Investigators had no cockpit-voice information, and only limited flight-data recordings, from the Bombardier Challenger 604 involved in an in-flight upset, apparently triggered by wake from an Emirates Airbus A380.
But pilot testimony indicates that the aircraft rolled several times after encountering the wake, injuring five of the Challenger's nine occupants.
The cockpit-voice recorder had already been overwritten by the time the aircraft landed at Muscat, while the jet lost inertial reference system data seconds into the upset and the flight-data recorder did not detail information on the aircraft's attitude, including pitch and roll.
However, the Challenger's captain, who had been flying, told the German-led inquiry that the aircraft had "shook briefly" before rolling heavily to the left. Its autopilot disengaged and the crew had manually attempted to stop the roll with right-bank input.
"But the [aircraft] had continued to roll to the left, thereby completing several rotations," says German investigation authority BFU.
Both pilots fastened their lap belts and the first officer, who additionally wore a shoulder belt, subsequently lost his headset during the roll. The upset also caused pages from the quick-reference handbook to be scattered around the cockpit.
"Since the sky had been blue and the ocean's surface almost the same colour [the captain] had been able to recognise the aircraft's flight attitude with the help of the clouds," says the inquiry, citing the captain's testimony.
The flight attendant told the investigation that she had been standing in the cabin before the upset, and four of the six passengers were also not seated.
"In her recollection the [Challenger] had turned three times around its longitudinal axis, during which the occupants had been thrown against the ceiling and the seats," says BFU. "Several of the passengers suffered injuries, some of which were bleeding. She herself suffered minor injuries."
Inspection of the aircraft after it landed at Muscat showed damage to the seats and panelling, and "traces of blood", the inquiry adds. Armrests on four seats were deformed or fractured and two oxygen masks had fallen from their housing.
One passenger suffered head injuries and a broken rib, while another had fractured a vertebra. Two other passengers and the attendant had injuries including bruising and a fractured nose.
Just after the upset began the left-hand General Electric CF34 engine started to lose thrust and the captain had observed that its interstage turbine temperature had increased to more than 1,000C. The crew shut the engine down.
The aircraft lost almost 9,000ft in altitude before the pilots recovered control at around 24,000ft using aileron control inputs and elevator deflection, according to the crew testimony.
"Based on the memory items the pilots were able to reactivate the [inertial reference system] in attitude mode," says the inquiry, adding that they were able to fly the aircraft towards waypoint KITAL, some 430nm north-east of the location of the upset. This waypoint had previously been on the aircraft's intended course.
The crew subsequently used cross-bleed of the right-hand engine to restart the left.
After restoring the second inertial reference system and eventually re-engaging the autopilot, the crew opted to divert to Muscat.
No warnings of clear-air turbulence had been issued for the Arabian Sea region in which the 7 January upset occurred, reinforcing the suspicion that the A380's wake had been responsible for the event. The inquiry had no available information from the A380's cockpit-voice or flight-data recorders.
BFU states that the Challenger reached airspeeds of 330kt during the descent. It states that the aircraft's maximum operating velocity between 26,570ft and 30,990ft is 318kt.
German business aviation firm MHS Aviation was the operator of the jet (D-AMSC) which suffered an overstressed airframe and was subsequently declared a write-off.
Source: Cirium Dashboard