Investigators have not been able to explain why the captain of a Diamond Aircraft DA62 calibration flight repeatedly breached minimum separation distances from commercial traffic at Dubai before a fatal wake-turbulence accident.

Analysis of the accident sequence indicates the UK Flight Calibration Services DA62, conducting approaches to runway 30L, had been 90s behind a Thai Airways International Airbus A350-900 descending to the parallel 30R.

Vortices from the A350’s left and right wing drifted with the crosswind at about 4.5m/s and respectively reached the 30L approach path after 74s and 87s, according to the United Arab Emirates General Civil Aviation Authority.

The analysis calculates that the vortices decayed to about 81% of their initial circulation strength at the point they reached the DA62 at 1,300ft altitude.

It rolled to the left, losing about 100ft in height, recovering after 9s. But 7s later it abruptly rolled to the left again, becoming inverted before diving steeply into the ground. None of the four occupants survived the accident, on 16 May last year.

A350 DA62 encounter

Source: UAE GCAA

Surveillance images show the A350’s left wing and (lower right) the initial DA62 upset and recovery

The DA62 had conducted 10 approaches. The nature of the work meant it flew 200-900ft lower than preceding aircraft operating to runway 30R, and the captain had the discretion to self-separate horizontally.

But the inquiry states that the captain – while concerned about A380 wake – appeared prepared to allow the light DA62 to breach normal ICAO separation between heavy and light aircraft, as well as the additional 3nm margin provided by Dubai air traffic control given the close proximity of runways 30L and 30R.

The captain had previously informed that Dubai air navigation service that he was experienced with calibration flights at the airport and was “content to be tighter” than normal wake separation.

“Other company pilots described him as safety conscious and they sometimes consulted him on flight operational issues,” says the inquiry.

“The investigation carefully considered possible causes for the [captain’s] misjudgment which led him to generally reduce the separation from other air traffic, but could not determine his reasoning.”

It states that, for the first five of the 10 approaches, the air traffic controller provided cautionary warnings of possible wake turbulence from heavy aircraft using runway 30R. But for the subsequent five approaches, following a controller handover, these cautions were no longer issued.

The inquiry says neither the provision of the advisories nor their absence influenced the captain’s judgement regarding self-separation from preceding aircraft.

“It is possible that the [captain] did not develop a mental picture of the surrounding air traffic, or simply underestimated the hazard,” it adds. “He may have compared the flight with other uneventful missions at single-runway airports.”

Two A380s and four Boeing 777s were among the aircraft followed by the DA62, which trailed them by just 5-6nm compared with the Dubai standard of 9nm for the 777 and 11nm for the A380.

“The [captain] had been provided with sufficient distance from preceding air traffic to establish a safe approach and was not provided with any time constraints by [controllers] that may have caused time pressure to complete the flight earlier,” the inquiry points out.

During the 10th approach the aircraft was 200ft below and 3.7nm behind the A350-900 – compared with the 6nm separation standard for ICAO and 9nm standard for Dubai – when it was struck by the wake vortex.

Airbus analysis for the inquiry states that, using ICAO separation, the vortex would have been 35-49% of its maximum strength at the point of the DA62 encounter, rather than around 70%.