The Transportation Safety Board of Canada (TSB) has determined that alternating rudder control by the captain of an Air Canada Airbus A319 that encountered wake turbulence at altitude while flying more than 18.5km (10nm) behind a United Airlines Boeing 747-400 at 36,000ft (10,973m) on 10 January 2008 stressed the aircraft's vertical fin beyond certification limit loads.

In a final report released 1 June, investigators reveal that rudder input by the captain in response to what he assumed to be a control system failure caused the rear vertical stabiliser attachment fitting to see 129% of the structure's limit load, and the rear fuselage fitting to see 121% of its limit load.

Airbus tests the vertical fin to 150% of its limit load during certification. TSB notes that an upper limit for what the fin can handle was not determined during A319 certification testing "due to test equipment limitations".

During the 18 second event, the control inputs included a series of oscillating altitude and roll angles as high as 55 degrees, according to the report. Though the aircraft remained intact, eight passengers and crew members received minor injuries, and three passengers received serious injuries due to falls and collisions with aircraft furnishings. The pilots declared an emergency and diverted to Calgary for an uneventful landing.

The incident shares much in common with the 12 November 2001 American Airlines Airbus A300 accident in New York, in which the first officer responded to a wake turbulence encounter with a Boeing 747-400 using full alternating rudder deflection. In that case, the control inputs combined with the aircraft's slideslip angle caused the vertical fin to see loads greater than its ultimate strength, snapping the fin from the fuselage and causing the aircraft to enter an uncontrolled dive to the ground. The rudder control system design was a key fixture in the US National Transportation Safety Board (NTSB) recommendations on the crash, as was the American Airlines upset recovery training programme.

The two aircraft share similar designs for rudder control, with maximum rudder travel limited progressively as a function of aircraft speed. During the Air Canada event, the pilot responded to the flight upset with six cyclic reversals of full rudder input from left to right. The cause of the pitch and roll upset in the A319 was later determined to be wake turbulence from the 747, though the separation between the two aircraft was greater than required minimum standards.

In addition to finding that existing separation criteria for wake turbulence avoidance "may not be effective in all cases", the TSB determined that unrestrained service trolleys pose a risk to passengers and crew, as do unrestrained flight manuals in the cockpit.

TSB also notes that in the A320 family series of aircraft, "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 load limits". TSB explains that since the time of the incident, Airbus has now defined load limits that should trigger tail plane inspections, information previously not defined.

As a result of the investigation, TSB issued two safety communications, one requesting that Transport Canada (TC) enter into discussions with ICAO, Nav Canada and the US FAA to "address ways to reduce possibilities of hazardous encounters with wake turbulence at cruising altitude or during en route climbs and descents". TSB is also requesting that TC "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".

Source: Air Transport Intelligence news