What happened
On 18 October 2017, a Boeing 737-800, registration EI-FJJ, operated by Norwegian Air International, was en route from Oslo to Alicante. While descending through FL190 in the vicinity of the CLS V/DME (Valencia), the aircraft encountered a conflict with another aircraft.
Both the Air Traffic Control (ATC) short-term conflict alert (STCA) and the on-board Traffic Collision Avoidance System (TCAS) triggered resolution advisories. During the execution of these maneuvers, the aircraft entered an area of intense convective activity characterized by a phenomenon known as convective training. The resulting vertical acceleration swings, which reached a maximum of 1.78 g, caused two flight attendants to fall. One of the crew members sustained a fractured tibia.
The investigation
The investigation examined the sequence of events leading to the conflict and the subsequent turbulence. It was established that the aircraft had previously requested a route deviation due to adverse weather.
Investigators analyzed the coordination between air traffic control sectors. The investigation found that the conflict was triggered when an aircraft in an adjacent sector was instructed to hold at a specific point, a decision made without sufficient coordination between the controllers of the upper (SUW) and lower (VAP) sectors. The investigation also reviewed the aircraft's flight recorder data, which showed that the TCAS RA maneuvers involved significant changes in bank angle and vertical speed, coinciding with the period of high vertical acceleration.
Findings
- The primary cause of the flight attendants' fall was the failure of controllers in two adjacent sectors to plan and coordinate, which led to the conflict alert.
- While maneuvering to resolve the conflict, the aircraft entered a region of severe turbulence.
- The conflict involved another aircraft that had been instructed to hold at CALLES after being transferred to a lower sector while still physically located in the upper sector's airspace.
- The instruction to hold was issued by the lower sector controller without coordinating with the upper sector controller.
- The aircraft's vertical acceleration fluctuated significantly during the TCAS RA, reaching peaks that caused the crew members to lose their footing.