Turbulence/windshear/microburst

The aircraft was operating as Flight QF004/145 from San Francisco to Sydney, cruising at flight level 390 in smooth, clear, moonless conditions. The aircraft Captain occupied the left control position and the Second Officer occupied the right. The weather radar system was serviceable and operating. No significant weather had been forecast or observed by the crew during the flight. Although the "fasten seatbelt" signs had earlier been turned off, passengers had been advised, by cabin announcements, that it was recommended that seat belts should be worn unless moving around the cabin. This information was also included in the passenger safety briefing cards as well as being stamped onto each seatbelt buckle. About six hours after leaving San Francisco, the Captain noticed stars he had observed from his left window were becoming obscured. Upon closer inspection he noticed cloud ahead, and to the left of, the aircraft. Although no weather radar returns were apparent, the Captain took the precaution of turning on the fasten seatbelt signs. Approximately 15 seconds later the aircraft encountered a short period of moderate to severe turbulence. A number of passengers and flight attendants were flung about the cabin, incurring minor injuries as they struck fixed objects. Damage to the aircraft was minor and confined to internal fittings. After considering the condition of the injured, the enroute medical facilities and the superficial nature of the aircraft damage, the Captain decided the flight should continue as planned. The aircraft subsequently landed at Sydney where further medical treatment was provided. Twenty passengers required short term treatment in hospital. The investigation of the incident revealed that the flight crew had operated the aircraft in accordance with approved company procedures. Their knowledge, and use, of the weather radar systems was considered satisfactory. All weather radar systems depend on the fact that water particles, either in suspension (cloud) or as rainfall, will reflect a radar energy beam to an extent generally dependent upon the density and size of the water droplets. The aircraft encountered vertical air currents associated with an isolated towering cumulus cloud, which evidently contained insufficient moisture to be detectable by weather radar.

Circumstances:

The pilot, in company with another aircraft, had been carrying out aerial top dressing operations from an airstrip located along the top of a ridge line. The conditions were calm, and the pilot, who had limited aerial agricultural experience, was relaxed and enjoying the flying. Returning to the airstrip at the completion of a sortie, the pilot made a glide approach and crossed the threshold just as the other aircraft was departing with a full load. Just before touch down, the aircraft encountered turbulence and dropped the left wing, then veered sharply to the left over the side of the ridge. The pilot attempted to regain the strip without increasing power.

However, the aircraft failed to respond and touched down on the sloping terrain at the side of the strip. During the landing roll he attempted to steer the aircraft up the slope, but it collided with a stack of timber which caused the right-hand main landing gear leg to separate. The pilot subsequently advised that he had been complacent and had not given sufficient attention to the landing. He had then encountered wake turbulence, generated from behind the departing aircraft, which had not had time to dissipate in the calm conditions. It had not occurred to him to apply full power and go-around.

Circumstances:

The pilot under check advised the tower controller that the pilot in command wished to demonstrate an engine failure after take-off from runway 33 and that they would land on runway 15. The controller gave approval for this sequence and issued a take-off clearance. Four minutes earlier, a Dash 8 aircraft had been cleared for take-off (this aircraft was making an intersection DEPARTURE) from the same runway. VH-CES was observed to turn back towards the runway and descend normally but contacted the ground adjacent to the runway right wing low and sideslipping to the right. The landing gear collapsed, and the aircraft came to rest facing north-east. Wind at the time was from 030` at 5-10 kts. The pilot reported that the aircraft had experienced a bump similar to that when encountering wake turbulence. The bump was encountered shortly after the aircraft commenced the turn back to the runway, and after the throttle was closed. The aircraft then experienced windshear and downdraft to the extent that the rate of descent could not be arrested before ground impact. VH-CES took off some 2-3 min after the Dash 8. Because the Dash 8 weighs less than 25,000 kg, no wake separation standard was required to be applied by the aerodrome controller. The wake produced by the Dash 8 right wing would, in nil wind conditions, travel away from the runway at about 5 kts. Under the crosswind conditions prevailing at the time, however, the wake could have remained in the vicinity of the runway. This, along with the significant weight difference between the two aircraft, could have led to the flight path of VH-CES being adversely affected.

Significant Factors:

The following factors were considered relevant to the development of the accident

1. Crosswind conditions prevailed.

2. Wake turbulence was generated by a departing aircraft.

3. The aircraft was probably affected by wake turbulence during approach.

On 31 January 2010, an American Aircraft Corporation Grumman Traveller AA-5 aircraft, registered VH-ERP, was being operated on a visual flight rules private flight from Temora, New South Wales to Canberra, Australian Capital Territory. At about 1630 Eastern Daylight-saving Time, on late final approach to runway 12 at Canberra Aerodrome, and at an altitude of about 150 ft above ground level, the aircraft experienced severe turbulence that resulted in a brief loss of control. The pilot recovered control and landed on runway 12.

The investigation determined that it was probable that the severe turbulence was generated by a combination of the wind conditions on the day and the position of the two buildings located about 220 m and 290 m upwind from runway 12. In addition, there were no standard criteria for assessing the potential local wind effect of aerodrome building developments on aviation operations, and no national building codes for aerodrome developments that address the phenomena of building-induced turbulence.

The aerodrome operator had commissioned pre-construction wind impact assessments of the two buildings to the north of runway 12. These reports concluded that the buildings would not result in adverse wind effects on aircraft operations. This conclusion was based in part on the assessment that use of runway 12 was unlikely in northerly wind conditions. However, operations to that runway remained possible in those conditions without any alert to affected pilots about possible risk. By contrast the Canberra Aerodrome information in the En Route Supplement Australia alerted pilots of the possibility of severe turbulence during touchdown on runway 35 in strong westerly winds.

Subsequent to this occurrence, the Department of Infrastructure, Transport, Regional Development and Local Government established the National Airports Safety Advisory Group (NASAG). NASAG's role is to examine airport planning issues, including the potential local wind effects of buildings on aircraft operations, and to develop a set of universal guidelines and policy material for application at state and local levels. In addition, Airservices Australia is progressing the installation of wind shear detection technologies at several aerodromes. There is the potential that one of those installations could be at Canberra Aerodrome.

On 9 July 2009, a Cessna 208B Grand Caravan aircraft registered VH-TFS, was being operated on a charter passenger flight from Lizard Island, Queensland (Qld) to Cairns, Qld. The flight was being conducted under instrument flight rules. At about 1250 Eastern Standard Time, the aircraft encountered severe turbulence. The pilot and two of the three passengers sustained minor injuries. The flight continued to Cairns and landed without further incident.

The Australian Transport Safety Bureau publication 'Staying Safe against In-flight Turbulence' (2009) provides some useful information on aircraft turbulence events. A full copy of that publication is available here.

On 29 August 2007, a SAAB Aircraft Company 340B-229 (SAAB) aircraft was being operated on a scheduled passenger service from Adelaide, SA to Mount Gambier. The aircraft departed from runway 05 soon after an Airbus A320-232 (Airbus) aircraft departed from the same runway. When the SAAB reached a height of 250 to 350 ft above ground level (AGL), the flight crew experienced abrupt, severe buffeting and an uncommanded roll to the left, followed by another roll to the right.

As a result of this occurrence, the aircraft operator advised that they reviewed their operating procedures relating to departures behind jet aircraft and will use the ATSB report as part of a safety promotion strategy directed at all company pilots. In addition, The Civil Aviation Safety Authority is reviewing the safety implications of this incident and is considering the development of a safety education program for flight crew and air traffic controllers.

At approximately 0955 Central Daylight-saving Time on 8 Jan 2007, the flight crew commenced the take-off roll on runway 23 in a Boeing Company 737-838 aircraft, registered VH-VXG, on a scheduled passenger service from Adelaide, SA to Alice Springs NT.

At a speed of approximately 140 kts, the crew reported an abrupt, uncommanded yaw. Corrective action was applied, engine parameters checked, and the takeoff was continued without further incident. The crew advised Air Traffic Control of the uncommanded yaw and contacted the operator's maintenance watch for advice. The crew subsequently returned the aircraft to Adelaide Airport. The wind at the time was reported to be light (approximately 3 kts) from the east.

Data from the aircraft's Flight Data Recorder was recovered and downloaded by the Australian Transport Safety Bureau (ATSB) for review. That review indicated that the input to the aircraft rudder was not uncommanded and that the rudder pedals moved proportionally to the rudder surface deflection at all times. An engineering examination of the aircraft did not identify any reason for the uncommanded yaw and the aircraft was released back to service.

Due to a previous, similar, event at Adelaide Airport on 15 Dec 2006 (ATSB occurrence 200607627), the aircraft operator sought advice from the aircraft manufacturer. The aircraft manufacturer reviewed the data from the Flight Data Recorder and concluded that the recorded event was not a result of an uncommanded aircraft rudder input, asymmetric thrust, nose-wheel steering or asymmetric brake application.

While the nature of the uncommanded yaw could not be positively identified, it is likely that the event was related to an atmospheric disturbance during the take-off run.

The ATSB continues to monitor such reported uncommanded yaw events and has reported similar events in the past (see occurrence reports 200607627, 200500994 and 199703237 available on the ATSB website: www.atsb.gov.au).

In the early hours of 22 June 2009, an Airbus Industrie A330 (A330), registered VH-QPI (QPI), encountered an area of severe turbulence associated with convective activity while en route from Hong Kong to Perth, Western Australia. As a result of the incident, a combined total of seven passengers and crew members received minor injuries. After consultation with medical and operational personnel, the pilot in command continued the flight to Perth. The aircraft suffered minor internal damage and, after a maintenance check, was returned to service.

The cloud associated with the convective activity consisted of ice crystals; a form of water that has minimal detectability by aircraft weather radar. Consequently, the convective activity itself was not detectable by QPI's radar. As the event occurred at night with no moon, there was little opportunity for the crew to see the weather.

The operator intends to upgrade the weather radar fitted to its A330 fleet, which will increase the fleet's capability to detect convective turbulence. Two other minor safety issues were identified during the investigation relating to the risks associated with the use of the pilot flight library when turbulent conditions are encountered, and the engagement of the manual latch to the cockpit door preventing timely access to the flight deck by other operational staff. The operator has taken, or is proposing, relevant safety action to address those issues.

At approximately 1416 Central Daylight-saving Time on 15 Dec 2006, the flight crew commenced a take-off roll in a Boeing Company 737-400 aircraft, registered VH-TJH, on a scheduled passenger service from Adelaide, SA to Melbourne, Vic.

At a speed of approximately 50 kts, the crew reported an abrupt, uncommanded left yaw. Corrective action was applied, and the take-off was continued without further incident. The remaining flight was uneventful.

The flight crew subsequently reported that they did not notice any rudder pedal movement coincident with the uncommanded yaw and recalled that there was a gusting crosswind from the left during the take-off.

Data from the aircraft's Flight Data Recorder was recovered and downloaded by the Australian Transport Safety Bureau (ATSB) for review. That review indicated that the input to the aircraft rudder was not uncommanded and that the rudder pedals moved proportionally to the rudder surface deflection at all times. An examination of the aircraft by the aircraft operators' maintenance engineers did not identify any reason for the uncommanded yaw and the aircraft was released back to service. Additionally, there was no other aircraft traffic in the vicinity that may have generated wake turbulence in the immediate timeframe prior to the take-off.

While the nature of the uncommanded yaw could not be positively identified, it is likely that the event was related to an atmospheric disturbance or turbulence during the take-off.

The ATSB continues to monitor such reported uncommanded yaw events and has reported similar events in the past (see occurrence reports 200700035, 200500994 and 199703237 available on the ATSB website).

As a result of this and a number of similar occurrences, the ATSB is reviewing past investigations and data held by the Bureau covering safety issues relating to the communication of weather information to aircrews and between Airservices Australia and the Bureau of Meteorology and the safety action taken by these organisations to mitigate known problems in this area.

The ATSB has previously published reports of the investigations into occurrences that involved flight by regular public transport aircraft into convective weather and other weather situations where the availability of accurate weather information and communication of weather information to flight crews was a factor. For further information, readers are directed to ATSB occurrence investigations 200100213, 200105157, 200201228, 200301941 and 200304400 and associated safety recommendations. Copies of these reports are available from the ATSB website, or from the Bureau on request.