On 13 September 2012, a Boeing 737 aircraft, registered VH-YIO (YIO), departed Bali International Airport (Denpasar), Indonesia on a scheduled passenger service to Brisbane, Queensland.
The aircraft became established in the cruise and the seat belt sign was turned off. Shortly after, the crew observed opposite direction traffic on the aircraft’s traffic alert and collision avoidance system (TCAS), about 1,000 ft above and slightly to the left. The crew observed the aircraft pass to the left. Airservices Australia surveillance data indicated the aircraft passed with about 0.9 NM lateral and 1,400 ft vertical separation.
Soon after, the First Officer reported that they felt ‘cobblestone’ like turbulence. The aircraft then experienced a wake induced roll, initially to the right to a maximum angle of 6.5° and then left to 40.4°, with a 40 ft loss in altitude. As the roll to the left commenced, the crew immediately responded by applying full right aileron deflection.
Surveillance data indicated that there was about 2.1 NM lateral and 1,400 ft vertical separation and the correct air traffic control separation standards were being applied at the time. Nil injuries were received.
This incident demonstrated the value of periodic recurrent training, allowing the crew to react to the wake turbulence encounter intuitively and promptly. Furthermore, it is a timely reminder of the benefits of having the seat belt fastened, even when the seat belt sign is turned off, so that injuries during a turbulence encounter can be minimised.