Norwegian B738 near Alicante on Oct 18th 2017, ATC turns aircraft, TCAS RA and turbulence injures two flight attendants

Last Update: May 26, 2020 / 20:59:54 GMT/Zulu time

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Photo of Norwegian Air International EI-FJJ, Boeing 737-800 — Norwegian Air International EI-FJJ, Boeing 737-800 (Photo credit: flybyeigenheer / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Oct 18, 2017

Classification
Accident

Airline
Norwegian Air International

Flight number
D8-5321

Departure
Oslo, Norway

Destination
Alicante, Spain

Aircraft Registration
EI-FJJ

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Norwegian Air International Boeing 737-800, registration EI-FJJ performing flight D8-5321 from Oslo (Norway) to Alicante,SP (Spain) with 178 passengers and 6 crew, was descending towards Alicante while being surrounded by cumulonimbus cloud. The aircraft had been cleared to descend to FL190 and was already level at FL190, when the crew received instruction by ATC to turn left 90 degrees shortly followed by a TCAS resolution advisory to descend below the cleared FL190. While descending between FL190 and FL180 on TCAS instruction the aircraft encountered turbulence causing injuries to two flight attendants. The aircraft continued to Alicante for a landing without further incident.

Spain's CIAIAC reported one flight attendants received a serious leg injury, the other a minor back injury when the aircraft turned left 90 degrees following an ATC instruction and descended from the assigned FL190 to FL180 due to a TCAS RA and thus passed through a zone of turbulence. After being clear of conflict the flight crew was informed about the injuries of the flight attendants and continued the flight for a landing at destination without further incident. The CIAIAC opened an investigation into the occurrence.

On May 26th 2020 Spain's CIAIAC released their final report concluding the probable cause of the accident was:

The investigation has determined that flight attendants 2 and 3 fell as a result of the failure of controllers in two adjacent sectors to plan and coordinate, which led to a conflict alert. While maneuvering to clear the alert, the aircraft entered an area of turbulence.

The CIAIAC analysed:

From the point of view of the passengers, the seat belt sign had been on since entering Spanish airspace, confirming that the weather factor had been anticipated by the crew. Despite keeping this sign on for the passengers, the captain had allowed the cabin crew to stand if necessary. In fact, before the event took place, the vertical acceleration was stable and the cabin crew that had been standing did not suffer any adverse effects. In other words, the preventive measures taken by the crew involving the weather are deemed to have been adequate and proportionate.

ATC was perfectly aware of the weather situation, and controllers had been managing traffic that had diverted precisely for this reason. However, despite being aware of the general situation, it is difficult to know the exact location of every phenomenon that can affect each of the flights under ATC’s control. Such was the case in this event, where ATC instructed an aircraft to turn left, unaware that said turn would bring the aircraft closer to a problem area. The crew also did not report why the turn was unfavorable for them, they simply reported that it was a bad course. If they had explained the reason for their hesitation to carry out the maneuver, ATC may have provided a different instruction.

The instruction to turn left was intended to resolve a developing conflict with a Vueling Airbus A320-200 flight 3968 from Bilbao to Valencia (named VLG in the following, the Norwegian B738 named IBK in the following), which was in contact with another controller and was being instructed to hold at CALLES. No coordination between the two controllers occurred. The CIAIAC wrote: "The event thus resulted from a lack of planning and coordination between the controllers in adjacent sectors SUW and VAP."

Following activation of short term collision alerts on both controllers' desks both started to issue instructions to revolve the developing conflict. The CIAIAC analysed:

The instructions included proper evasive maneuvers for both aircraft, although the terminology employed in both cases did not include appropriate information on the urgency and reason for the instructions:

- In the case of VLG, the phraseology did not state that the instruction was due to a conflict with another traffic, or the urgency of the maneuver.

- In the case of IBK, the controller did specify that the maneuver was for conflicting traffic, but the urgency was not specified.

The fact that the two controllers each issued instructions to their respective aircraft involved shows that they were monitoring the progress of the conflicting traffic: in fact, the second instruction to VLG was amended in response to the changing situation. In all, two messages were provided to each aircraft before the TCAS RA was issued.

The resolution instructions provided by ATC to the aircraft were unable to avoid the conflict. The times between the execution of ATC’s instructions until the TCAS RA was issued were 33 s (VLG) and 29 s (IBK). The flight paths were not changed during this time. By the end of this period, VLG was starting to change its course and vertical speed, and IBK was just then starting to change its course, since the crew did not select the new heading until 20 s after receiving the instruction from ATC.

The maneuvers of the aircraft did not avoid the conflict and the TCAS issued an RA when the aircraft were 4.5 NM and 700 ft apart. The RA maneuver was executed as per the airline’s operating procedures, with the crew disengaging the autopilot and autothrust and adjusting the pitch angle to achieve the descent ordered by the maneuver. The reaction times expected (under 5 s) following the appearance of a TCAS RA were also satisfied in this case. The resulting flight profile was as follows:

- A descent at a rate that reached -3536 fpm. This value exceeded the -1500 to -2500 fpm range recommended by Eurocontrol and EASA guidelines to carry out TCAS maneuvers.

- A descent as the aircraft increased the left bank angle to -28.5º, as a result of the instruction issued 29 s earlier by ATC. As regards the compatibility of banking while executing a TCAS maneuver, EASA and Eurocontrol guidelines state that the bank must be stopped to achieve a descent rate that is as precise as possible. In contrast, the ICAO states that the ATC and TCAS instructions should be followed to the extent possible. The fact that the aircraft was turning could have affected the crew’s ability to adjust the descent rate.

The geometry of the two flight paths during the descent maneuver caused the TCAS to issue a new instruction to stop the descent of IBK and level off at its current altitude. This new instruction was executed immediately by the crew, which decreased the descent rate, and the pitch and bank angles. All the RAs cleared after 9 s. The fall of the two cabin crew and the closest point of approach of the two aircraft (2 NM and 1500 ft) occurred during this second stage, after which the aircraft increased their separation as they continued in opposite directions.