Norwegian B738 enroute on Jul 25th 2021, turbulence injures flight attendant

Last Update: October 18, 2022 / 08:24:42 GMT/Zulu time

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Incident Facts

Date of incident
Jul 25, 2021

Classification
Report

Flight number
DY-3646

Destination
Nice, France

Aircraft Registration
SE-RPE

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Norwegian Air Sweden Boeing 737-800, registration SE-RPE performing flight DY-3646 from Copenhagen (Denmark) to Nice (France) with 180 passengers and 6 crew, was enroute at FL370 near Milan being handed off to Milan Area Control, when the aircraft needed to avoid (forecast) storm cells. After the crew hand deviated around what they thought was the last storm cell along their way, the crew turned direct towards their next waypoint and to descend to FL340 as cleared by ATC, when they discovered another storm cell which so far had been invisible to them. The crew was able to warn the passengers and cabin crew, the airspeed dropped from 250 to 228 KIAS within 14 seconds, then the aircraft encountered severe turbulence for about 7 seconds, and the airspeed increased again to 272 KIAS within another 13 seconds. During the turbulence encounter the aircraft encountered vertical accelerations between 0.27G and 1.79G, roll angles up to 35.5 degrees. Cabin crew subsequently informed the flight crew a flight attendant in the aft galley had broken her leg. The crew informed ATC a cabin crew suspected to have broken her leg and requested an ambulance for their arrival. The aircraft continued to Nice for a landing without further incident.

Late on Oct 17th 2022 the French BEA released their final report concluding the probable causes of the accident were:

Scenario

When the crew contacted the Milan ACC, 20 min before the occurrence, they were already engaged in storm cell avoidance manoeuvres. Other crews were also avoiding storm zones. This weather situation had been forecast. The meteorological file contained information about it.

The Milan ACC controllers regularly asked the crew to call back when they were able to head to BORDI waypoint, which is the point of tacit transfer between the Milan ACC and Nice approach. The controllers were not able to see the meteorological information on their screens. They could not see the exact location of the storm zones that prevented the crew from heading to the requested waypoint for over 20 minutes.

After circumnavigating what they thought to be the last storm cell, the crew stated that they were able to head to BORDI waypoint. Approval was granted by the air traffic controller. On turning, they suddenly saw on the weather radar and had in sight, storm cloud developments embedded in the cloud mass, immediately behind the cell they had just circumnavigated. It is possible that these storm clouds may have been hidden by the previous storm cell and that the onboard weather radar was unable to detect them earlier. It is also possible that they could not be detected by the onboard radar when the cell was in the radar’s detection beam. Lastly, it is possible that they may have been developing and appeared only shortly before on the aircraft’s path.

The crew were unable to avoid this cell and expected turbulence. They alerted the passengers. The aircraft passed through a zone of strong windshear and turbulence due to the presence of convective uplifts in a predominantly very strong southwesterly air flow. Flight through this zone was short but very turbulent. Strong vertical and horizontal accelerations associated with windshear and variations in speed followed in succession in the space of around seven seconds.

It is very probable that it was at the beginning of this phase that a cabin crew member, who was busy securing equipment in the aft galley, was thrown to the ground. His foot then became trapped under an escape slide housing and his ankle broke during the vertical accelerations that followed.

With respect to the onboard weather radar the BEA analysed:

The radar image obtained is based on three parameters: the gain, the tilt (angle between the horizontal line and the middle of the radar beam) and the scale selected by the pilot on his Navigation Display (ND). The radar beam opening angle is 3°.

The weather radar is designed to detect water in liquid form (rain or wet hail) by measuring precipitation levels. According to the levels detected and the gain selected, echoes appear on the ND in different colours. The weather radar barely detects water in solid form, such as ice crystals or dry snow.

The tilt adjustment determines the area passed through by the radar beam and, consequently, the echoes that are detected and displayed on the ND. The gain adjustment can then be adapted to the reflectivity of the precipitation encountered. It should be noted that clouds located ahead of the aircraft, which the beam does not pass through, will not be seen on the radar.

Moreover, the structure of cumulonimbus clouds requires an adjustment of the tilt and of the scale as the aircraft gets closer to a convective cell. Indeed, the reflectivity of precipitation inside a cumulonimbus depends on the temperature. The most reflective zones (liquid precipitation) are located under the 0°C isotherm. The reflectivity is high in these zones. Between the 0°C isotherm and the 40°C isotherm (known as the “radar top”), medium reflectivity is observed, based on the liquid water/ice crystal ratio. When the temperature is below -40°C, the reflectivity is very low (ice crystals).

The radar can operate in manual or automatic mode. The gain and the tilt can be adjusted separately. The operator, Norwegian Air Sweden recommends activating “WX + T” mode (detection of cloud layers and turbulence) in automatic mode. The T mode consists in an analysis of the doppler signal of the radar echo, which is used to estimate the movements of water droplets or ice particles detected, and variations in their movements, thus enabling detection of turbulence.

The radar does not detect cells hidden by other cells, and new cells can appear depending on changes to the aircraft’s path. In addition, these cells can be difficult to detect due to an insufficient amount of water.

Weather radar adjustments are not saved in the FDR parameters, so it was not possible to determine its configuration during the flight.
Incident Facts

Date of incident
Jul 25, 2021

Classification
Report

Flight number
DY-3646

Destination
Nice, France

Aircraft Registration
SE-RPE

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

This article is published under license from Avherald.com. © of text by Avherald.com.
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