An aircraft Atrak Airbus A320 (EP-TTA) was ready to take-off from Tehran Mehrabad’s runway 29L for a service to Mashhad on 13 August last year.
Flight-data recorder information showed different thrust-lever positions – the left had been reduced to 2.8° while the right was increased to 5.6° – resulting in differing engine-pressure ratios and turbine speeds. Both thrust levers were symmetrically advanced. So while engine-pressure ratio for the right-hand International Aero Engines V2500 increased “immediately”, the left-hand powerplant increased “at a slower rate”.
Though the operating manual of A320 requires the engine-pressure ratios to be stabilised at a level of 1.05 before the take-off levers are pushed forward, the pilots showed indifference regarding the manual. The first officer had been replying to air traffic control and was not concentrating on the engine parameters when the aircraft began to roll. It initially started to drift to the left and then to the right as the captain used the rudder pedals and thrust levers in an attempt to keep the A320 on the centreline.
Cockpit-voice recorder data indicates that the first officer – who had more time on the type than the captain – requested that the take-off be aborted but was overruled.
The right-hand lever was advanced nearly to the high-power go-around setting when the aircraft reached 26kt, while the left-hand lever was reduced to idle. Then the jet began veer sharply to the left and exited the runway’s left side about 300m from the edge of the runway. Even after the aircraft came to a halt in rough ground the flight-data recorder registered advancements in the thrust levers and increases in engine power. Analysis of the take-off roll showed that the crew had also used the nose-wheel steering tiller – against standard procedures – to try to correct the aircraft’s course, a decision which the inquiry describes as “dangerous”. The inquiry categorically mentioned that the departure should have aborted.
Luckily none of the 154 passengers or nine crew members was injured. But there were damages to the nose-wheel assemblies and engine blades.