Description

On 17 January 2021, a Boeing 777-300 (A6-EPN) being operated by Emirates on a scheduled international passenger flight from Dubai to Beirut as EK957 was in the descent to destination in day VMC when it encountered severe clear air turbulence which resulted in one serious and two minor injuries amongst the 50 passengers and minor injuries to four of the 12 cabin crew. The flight to destination was completed and on arrival the seriously injured passenger was hospitalised. A post flight inspection confirmed that the aircraft had not sustained any damage. 

Investigation

The event occurred within the Nicosia FIR and Cyprus was thereby identified as the ‘State of Occurrence’ and notification was therefore sent to the Cyprus Aircraft Accident and Incident Investigation Board (AAIIB) the same day. The subsequent Investigation was then conducted by the UAE GCAA Air Accident Investigation Sector, the corresponding agency of the State of the Operator and the State of Registry of the aircraft. Relevant data were obtained from both the FDR and the QAR but as the aircraft had continued to operate for more than two hours following the turbulence encounter, relevant data from the CVR had been overwritten. Assistance in the analysis of the FDR and QAR data was provided by Airbus.     
No information was recorded as to the flying experience of either the Captain, who was PF for the flight, or First Officer operating the aircraft during the accident involved.

What happened

The Operational Flight Plan (OFP) was downloaded by the flight crew 2 hours and 45 minutes prior to a delayed departure for the four hour flight to Beirut. Having examined the SIGWX chart provided with the OFP (see below) and noted the possibility of “occasional, isolated and embedded CB cloud” on track, the Captain advised the cabin crew of the possibility of in-flight turbulence during the descent. In their interview during the Investigation, the flight crew stated that after crossing the Egyptian coast northbound in the cruise at FL 360, CB cloud had been visible approximately 80 nm ahead of the aircraft. After transfer to Nicosia ACC, they were given advisory information to deviate to the right of track so as to avoid this area of convective weather ahead and invited to request any further deviation if required. At this point, the Captain recalled that the weather radar was displaying returns approximately 120 nm ahead in the 12 o’clock relative position and at approximately 50 nm ahead between the 9 and 11 o’clock relative position.

The SIGWX Chart provided to the flight crew prior to flight. [Reproduced from the Official Report]

About 40 minutes from destination, the seat belt signs were turned on and the cabin crew were instructed to begin securing the cabin in anticipation of turbulence ahead. Half a minute later, descent from FL 360 was commenced but after only 800 feet of descent, clear air turbulence began to become evident and the First Officer made a PA announcement “cabin crew take your seats”. The Captain stated that soon after this announcement had been made the turbulence had increased rapidly in severity in parallel with a sudden increase in airspeed and violent jolts had made the flight instruments difficult to read.  

The flight mode VNAV PATH was engaged and to avoid an overspeed, speed intervention was selected with a reduction towards 280 knots. However, as the speed continued to increase, the speedbrake was intermittently extended for additional control. In the event, the AP remained engaged and there were no overspeeds. However, because the vertical speed momentarily exceeded 5,000 fpm, VS mode was then selected.

Approximately 3½ minutes into the descent, a request was made to ATC for a further deviation to “20 nm right of track due to weather” which was approved with a turn onto 070°. At almost the same time, whilst the cabin crew were still in the process of securing the cabin, turbulence began without warning and quickly became worse. ATC were advised at around 0804 UTC that the turbulence being experienced was “moderate to heavy” but after a further 3½ minutes had elapsed ATC were then informed that the turbulence had decreased and was now “light” with the worst of it having been encountered between FL300 and FL270. Once below FL 240, the flight crew contacted the SCCM to assess the situation and were advised of the injuries which had been sustained. Most of those injured, including the seriously injured passenger, had been in or very close to the rearmost cabin section. The light turbulence continued for the remainder of the flight which was without further event and a landing at Beirut occurred approximately 35 minutes later.

An examination of available FDR data showed that the significant turbulent conditions occurred during the five minute period beginning two minutes after descent had commenced as the aircraft was descending from FL 292 to FL 222. Within this period, there were two intervals during which the severity of the turbulence was greatest. The first of these intervals began 3½ minutes after the beginning of descent and lasted for 48 seconds as the aircraft descended from FL284 to F255 and included a peak negative vertical acceleration of -0.49 followed immediately by a peak positive vertical acceleration of +1.72 g. The second of these intervals began one minute after the first ceased and lasted 17 seconds during which the aircraft descended from FL246 to FL238 and included a peak positive vertical acceleration of +1.71 g, and peak lateral accelerations of -0.19 g (to the left) and +0.35 g (to the right).

Why it happened

Boeing provided an analysis of the atmospheric conditions which it was likely had been affecting the aircraft at the time of the CAT encounter based on satellite images, atmospheric sounding, radio frequency lightning triangulation, convective available potential energy and Ellrod Index. The latter is used to assess turbulence potential attributable to wind shear along jet streams and weather fronts. An Ellrod Index mapping of the area where the aircraft was is shown below with the exact location where severe turbulence occurred marked. This location was in an area with an Ellrod value above 30 and next to one with a value above 50, which Boeing atmospheric physicists advised is capable of producing strong turbulence. Modelling indicated that the jet stream had reached speeds of about 170 knots just to the south and above the location of the aircraft.  

In summary, the aircraft position in relation to that of the jetstream at the time of the CAT encountered was presented as follows:

• The aircraft was passing through significant jetstream shear on the northern underside of a jet core; 
• Wind speed was changing both vertically and horizontally as the aircraft descended out of and passed north of the 170 knot jetstream; 
• The aircraft flew just above, or perhaps through the tops of, a line of thunderstorms at the same time.

The Ellrod Index showing the relative potential for CAT with the aircraft position indicated. [Reproduced from the Official Report]   

The Investigation discovered that, whilst most turbulence-related SIGMETs valid around the time the flight operated for the route taken were about convective (thunderstorm related) turbulence rather than CAT. However, there was one exception, a SIGMET about CAT in the area where it was encountered which, whilst it was received at the Emirates flight despatch/flight watch centre almost three hours prior to the turbulence episode, was not then sent to the flight by ACARS until it was too late to be of use. This delay occurred because the automatic software interface did not recognise a non standard format used by the originating Meteorological Watch Office at Larnaca. The existence of this SIGMET was also not communicated to the flight by Nicosia ACC. It was found that the operator’s SMS had not identified the potential for incompatibility between the format of inbound weather messages and the capability of the proprietary conversion software.

A review of the conduct of the flight and cabin crew in respect of the actual circumstances which prevailed during the flight concluded that both their performance and the procedures they followed were not contributory to the occurrence in any way. There were also no problems controlling the aircraft with the AP remaining engaged throughout. 

The Cause of the accident was formally documented as “the significant vertical g-forces imposed on the aircraft because of severe clear air turbulence, which caused an unsecured passenger to be forcefully lifted off her feet and impact cabin furnishings which resulted in her right foot sustaining a serious fracture injury”.

Three Contributory Factors were also identified as follows:

1. The influence of the jet stream and thunderstorm in the flight path area was significant enough to produce severe turbulence. 
2. The flight crew decision making lacked (the benefit of access to) critical information contained in SIGMET 3 issued the Larnaca Meteorological Watch Office (MWO) for the period from 0530Z to 0930Z which forecasted severe turbulence between FL260 and FL390, south of latitude N 34° 30’. 
3. The format of the SIGMET issued by the Larnaca MWO in a format which differed from the recommendations contained in ICAO Annex 3 and EU regulation No. 2020/469 such that the Weather Services International ‘Fusion’ application (WSI Fusion) used by the aircraft operator’s flight dispatch department did not generate a sufficiently timely alert for the SIGMET 3 report issued by the Larnaca MWO.

One formal Safety Recommendation was issued as a result of the Investigation as follows:

• that the ICAO engage the European Union Aviation Safety Agency (EASA) and the ICAO meteorology subject matter experts for the purpose of addressing the significance of Meteorological Watch Offices standardising the publication of SIGMET to avoid incompatibility with particular airline software applications employed to read SIGMETs. [SRxx/2021]

Editor's Note: At Initial Issue, the Official Report did not give a number to the Safety Recommendation issued.

Three in-text and un-numbered ‘safety improvement’ Recommendations were also made as follows:

1. that Emirates enhance the training of dispatchers on the software application employed to read and compile SIGMET messages for internal distribution using this occurrence as evidence-based training. 
2. that Emirates considers the risk of SIGMET format incompatibility in its safety management system and takes mitigation action to prevent mis-compilation by the Weather Services International (WSI) Fusion application used for processing messages. 
3. that Larnaca Meteorological Watch Office revises the format of issued SIGMETs and aligns it with the templates contained in the recommendations provided in ICAO Annex 3 - Meteorological Service for International Air Navigation, Appendix 6, Table A6-1A and with Appendix 5A to the European Union Aviation Safety Agency regulation (EU) No. 2020/469.    

The Final Report was issued on 29 July 2021.

Related Articles

• Clear Air Turbulence (CAT)
• Jet Stream
• Weather Radar
• SIGMET
• Weather Forecast
• Adverse weather deviations - ATC clearance and pilot and controller actions