On 4 December 2012, at about 1618 Western Standard Time, a Qantas Airways Boeing 737‑838 aircraft, registered VH‑VZL, was taking off on runway 06 at Perth Airport, Western Australia on a flight to Canberra, Australian Capital Territory.
During the take-off run, approaching the take-off reference speeds, the airspeed stopped increasing and did not start increasing again for several seconds. The captain noticed that the wind vector on the navigation display was showing a tailwind of about 20-25 kt. The captain disconnected the auto-throttle and ‘fire-walled’ the thrust levers. During the initial climb, the first officer performed a windshear escape manoeuvre.
While there was some cumulonimbus cloud activity about 20–30 NM north of the airport, there were no indications of an impending wind change before take-off.
Before take-off, both pilots checked the windsocks, which showed that headwind conditions existed. Late in the take-off run, a significant wind change occurred, and the aircraft began to experience tailwind conditions of about 20 kt. As the performance calculations had assumed nil wind for take-off, the aircraft failed to achieve the predicted take-off performance.
This incident serves as a reminder to pilots that significant wind changes can occur during take-off, can be difficult to predict, and can occur in the absence of thunderstorm activity. The wind conditions at each end of a runway may differ significantly so that headwind conditions can exist at one end and tailwind conditions at the other end.
Although it did not assist in this case, it is important to monitor the available windsocks before take-off as it is the final opportunity to detect wind changes before the take-off roll begins.