On 4 May 2016, an Airbus A330-200 (A6-EYN) being operated by Etihad Airways on a scheduled international passenger flight from Abu Dhabi to Jakarta Soekarno-Hatta with an augmented crew and in the cruise at FL 390 in day VMC encountered a very brief period of severe turbulence which was not anticipated by the flight crew. One member of the cabin crew and six passengers sustained serious injuries and three cabin crew and 14 passengers sustained minor injuries. There was also some damage to the cabin interior. As the flight was nearing its destination, it was decided that the flight would continue as planned and the seriously injured passengers were hospitalised on arrival.
The location where the turbulence episode occurred. [Reproduced from the Official Report]
An Investigation was carried out by the Indonesian National Transportation Safety Committee (NTSC/KNKT). Data from the 1172-parameter FDR and the 2 hour CVR were successfully downloaded and used to support the Investigation.
It was noted that the Captain had a total of 11,280 flying hours which included 2,337 hours on type. The First Officer had a total of 2,656 flying hours which included 1,176 hours on type. A third pilot who was also included in the crew was occupying a supernumerary crew seat in the flight deck and had 2,280 hours total flying experience which included 568 hours on type.
It was established that the forecast chart issued by the World Area Forecast Centre (WAFC) London which was available to the crew at pre-flight briefing showed that thunderstorms and Cumulonimbus clouds reaching above FL 470 around Sumatra Island should be expected. After an uneventful flight, the aircraft was at FL 390 over Bangka Island just east of southern Sumatra and about 15 minutes from top of descent when the Captain called the Cabin Manager to the flight deck to receive an arrival brief. Having reported noting the presence of “Cumulus cloud” on the flight path and having just decided to deviate around it by an unrecorded margin and with the Cabin Manager now on the fight deck, the aircraft suddenly encountered a short episode of severe turbulence.
FDR data showed that for a period of 22 seconds, the aircraft altitude fluctuated between 250 feet below the cleared level and 450 feet above it at vertical speeds of between -2,500 and +2,000 fpm whilst pitch attitude ranged from -5° and +2°. Vertical acceleration during this period varied between -0.65g and +1.95g. Two seconds prior to the end of this episode, Normal Law changed to Alternate Law and, with the AP no longer engaged, the aircraft reverted to manual flight path control. The Captain, as PF, was then recorded as making pitch and roll inputs to his side stick of between -7.5° and 7.4° and between -19.3° and 14.9° respectively over a period of 6 seconds which ceased after Normal Law was reinstated and eventually, the AP was successfully re-engaged. There were no further turbulence episodes. Although the First Officer put the seat belt signs on as the turbulence began, this did not prevent a range of injuries to un-secured passengers and cabin crew.
The satellite image of cloud top temperatures reproduced below shows that the turbulence occurred in the vicinity of a vigorous thunderstorm cell with a sensed cloud top temperature well below -60° C. A detailed analysis of the FDR data carried out by Airbus for the Investigation showed that as the aircraft entered the area of turbulence at a speed of M 0.856, speed had rapidly dropped to M 0.844. It was considered that the brief reversion from Normal Law to Alternate Law had probably been due to a drop in airspeed of more than 30 knots within one second (recorded as from 258 knots to 227 knot) as a consequence of a change in wind component from a 10 knot headwind to a 39 knot tailwind in the same time interval.
The satellite weather image at the time of the turbulence episode (aircraft location indicated). [Reproduced from the Official Report]
It was not possible to determine how close the aircraft was to the observed convective cell when the turbulence episode occurred but it was noted that the Airbus FCTM and the relevant FAA Advisory Circular AC 120-88A both remind pilots that CAT outside of but associated with visible convective cells may occur up to 20nm away from them.
It was found that the Etihad ‘Safety & Emergency Procedure Manual’ (SEPM) included a requirement for the Captain of an aircraft which has been involved in an abnormal event to call the Cabin Manager to the flight deck and provide a NITS briefing, which in the investigated event would have served to organise an effective response prior to and after landing. However, no evidence that such a briefing had been conducted was found.
It was also noted that the Etihad Operations Manual required that a PAN be declared to ATC in the event that there is a condition concerning the safety of the aircraft or some person on board or within sight, which does not require immediate assistance but which might benefit from priority or other special service. However, the Captain had considered that in this instance the serious injuries sustained by some passengers did not warrant such action and judged that notification to the Company Ground Handling agent at the destination was sufficient.
The commonly-found Operations Manual (OM) policy that when the seat belt sign is switched off, cabin crew shall recommend passengers to “keep their seat belts fastened when seated” was included in the Etihad OM but since most of the injured passengers had not been secured during the turbulence episode, it was concluded that after more than 5 hours flying in clear weather with no turbulence since Departure, cabin crew attention to checking this had most likely been “degraded”.
The Conclusion of the Investigation was that at the time of the upset the aircraft had been flying within the turbulence area of a thunderstorm which the crew had not anticipated and (therefore) no warning had been provided to the cabin crew and passengers.
Three Safety Recommendations were made at the conclusion of the Investigation as follows:
- that Etihad Airways reviews the effectiveness of their use of seatbelt policies to ensure that passengers fasten their seatbelt when seated.
- that Etihad Airways ensures that their pilots conduct a ‘NITS’ briefing after an abnormal flight event as described in section 8.1.2 of their Safety Emergency & Procedure Manual.
- that the Indonesian Directorate General of Civil Aviation (DGCA) requires Indonesian aircraft operators to review the effectiveness of their use of seatbelt policies to ensure that passengers fasten their seatbelt when seated.
The Final Report was released on 29 December 2017.