German investigators have disclosed that lateral offset procedures were not apparently permitted on the airway being flown by an Airbus A380 and a Bombardier Challenger business jet involved in a serious wake-turbulence upset over the Arabian Sea.
Strategic lateral offset is a technique by which aircraft fly slightly off the centreline of an airway to reduce the risk of a mid-air collision or wake encounter, particularly in regions outside normal air traffic surveillance.
German investigation authority BFU has identified a Sydney-bound Emirates Airbus A380 (A6-EUL) as the aircraft suspected to have triggered the upset to the Challenger 604 on 7 January.
Three crew members and six passengers had been on board the Challenger. Two passengers suffered severe injuries, including broken bones, while two others, and the flight attendant, received either bruising or fractures. Four others, including the two pilots, were unharmed.
The business jet, cruising at 34,000ft, lost 8,700ft in altitude in the space of 32s during the upset, the onset of which occurred less than a minute after the A380 passed overhead, in the opposite direct, at the adjacent cruise level of 35,000ft.
During the descent the aircraft's airframe exceeded the design certification load limits. Although the aircraft landed safely at Muscat, with no apparent external damage, BFU says Bombardier determined the airframe "could not be restored to an airworthy state".
"The aircraft is considered to be damaged substantially," it adds.
BFU confirms that the Challenger – flying from Male in the Maldives to Al-Bateen in the United Arab Emirates – had been operating along airway L894, in the Mumbai flight information region.
India's aeronautical information publication lists 17 routes in the Mumbai FIR on which strategic lateral offset procedures are permitted.
But BFU says airways L894 was "not among them".
The encounter occurred in the vicinity of the DONSA waypoint some 500nm from the nearest land.
BFU says the A380's collision-avoidance system initially detected the Challenger approaching from the opposite direction at a distance of 6nm. The two jets passed one another some 22s later.
Around 48s after the pass, says the inquiry, the Challenger encountered the wake vortex. The A380, by this point, was already 15nm distant to the south-east.
The Challenger initially began to roll to the right, subsequently banking sharply to 42°, and the aircraft experienced strong fluctuations in vertical acceleration and pitch, from 9° nose-up to 20° nose-down, as well as a loss of airspeed from 277kt to 248kt.
Flight-data recorder information captured aileron and rudder deflections to the left, and a roll to 31° left bank. Some 13s after the upset began the aircraft lost the validity of its inertial reference system parameters, and the flight data subsequently recorded zero pitch and roll information.
Testimony from the flight attendant indicates, however, that the aircraft rolled several times during the upset, throwing its occupants against the ceiling.
BFU states that the aircraft's airspeed, which had been around 277kt in the cruise, initially dipped to 248kt before rising to 330kt as the jet descended. Spoilers were extended and then retracted and the left engine, which had been losing thrust, was shut down.
The crew managed to regain control of the aircraft and arrest the descent.
Mumbai air traffic controllers were advised of the occurrence by the crew about 17min after the initial upset, and the pilots declared an emergency, stating that they would divert to Oman. The crew restarted the left engine around 20min later, and the aircraft was flown at 25,000ft. The pilots re-engaged the autopilot about 40min after the engine restart.
BFU says the Challenger landed at Muscat airport around 2h 25min after the wake encounter. The A380 had, in the meantime, continued its flight to Sydney, apparently unaffected by the incident.
Although the German authority has not disclosed the operator of the A380, it has confirmed the airframe involved was MSN224, which Flight Fleets Analyzer confirms as an Emirates aircraft, powered by Engine Alliance GP7200s, which had only been delivered to the Dubai-based carrier less than three weeks earlier.