A319 / B744, en-route near Oroville WA USA, 2008
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On 10 January 2008, an Airbus A319-100 being operated by Air Canada on a scheduled passenger flight from Victoria BC to Toronto in night Visual Meteorological Conditions (VMC) was west of Oroville Washington USA when it was subjected to sudden jolts and then a series of rolls which resulted in an unintended descent of 1400 feet before straight and level flight was regained. During the episode, cabin service was in progress and seat belt signs were off. Cabin service carts were lifted off the floor and struck the ceiling above occupied passenger seats before coming back to rest in the aisle. The flight crew, suspecting a flight control abnormality, declared an emergency and diverted uneventfully, and without further use of the autopilot, to Calgary. Three of the 88 occupants sustained serious injuries and eight sustained minor injuries as a result of falls and collisions with aircraft furnishings.
An Investigation was carried out by the Canadian TSB. It was established that the aircraft had been climbing in accordance with its ATC clearance from FL 350 to cruise altitude FL 370 behind a United Airlines operated Boeing 747-400 en route from Hong Kong to Chicago and level at FL 370 and that the incident had occurred as the A319 had passed FL 366 at a distance of 10.7 nm behind the Boeing. The relative tracks are shown on the diagram below taken from the official report.
An examination of DFDR data indicated that “external forces applied to (the A319) disturbed its steady-state flight conditions, and that the upset was not initiated by the aircraft flight control systems. Any auto flight control inputs during the early stages of the event were attempts by the aircraft systems to return the aircraft to a wings-level climb.”
Following comparison of aircraft trials and Incident FDR data, it was established that the A319 had been “affected by turbulence associated with wake vortices from (the B744). With a wind no more than 12º off the tail of both aircraft, the track of (the A319) was aligned with the descending vortices. Air stability, with little mechanical turbulence, promoted longevity of the rotational energy contained in the vortices. This energy was sufficient to initiate significant attitude changes in (the A319), including those in the pitch axis, which resulted in negative g forces and displacement of persons and objects in the cabin.”