Iran Air A300 veer-off probe seeks low-speed yaw rethink

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Swedish investigators are recommending the possible introduction of new requirements on low-speed yaw stability after a departing Iran Air Airbus A300-600 veered off a Stockholm runway.

The twinjet (EP-IBB) suffered a failure in its left-hand General Electric CF6 engine shortly after commencing its take-off roll, and the crew lost directional control.

Bound for Tehran, the A300 was cleared to execute a rolling take-off from runway 19R. Its captain, ironically, warned the first officer, who was flying, to ensure that the aircraft was lined up before accelerating, to avoid the risk of skidding off.

About 11s after take-off thrust was applied, with the A300 some 250m (820ft) into its roll, a muffled explosion was heard and the pilots retarded the thrust levers. The aircraft, travelling at 59kt, veered to the left and left the runway 400m from the threshold.

"The pilots were unable to correct the veer that had arisen," says Swedish investigation authority SHK. The jet's nose-wheel became embedded in the ground and the A300 halted 40m from the runway edge.

SHK's inquiry has focused not only on the engine failure but the inability of the crew to stabilise the aircraft's course. It puts the minimum ground-control speed - the speed at which the pilots would have had sufficient rudder authority to maintain directional control after engine failure - at 113kt.

This figure is far above the airspeed the A300 had attained. SHK says there are "deficiencies" in the certification process for large aircraft, with wing-mounted engines, in such circumstances, and is recommending that European and US certification authorities investigate the "prerequisites for introducing requirements" on yaw stability.

SHK discovered that the pilots' braking was "unintentionally asymmetrical", with a higher brake pressure on the side more likely to exacerbate the problem. But it adds that the inquiry has been unable to determine, with any certainty, whether this affected the jet's movement pattern.

While braking conditions on runway 19R were listed as good on the date of the incident, 16 January 2010, SHK points out that the runway was contaminated in the icy weather conditions and its friction level probably "fell short" of reported values.