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 FL350 to cruise altitude FL370 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 FL366 at a distance of 10.7 nm in trail 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. 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 data with Incident FDR data, it was established that an initial disturbance had been due to wake vortices from the aircraft ahead. 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 pitch attitude change which "resulted in negative g forces and displacement of persons and objects in the cabin.”
However, with the aircraft commander as PF, before the autopilot had countered the initial un-commanded roll perturbation, the PF disconnected the autopilot and autothrottles and attempted manual correction. Four further rolls of up to 55° wing down then followed with the whole disturbance lasting 18 seconds and involving multiple side stick and rudder pedal inputs, some of which “exacerbated the situation”. It was noted that the event was the aircraft commander’s first experience of significant wake turbulence upset of this degree and he had “believed that there was a serious aircraft-induced flight control problem”.
It was considered that both the encountered wake turbulence and the subsequent pilot-induced control inputs had contributed to the displacement of unrestrained passengers and objects in the cabin. It was also found that the lateral accelerations had imposed air loads on the vertical stabiliser structure following pilot inputs had exceeded its certified limits and that “some pilot actions intended to rectify the upset (to the A319) were similar to those that contributed to damage to the vertical stabilizer attachment fittings (in the fatal accident to the American Airlines A300) in 2001.” See A306, vicinity JFK New York USA, 2001
It was considered by the Investigation that since “the low expectancy of experiencing such an upset event in cruise flight combined with little or no warning of entering wake vortices likely startled the pilots, who then responded with potentially hazardous flight control inputs. Advance warning of possible wake turbulence could sensitize pilots and prepare them for such an encounter.”
It was additionally noted that, during the incident, “the automatic reversion to alternate (flight control) law due to differences in computed angle of attack values was also a product of pilot control inputs.”
Map of Flight Paths - ACA190 (A319), UAL896 (B744)
In December 2008, the TSB issued two ‘Aviation Safety Advisories’ addressed to the Canadian National Aviation Authority (NAA), Transport Canada.
- A08W0007-D2-A1, Wake Turbulence Encounters During En Route Climbs and Descents noted that “as shown in this and other occurrences, the current provisions for wake turbulence avoidance may not be effective in all cases, especially for aircraft operating below and behind a preceding larger aircraft while en route. Unanticipated encounters with wake vortices may continue, increasing the risk of injury to passengers and cabin crew, and damage to aircraft” and suggested that Transport Canada may wish to enter into discussions with ICAO, Canadian ANSP Nav Canada and the FAA “to address ways to reduce the possibilities of hazardous encounters with wake turbulence at cruising altitude or during en route climbs and descents.”
- A08W0007-D1-A1, Pilot Training for Upset Recovery in Transport Category Aircraft noted that “as demonstrated in this occurrence, pilots of transport category aircraft can make inappropriate rudder inputs during upsets that involve high bank angles. This can result in aircraft movements that can cause injury to passengers and cabin crew and, as demonstrated in American Airlines flight 587 in 2001, damage to structure. A review of upset training programs of Canadian operators indicates that some of the programs do not contain scenarios involving recovery from large bank angles.” It further suggested that “Transport Canada may wish to communicate to transport category aircraft operators in Canada the necessity to include roll scenarios in upset training and the appropriate use of rudder control during recoveries.”
Findings of the Investigation
The following are the full statement of the Findings of the Investigation taken verbatim from the Final Report:
Findings as to Causes and Contributing Factors
- Although the aircraft were separated by more than the minimum separation standard, when ACA190 was cleared to climb, the wake vortices from UAL896 had not dissipated.
- The wingtip vortices of the heavier aircraft contained sufficient energy to significantly destabilize ACA190 in pitch and roll, which contributed to displacement of persons and objects in the cabin.
- During recovery from the upset, pilot rudder and sidestick control inputs resulted in aircraft sideslip and g loadings. These contributed to the displacement of occupants and objects in the cabin, as well as placing lateral accelerations and aerodynamic loads on the vertical stabilizer structure to beyond certified limits.
- Annual recurrent A319/A320 pilot training at Air Canada did not consistently include reference to the hazards of pilot rudder pedal reversals during upset recovery at high airspeeds. This increased the likelihood that pilots would make inappropriate rudder pedal inputs during upset recoveries.
Findings as to Risk
- Wake vortices increase the risk of injury to passengers and cabin crew, and damage to aircraft.
- There is no industry-wide practice of restraining service trolleys when in use in aircraft aisles. An examination of the risks associated with unsecured trolleys by manufacturers and operators may be warranted to identify mitigation strategies.
- During the event, unrestrained manuals were moved from their stowage compartments and were strewn about the cockpit. These materials presented a risk of injury to the flight crew and damage to flight deck switches and controls.
- In the Airbus A318/A319/A320/A321 series, it may be possible for a pilot to apply rudder control inputs that result in aerodynamically generated structural loads in excess of certification design limits and approaching ultimate loads.
- At the time of the occurrence, Airbus documents supplied to A318/A319/A320/A321 series operators did not include lateral load limits that would trigger maintenance action for vertical tailplane inspection.
The Final Report of the Investigation was released 8 April 2010 and may be seen in full at SKYbrary bookshelf: TSB Canada Aviation Investigation Report A08W0007
No Safety Recommendations were made.