Boeing Co 737-376, VH-TAU

Investigation number

200105157

Occurrence date

22/10/2001

Location

Alice Springs, Aero.

Report release date

16/08/2002

Report status

Final

Investigation status

Completed

Aviation occurrence type

Turbulence/windshear/microburst

Occurrence category

Serious Incident

Highest injury level

None

Safety Action Analysis Summary

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].

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.

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).

Aircraft Details

Manufacturer

The Boeing Company

Model

737

Registration

VH-TAU

Serial number

23486

Operation type

Air Transport High Capacity

Departure point

Sydney, NSW

Departure time

0930 hours CST

Destination

Alice Springs, NT

Damage

Nil