Description

On 10 July 2019 an Airbus A380-800 (A6-EEM) being operated by Emirates on a scheduled international passenger flight from Auckland to Dubai as EK449 with an augmented crew was in the cruise east of Chennai in night VMC at FL 400 when it encountered severe turbulence unexpectedly which resulted in one serious and 13 minor injuries amongst the 378 passengers and minor injuries to 13 of the 25 cabin crew. Impact damage to the cabin ceiling panels and detachment of several ceiling mounted exit signs also occurred but there was no other damage to the aircraft or its systems. The flight was completed to destination where the seriously injured passenger was hospitalised.

Investigation

A comprehensive Investigation was carried out by the UAE GCAA Air Accident Investigation Sector. Relevant data were obtained from both the FDR and the QAR but as the flight had continued for more than two hours following the turbulence encounter, relevant data from the CVR had been overwritten. Assistance in the analysis of the FDR data was provided by Airbus.

The 52 year-old Captain had a total of 10,922 hours flying experience which included 1,091 hours on type and the 35 year-old First Officer a total of 9,742 hours flying experience which included 5,345 hours on type. The 50 year-old augmenting Captain had a total of 7,890 hours flying experience which included 4,250 hours on type and the 43 year-old augmenting First Officer a total of 5,259 hours flying experience which included 4,522 hours on type. The augmenting crew were in control of the flight at the time the turbulence occurred.

What Happened

Pre flight planning by the four pilots involved included significant weather charts which were issued 12 hours in advance of their validity period which included the time when the turbulence episode occurred. The flight crew subsequently stated that they had not considered that there was anything significant about the forecast en-route weather and were aware of the potential consequences of their routing across the Bay of Bengal during the monsoon season. Once in flight, they had not sought updated information on the adverse weather forecast in the area of the Bay of Bengal from ATC nor been provided with any by the aircraft operator. They noted that during the pre flight briefing, the Cabin Manager had been told that turbulence en-route was likely.

Approximately one hour prior to the turbulence encounter, the operating crew had handed over control to the augmenting crew in order to begin their planned rest in the flight crew rest compartment. During the handover briefing, the off-going crew had noted that there was weather activity ahead close to waypoint ‘IDASO’ and that other flights had been requesting deviations to avoid it. The augmenting First Officer took over as PF and it was confirmed that the weather radar ‘WXR’ and ‘TURB’ functions were set to ‘AUTO’ and the weather radar display on the Navigation Displays (ND) was enabled.

With the aircraft approximately 40 nm (five minutes) away from where the turbulence episode would subsequently occur, recorded data showed that magenta areas on the weather radar 40 nm ahead of the aircraft and 20 nm either side of the intended aircraft track between FL350 and FL450 had become permanent. At this time, the weather radar manual mode was not selected but the Captain decided to switch on the seat belt signs as a precaution. However, when doing so, he did not alert the Senior Cabin Crew Member (SCCM) by interphone and since none of the cabin crew noticed the signs coming on or heard the chime which accompanies this, the usual cabin PA for passengers to return to their seats and fasten their seat belts was not made. It was noted that at this time, AP2 and the A/THR (autothrottle/autothrust) were both engaged, the latter in ‘MACH’ mode, both FDs were displayed and the thrust levers were in the Maximum Climb (CL) detent. The thrust levers remained in this position throughout the subsequent turbulence encounter and the AP remained engaged. Approximately 13 hours into the 17 hour flight, close to the previously mentioned waypoint ‘IDASO’, an unexpected sudden onset turbulence episode began with severe turbulence for the first 20 seconds with FDR data indicating that the aircraft had entered an area with significant wind velocity variations.

The pilots stated that after the seatbelt signs had been switch on, the ND weather radar range for the Captain had been set to 160 nm and that for the First Officer to 80 nm and that the gain control had been set 85% for the Captain and 50% for the First Officer. The Captain also stated that a decision to deviate is normally taken before the aircraft is 40 nm from any significant weather returns and that on the basis that their track would take them “through a clear area with few clouds” and the weather radar returns were off track, he had decided to continue as planned. Both pilots had noted that having dimmed the flight deck lighting, they had been able to observe lightning activity in the area with discharges about every 90 seconds. This had allowed them to see the cloud tops and weather and the weather below the aircraft and that the aircraft would remain clear of cloud. They added that they had seen a thunderstorm squall line in the area which they said was “about 80 miles long and directly across the aircraft flight path” but said that as there was no precipitation showing on their NDs, they considered that no avoidance was required as the weather was below the aircraft level and off-track. They believed that the turbulence encountered was “downwind of the cumulonimbus clouds seen and had been clear air turbulence”.

The flight crew reported that when the turbulence began, the airspeed had suddenly started to increase towards the MMO (M0.89) and the PF had responded by deploying the speedbrakes and reducing the Mach target from M0.84 to M0.72. Although they stated that they did not observe any MMO exceedance, examination of the flight data showed that there had been three transient ones, during one of which the speed had increased from M0.836 to M0.903 in just five seconds triggering a brief overspeed warning. This particular speed increase occurred concurrently with a rapid decrease in the detected tailwind component from 54 knots to 17 knots.

When the turbulence ceased after just over four minutes, the flight crew stated that they had initially assessed it to have been ‘moderate’ but after they began receiving calls from the cabin crew about injuries and cabin damage (see two illustrative examples below) they had revised this to ‘severe’. The Investigation noted that the evidence assembled during the Investigation confirmed the definition of severe turbulence provided in ICAO Doc 4444 ‘Procedures for Air Navigation Services’ but no report of severe turbulence was subsequently made to ATC.

Damage to the main deck aft cabin ceiling panels caused by occupant impact. [Reproduced from the Official Report]

The Captain called the augmenting Captain from his crew rest compartment for a briefing and both members of the operating crew then returned to the flight deck. Recorded flight data showed that during the episode, vertical acceleration had ranged between -0.35g and +1.65g, the angle of attack had varied between -1.0° and + 5.5°and that the AP had remained engaged throughout with in excess of ten activations of the Load Alleviation Function (LAF) which, as designed, had helped to reduce the fatigue and static loading on the wing structure.

At the time the turbulence began, 15 of the 25 cabin crew, including the Cabin Manager, were ‘on duty’ and 13 of them were standing whilst performing their normal duties and the rest were taking their scheduled rest in the cabin crew rest compartment in the aft cabin with their waist seat belts fastened. Despite having the belts provided fastened, seven of these crew had hit their heads on the compartment ceiling. The Cabin Manager stated that as he and the two other cabin crew with him in the first class cabin had tried to reach their jumpseats, it felt like the aircraft had “dropped”. He began receiving reports from his crew which included the news that four of them in the upper deck aft galley had been injured when they “flew up” and hit the ceiling. Overall, it became apparent that the worst consequences of the turbulence had been in the aft cabins of both decks. The flight crew established contact with the Operator’s ground medical team to assess the condition of those injured and the Captain decided that the flight to destination should be completed and this was achieved without further event. It was found after the flight that the Compliance Monitoring System (CMS) post flight included a fault code which indicated that an overspeed loads analysis was required.

Although it was not possible to obtain satellite imagery of the weather system which had led to the turbulence, a comparison of the forecast provided to the crew prior to departure and a later one showed that although the pre-departure forecast (see the illustration below) had envisaged that the flight track across the Bay of Bengal would pass between two areas of occasional or isolated embedded cumulonimbus, by the time the flight got there, the area of convergence left of track on the pre departure forecast had been moving east at approximately 15 knots and had moved sufficiently for the track to pass through it where the severe turbulence had occurred. It also appeared likely that at about the position where the turbulence occurred, the flight would have been in the vicinity of a 120 knot easterly Jetstream expected to occur between FL 420 and FL 550. However, no related SIGMETS had been issued.

Part of the significant weather chart provided to the flight crew for pre flight briefing. [Reproduced from the Official Report]

The Cause of the accident was formally documented as “the severe turbulence acceleration forces in clear air imposed on the aircraft as it flew in an area affected by convective activity resulting in several unsecured passengers and cabin crew members forcefully impacting cabin furnishings".

A total of five Contributory Factors were also identified as follows:

1. The flight was planned north of an area with forecasted convective activity containing embedded cumulonimbus clouds.
2. The flight crew did not request updated weather information from air traffic control or pilot reports as the aircraft approached the area affected by the convective activity over the Bay of Bengal.
3. The wet turbulence area/s displayed in magenta on the navigation display did not prompt the flight crew to use the WXR best capabilities by using WXR manual mode, enabling a more accurate assessment of the distance margin with the area of greatest threat.
4. After turning the seat belt sign ON, the flight crew did not communicate with the Cabin Manager to secure the passenger cabins before the onset of the turbulence.
5. The Cabin Manager and other on duty cabin crew members were not aware that the fasten seat belt sign had been switched ON in spite of the fasten seat belt sign flashing for five seconds and fasten seat belt chime sounding.

Safety Action taken by Emirates whilst the GCAA Investigation was in progress was noted to have included, but not been limited to, the following:

• A flight crew eLearning module has been developed which includes the Weather Radar Differences Course for both the Airbus A380 and the operators other in-service type, the Boeing 777. Since its introduction, this module has also been updated to reflect changes in the A380 weather radar contained in a subsequent Airbus update to the A380 FCOM and a video has been added to the module.
• An eLearning module has been developed for both flight and cabin crew which includes a discussion on enhancing their communications in respect of the use of the seatbelt signs.
• Fourteen additional languages have been added to the seatbelt sign announcement and the IFE system now gets a simultaneous seatbelt awareness message when the seat belt signs are switched on because of turbulence.
• The volume of the seatbelt chime has been increased.

A total of six Safety Recommendations were issued as a result of the Investigation as follows:

• that Emirates review and enhance flight planning taking into consideration known historical geographical locations that are affected by seasonal en-route significant weather and meteorological conditions. [SR75/2020]
• that Emirates standardise and improve the accessibility of the lavatory handholds, the accessibility and identification of handholds in the wet and dry galleys and the accessibility of the handholds in the showers. [SR76/2020]
• that Emirates implement measures to mitigate the risk of crew members suffering head injuries during turbulence while resting in the crew rest compartment. [SR77/2020]
• that Airbus enhance the explanation and guidance in the FCOM and FCTM of the detection of wet turbulence so that there is clear understanding and actions required by pilots. [SR78/2020]
• that Airbus enhance the cockpit alerting system to include when wet turbulence is detected by the weather radar turbulence function. [SR79/2020]
• that Airbus facilitate the flight data recording of adverse weather precipitation detected by the weather radar as displayed on the navigation display so that this information can be used to enhance pilot training. [SR80/2020]

The Final Report was issued on 12 August 2020.

Related Articles

• Clear Air Turbulence (CAT)
• Jet Stream
• Weather Forecast
• Weather Radar
• In-flight Seat Belt Requirements
• Turbulence