Near-simultaneous clearance for departure of a QantasLink Bombardier Q400 and the release of a weather balloon preceded an encounter between the two, Australian investigators have found.
The balloon passed some 10-20m (30-65ft) off the starboard wing-tip at around 700ft, after the pilots sighted it and immediately initiated a left turn.
Bureau of Meteorology personnel released hydrogen-filled weather balloons - capable of ascending at 1,000ft/min - four times each day from Perth.
Under an agreement with Australia's air traffic services, the meteorology bureau must contact Perth control tower to co-ordinated the release.
During a short inquiry, the Australian Transport Safety Bureau found that the tower had been contacted at the same time as the Q400, bound for Geraldton, lined up on runway 03.
Permission was granted for the balloon release, state the investigators, but the Q400 crew received take-off clearance "immediately after".
The Q400 crew had not seen any balloon activity, nor received any advice of such, while the meteorological officer had not seen any aircraft depart before the release. But the officer expressed concern that strong easterly winds might take the balloon into the Q400's flightpath.
None of the 58 occupants of the Q400 (VH-LQG) was injured in the 26 October 2012 event.
Dutch investigators have recently been analysing two close encounters last year between weather balloons and aircraft - a KLM Boeing 777-200 and an Arkefly 767-300 - climbing out of Amsterdam.