Iranian investigators believe an Atrak Air Airbus A320 crew failed to stabilise initial engine parameters before setting take-off thrust, resulting in the aircraft's departing the runway during take-off at Tehran.
The aircraft (EP-TTA) had lined up for take-off from Tehran Mehrabad's runway 29L for a service to Mashhad on 13 August last year.
Flight-data recorder information shows the thrust-lever positions, at this point, were different – 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, says the Iranian Civil Aviation Organisation, but while engine-pressure ratio for the right-hand International Aero Engines V2500 increased "immediately", that for the left-hand powerplant increased "at a slower rate".
In its analysis of the event the inquiry states that the A320 flight crew operating manual requires the engine-pressure ratios to be stabilised at a level of 1.05 before the take-off levers are pushed forward.
"When engines are set to take-off thrust without stabilisation, due to different initial engine conditions, different engine spool-up and thrust power may be created in the first short time period," says the inquiry.
It adds that the pilots showed a "lack of attention" regarding the manual.
While the crew had received take-off clearance, 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 substantially to the right as the captain used the rudder pedals and thrust levers in a vain 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.
When the aircraft reached 26kt the right-hand lever was advanced nearly to the high-power go-around setting while the left-hand lever was reduced to idle. The jet then began veer sharply to the left and it exited the runway's left side about 300m from the threshold.
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 crew, it says, should have aborted the departure.
None of the 154 passengers or nine crew members was injured but the nose-wheel assemblies, in particular, as well as engine blades received damage.