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).
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.
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.
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].
|Date:||22 October 2001||Investigation status:||Completed|
|Time:||1132 hours CST|
|Location:||Alice Springs, Aero.|
|State:||Northern Territory||Occurrence type:||Turbulence/windshear/microburst|
|Release date:||16 August 2002||Occurrence class:||Environment|
|Report status:||Final||Occurrence category:||Serious Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Nil|
|Departure point||Sydney, NSW|
|Departure time||0930 hours CST|
|Destination||Alice Springs, NT|
|Role||Class of licence||Hours on type||Hours total|
|Pilot-in-Command||ATPL 1st Class|