conform to recommended practices unless a difference has been filed with ICAO.

Annex 11 contained the standards and recommended practices that relate to the provision of air traffic services to the aviation sector. Paragraph 4.3.7 of Annex 11 detailed the information to be included in ATIS messages, and paragraph 4.3.7 k) required that messages contain:

`…other essential operational information.'

Paragraph 4.3.7 s) required ATIS messages to contain:

`…any available information on significant meteorological phenomena in the approach, take-off and climb-out areas including wind shear, and information on recent weather of operational significance.'

ATIS "Sierra" contained no information regarding the likelihood of turbulence in the touchdown zone of runway 35 at the time of the occurrence.

Canberra International Airport is a licensed aerodrome, and particulars about the aerodrome are required to be published in the Aeronautical Information Publication (AIP) Enroute Supplement Australia (ERSA).

The Civil Aviation Safety Authority (CASA) publishes Civil Aviation Advisory Publications (CAAPs) which provide guidance on the preferred method for complying with the Civil Aviation Regulations. CAAP 89O-1 (2), issued in November 2000, related to publishing aerodrome information and reporting changes to that information. That information included any event that affected the safety of aircraft using the aerodrome.

At the time of the occurrence, both the ERSA and the location briefing material for Canberra aerodrome contained no information to caution pilots of the likelihood of turbulence from the hangar located adjacent to, and to the west of, the touchdown zone of runway 35.

There have been two similar events of building-induced turbulence in the UK. One resulted in a B747 leaving the runway at London Heathrow airport, and was described in the UK Air Accident Investigation Branch (AAIB) Bulletin No. 5/2002. The other involved an A300 that sustained a podstrike at London Gatwick airport, and was described in AAIB Bulletin No. 6/2002.

The National Transportation Safety Board (NTSB) of the USA has had no reported occurrences of building-induced turbulence, nor has the NTSB identified that condition as a safety concern.

There are presently no building codes or standards in Australia that address the phenomena of building-induced turbulence with respect to proposed buildings to be located on or adjacent to aerodromes.

Occurrence summary

Investigation number	200205179
Occurrence date	05/11/2002
Location	Canberra, Aero.
State	Australian Capital Territory
Report release date	13/03/2003
Report status	Final
Investigation type	Occurrence Investigation
Investigation status	Completed
Mode of transport	Aviation
Aviation occurrence category	Turbulence/windshear/microburst
Occurrence class	Incident
Highest injury level	None

Aircraft details

Manufacturer	The Boeing Company
Model	737
Registration	VH-TJG
Serial number	24432
Sector	Jet
Operation type	Air Transport High Capacity
Departure point	Melbourne, VIC
Destination	Canberra, ACT
Damage	Nil

Boeing Co 737-376, VH-TAU

Summary

The Boeing 737 aircraft was operating a scheduled passenger service from Sydney to Alice Springs. At 1118 Central Standard Time, the flight crew advised air traffic control (ATC) that they had commenced a descent from flight level (FL) 310 to Alice Springs.

The general weather situation in the Alice Springs area was influenced by an unstable air mass with a trough developing to the west of Alice Springs. The aerodrome forecast issued to the crew prior to departure from Sydney indicated strong, gusty north-westerly winds during the day with moderate turbulence below 5000 ft expected after 1300. Storms were forecast to develop by 1130.

Distant lightning and showers had been reported to the west of Alice Springs from 0330. From that time, weather radar picture (RAPIC) imagery indicated that showers and thunderstorms were moving in a south-westerly direction at 15 to 25 kts. The RAPIC imagery available to the air traffic controllers was updated every 10 minutes. The shower/thunderstorm activity was mostly developing and decaying within 30-60 minutes, with breaks between the cells.

Showers developed in the vicinity of Alice Springs airport by 1100. An aerodrome special weather report (SPECI) was issued at 1109 due to the wind gusting to 26 kts. The visibility was 10 km or greater with one octa of cumulonimbus cloud and four octas of cumulus cloud at 9,000 ft. The report also indicated that there were showers in the area.

At 1119, the aerodrome controller advised the crew of a military aircraft inbound to the airport that `a storm or shower was passing through at the moment' with the visibility being greater than 10 km and a cloud base of 9,000 ft. That aircraft subsequently landed on runway 30 and did not report encountering turbulence during the approach.

Another SPECI was issued at 1120 due to a thunderstorm with a base of 9,000 ft and reduced visibility of 3,000 m to the north west of the airport. The 1120 RAPIC image showed a large area of rain associated with thunderstorm activity to the southeast and southwest of the airport between 2 and 13 NM. The thunderstorm activity had developed earlier over the MacDonnell Ranges to the west of the airport, and had moved in a south-easterly direction at around 15-25 kts.

At 1125, the aircraft encountered light to moderate turbulence as it passed through FL110, and the frequency of the turbulence increased as the descent continued. One minute later, another SPECI was issued, which reported that the wind was 250 degrees at 27 kts, with gusts to 37 kts. The visibility had reduced to 3,000 m to the west of the airport due to rain from thunderstorms.

The aircraft was 27 NM from the airport when the aerodrome controller advised that there were showers to the northwest and south of the airport but that it appeared `fairly clear for straight in for a final 30'. The flight crew concurred, noting that the weather was on their left. The rainfall was also observed by the crew on the aircraft weather radar that was displayed on the electronic horizontal situation indicators located on the aircraft instrument panels.

At 1128, the aerodrome controller broadcast terminal information Foxtrot, which advised that the wind was 250 degrees at 15 kts, the visibility was reduced to 6 km in passing rain showers, with heavy rain showers to the northwest. The 1130 RAPIC image showed that the thunderstorm activity had moved further to the southeast with areas of moderate rain recorded between 4 and 19 NM from the airport adjacent to and over the approach path for runway 30.

The aircraft experienced three encounters with moderate to severe turbulence between 1131 and 1132 as it descended through 4,700 ft (approximately 2,900 ft AGL). The encounters occurred at about 10 NM from the airport on the extended centreline for runway 30. At that stage the area of moderate rain was about 3 NM to the left of the aircraft with another area of moderate rain ahead on the approach path.

The flight crew conducted a missed approach at 1132. They advised the aerodrome controller that they had encountered severe turbulence and were turning right to remain clear of a thunderstorm. The aircraft sustained further encounters with moderate to severe turbulence until 1133 during the turn. Wind information recorded on the aircraft flight data recorder showed that the wind had been 325 degrees at 31 kts before the turbulence and had backed to 260 degrees at 45 kts during the encounters.

The crew subsequently advised the controller at 1134 that they had received a `very severe and nasty whack', and followed up with the comment that what they had encountered was in the `downburst type of category'. At 1136 the tower broadcast terminal information Golf and, due to the information being prepared before the missed approach, it did not contain any reference to pilot reports of severe turbulence. As there were no other aircraft in the vicinity, the terminal information was not immediately updated to include the report of severe turbulence.

The crew manoeuvred the aircraft to the north and southwest of the airport before conducting a landing on runway 12 at 1147 without further incident. After landing, the crew advised the controller that conditions on approach for runway 30 were `absolutely violent'. At 1201 the aerodrome controller broadcast terminal information Hotel, advising that thunderstorms were in the area and severe turbulence had been reported below 5,000 ft in the circuit area. That information was subsequently provided at 1203 to the crew of another aircraft inbound to Alice Springs from the north.

The procedures relating to the provision of weather information to pilots by ATC were set out in the Manual of Air Traffic Services (MATS) issued by the Department of Defence and Airservices Australia. Part 5; section 1 of MATS contained instructions regarding information to be provided to pilots by air traffic control including a hazard alert service.

The hazard alert service was required to contain information assessed by ATC as being of an unexpected and critical nature. That information would be based on the surveillance and assessment of various reports including SIGMET and AIRMET forecasts, amended forecasts, RAPIC information, observations and reports indicating weather conditions at the destination had deteriorated below the IFR or VFR alternate minima.

During the descent and approach of the aircraft, there were no SIGMETs or AIRMETs issued for the Alice Springs area, nor were the SPECIs indicating that the weather conditions had deteriorated below the IFR alternate minima. The Bureau of Meteorology (BoM) had no record of Alice Springs ATC contacting either the Darwin Regional Forecasting Centre or the Alice Springs Weather Service Office about the crew's report of severe turbulence.

Research into convective activity in dry semi-arid environments, similar to Alice Springs, has shown that storms with high bases can produce strong downdrafts (Caracena, Holle & Doswell, 2001). Studies have also shown that storms producing little or no surface rain (<0.25 cm) can produce dry microbursts. In extremely dry situations the storms may not produce lightning even though the high based cumulus clouds have a fibrous appearance and a prominent anvil (Caracena, Holle & Doswell, 2001). Researchers have also noted that radar echoes may be very misleading in determining the likelihood of dry microburst activity (Caracena, Holle & Doswell, 2001).

Analysis

The aircraft was operating in an environment that was conducive to turbulent conditions. During the encounter with the severe turbulence between 1131 and 1133, the aircraft was located about 3 NM from the edge of a convective cell with a high base. The severe turbulence encountered by the aircraft was probably associated with strong convective outflows from that cell.

The crew considered that the meteorological information issued to them prior to departure from Sydney and also by the Alice Springs aerodrome controller, was sufficient for them to determine the weather conditions likely to be encountered during the approach to runway 30. The crew's decision to continue the approach near convective activity was based on their subjective assessment of that information and the actual weather conditions.

The provision of weather information to the flight crew by the aerodrome controller was in accordance with the requirements of the MATS. There were no current SIGMETs or AIRMETs during the descent and approach of the aircraft and therefore the aerodrome controller was not required to issue a hazard alert to the crew.

The transmission from the crew at 1134 referred to downbursts but this information was not clarified by the controller to determine whether a report of windshear conditions was required in the next terminal information broadcast. Information Golf, broadcast at 1136, did not contain any reference to pilot reports of severe turbulence in the circuit area.

There was no record that the crew's reports were passed on to BoM for processing. MATS was ambiguous and did not provide clear guidance as to what action should be taken by air traffic controllers following the receipt of a pilot report concerning severe turbulence in the terminal area. Despite the ambiguity, the controller assessed the reports, and he subsequently included them in information Hotel issued at 1201. The controller also alerted the crew of the next aircraft operating in the Alice Springs terminal area about the report.

This occurrence highlights the need for air traffic controllers and flight crews to be aware of the hazards associated with convective activity. It is the second occurrence in a twelve-month period involving high capacity aircraft operating into airports affected by convective activity. The first occurrence involved a windshear encounter at Brisbane Airport in January 2001. A more detailed analysis of the hazards associated with convective activity in terminal airspace is provided in ATSB Air Safety Occurrence Report 200100213.

Safety Action

Local Safety Action

As a result of this occurrence, the air traffic services provider will be preparing a refresher training package for use by aerodrome controllers.

Reference:

Caracena, F., Holle, R.L., & Doswell, C.A., 2001, Microbursts A Handbook for Visual Identification (online). U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Washington. http://www.cimms.ou.edu/~doswell/microbursts/Handbook.html
[Accessed 22 January 2002].

Occurrence summary

Investigation number	200105157
Occurrence date	22/10/2001
Location	Alice Springs, Aero.
Report release date	16/08/2002
Report status	Final
Investigation status	Completed
Mode of transport	Aviation
Aviation occurrence category	Turbulence/windshear/microburst
Occurrence class	Serious Incident
Highest injury level	None

Aircraft details

Manufacturer	The Boeing Company
Model	737
Registration	VH-TAU
Serial number	23486
Operation type	Air Transport High Capacity
Departure point	Sydney, NSW
Destination	Alice Springs, NT
Damage	Nil

Wake turbulence, Fairchild Industries SA227, near Perth Airport, Western Australia, on 6 April 2021

Brief

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.

__________

Western Standard Time (WST): Coordinated Universal Time (UTC) + 8 hours.
Wake turbulence: Turbulence due to wakes behind large aircraft with a powerful downward motion.
Analysis of Wake Turbulence Occurrences at Sydney Airport 2012-2016

Occurrence summary

Mode of transport	Aviation
Occurrence ID	AB-2021-011
Occurrence date	06/04/2021
Location	near Perth Aerodrome,
State	Western Australia
Occurrence class	Serious Incident
Aviation occurrence category	Turbulence/windshear/microburst
Brief release date	17/05/2021

Aircraft details

Manufacturer	Fairchild Industries Inc
Sector	Turboprop
Operation type	Air Transport Low Capacity
Departure point	Perth, Western Australia
Destination	Forrestania, Western Australia
Damage	Nil

Turbulence event involving a Cessna 182Q, near Texas Aerodrome, Queensland, on 24 January 2020