Icelandic investigators have concluded that a fatigued Sukhoi Superjet crew advanced the wrong throttle lever during a single-engined go-around test before the aircraft overran the runway.
The inquiry into the 21 July 2013 accident at Reykjavik’s Keflavik airport found that, although the crew was well-rested before the original planned duty time, “significant and repeated” delays meant this was not the case at the time of the testing.
Sukhoi’s test crew was executing a series of early-morning approaches to Keflavik as part of a Category IIIa certification programme for the Superjet 100.
Seven approaches and go-arounds were conducted to runway 20 before testing moved to runway 11, in order to check automatic flight control system performance in crosswinds with the starboard engine shut down.
The aircraft had been conducting its second test approach to runway 11 – its autopilot and autothrottles engaged – with the crew intending to simulate a starboard engine failure at 25ft, followed by a go-around.
At a height of 17ft the right and left throttle levers began to move to the idle position, in accordance with the logic of the automatic flight-control system.
When the aircraft had descended to 10ft, a certification specialist in the cockpit shut down the starboard engine. This halted the right-hand lever but the left-hand lever continued retarding to idle.
The flying pilot disengaged the autopilot at a height of 4ft, and the left-hand autothrottle started advancing the left-hand throttle.
But as the pilot attempted to activate the take-off/go-around switch, the aircraft’s main landing-gear contacted the runway – causing the left-hand autothrottle to disengage, as designed, leaving the left-hand throttle set at only 16°.
The pilot noticed the go-around mode had not engaged, as a result of the touchdown, and he started to execute a manual go-around. But he set the right-hand throttle lever – for the inoperative starboard engine – to the go-around position and ordered retraction of the landing-gear.
Without the necessary thrust to perform the go-around, the aircraft’s airspeed declined and the jet – having reached only 27ft – descended and struck the runway with its undercarriage stowed.
The pilot had realised, moments before the impact, that he had been attempting to control the inoperative engine but, despite quickly setting the left-hand throttle to maximum, he was unable to halt the descent. After hitting the ground the jet skidded off the end of runway 11 and came to rest some 163m away.
Icelandic investigation authority RNF says the crew exceeded maximum duty time because the time for the flight-test campaign was about to end and the weather forecast for the following three days would not have allowed the testing to continue.
RNF carried out fatigue calculation and estimates that the crew suffered a 46% degradation in task performance at the time of the accident.