Libyan investigators have determined that poor co-ordination between the pilots of an Afriqiyah Airways Airbus A330-200, spurred by sensory illusion, preceded the fatal go-around crash at Tripoli.

While the first officer was the flying pilot, the inquiry found that the captain began making inputs to his sidestick control as the aircraft aborted its non-precision approach to runway 09.

The aircraft climbed to just 450ft during the go-around before it descended and crashed short of the runway threshold - striking the ground at 260kt with a descent rate of 4,400ft/min - killing all but one of the 104 occupants.

Libya's Civil Aviation Authority, in its final report into the 12 May 2010 crash of flight 8U771, says the pilots had opted to continue descending through the minimum descent altitude of 620ft.

But the inquiry determined that the crew had not acquired visual ground references before proceeding with the final approach.

The aircraft descended to 280ft above ground before a terrain-awareness warning sounded, and the captain ordered a go-around.

Immediately the autopilot was disengaged. The first officer initially made a nose-up input, and the thrust levers were set to go-around power.

"The go-around was initiated without undue haste," says the inquiry. But while the pilots initially appeared co-ordinated, it says, the captain was probably "destabilised" by the terrain warning.

Some of the go-around call-outs, such as "positive climb", were not made and the inquiry says the first officer "questioned" the captain on several occasions, indicating a need "more active participation" from the non-flying pilot.

The aircraft pitched to 12.3° nose-up, and the crew retracted the landing-gear and flaps. But the co-pilot started making nose-down inputs 4s after the autopilot disconnection.

"These inputs are consistent with the high pitch attitude he could have perceived, typical of a somatogravic perceptual illusion," says the inquiry. Somatogravic illusion is the false perception of excessive pitch - caused by sensory misinterpretation in the absence of visual cues - which can prompt an instinctive nose-down response.

Pitch-down inputs were applied for 21s, causing the A330's pitch attitude to reduce to 3.5° nose-down. The inquiry suggests the co-pilot was focused on the aircraft's speed, rather than its attitude, following an incident 14 days earlier when an overspeed warning activated during a go-around.

"At no time was the go-around pitch attitude controlled, nor did the [first officer] follow the instructions from the flight director," it states, adding that fatigue could have played a role in the crash by causing him to focus solely on the airspeed.

Analysis found that the captain was also applying inputs to his sidestick, matching the first officer's, although not sufficient to trigger a dual-input warning.

"This action appears to be intended to provide assistance, without the captain intending to fly the aircraft by himself, without showing a lack of trust in [the first officer]," says the inquiry. But it says this distracted the captain and led to "ambiguity" as to who was controlling the aircraft.

As the aircraft lost height the terrain-awareness system issued a succession of sink and ground-proximity warnings. But the captain responded with a "sharp" nose-down input, says the inquiry, adding that he might have been subject to somatogravic illusion or was similarly focused on the A330's speed.

He then took control of the aircraft, without warning, via the sidestick priority button and maintained the nose-down input, while the first officer was simultaneously - and in vain - pulling back on his own sidestick.

Just 2s before impact, at a height of 180ft, the captain also pulled his sidestick fully back, suggesting both pilots were aware of the aircraft's impending collision with the ground, but were unable to arrest the descent.

Source: Air Transport Intelligence news