On 31 July 2021, a Boeing 737-800 (EI-EPC) being operated by Ryanair on a scheduled international passenger flight from Fez to Barcelona and in a daylight descent passing FL180 in an area of forecast moderate turbulence encountered a two minute episode of convective turbulence close to storm clouds but still in VMC. When the indicated airspeed increased towards Vmo, the autopilot was disconnected and an abrupt manual pitch up followed immediately by an equally abrupt pitch down led to serious injuries to two of the cabin crew and one of the 172 passengers. The three injured individuals were taken to hospital by ambulance on arrival.
An Accident Investigation was carried out by the Spanish Commission for the Investigation of Accidents and Incidents (CIAIAC). Relevant flight data was available from the CVR and QAR. Crew statements and recorded ATC data were also available.
It was noted that the flight crew consisted of a 29 year-old Captain acting as PF who had been issued with an ATPL (airline transport pilot licence) just over 2 years previously and had since been promoted to Captain. He had a total of 4,041 hours flying experience but how much of that had been gained in command on type and in total on type was not recorded. His type training had explicitly included “procedures upon entering an area of severe turbulence, recovery from overspeed and awareness of g-load factor”. The 26 year-old First Officer had a total of 1,692 hours flying experience on all types. An off-duty pilot employed by the operator was occupying the supernumerary crew seat in the flight deck. The accident flight was the third out of four which the crew had been rostered for.
A SIGMET indicated storms were likely in the area enclosed by the red line. [Reproduced from the Official Report
It was established that about an hour after takeoff, with the cabin crew already informed that the forecast weather conditions meant that it was likely that turbulence would be encountered during the descent, descent had commenced towards the ‘MATEX’ waypoint to join the MATEX 2E STAR (see the illustration below) for an eventual landing on runway 07L at Barcelona. For the initial descent to FL200 with the AP engaged, ‘MCP SPD’ was used. Since ATC had delayed the start of descent in order to comply with the altitude restrictions of the intended STAR, the crew had set idle thrust (which automatically reset the A/T mode to ‘armed’ and then began to gradually increased the speed selected on the MCP to increase the rate of descent which was taking place in the presence of a 70 knot tailwind component. Due to Covid-related considerations, the seat belt signs remained on throughout the flight.
As the aircraft was descending through FL280 outbound from MATEX towards the ‘BL028’ waypoint, cumulonimbus could be seen on the aircraft weather radar and an avoidance turn to the left was requested and approved to fly a radar heading of 065º. The cabin crew were immediately reminded of the risk of turbulence and instructed to secure the cabin for landing. Further descent to FL080 was then given along with clearance to proceed direct to waypoint ‘ASTEK’, the IF for the runway 07L approach “if flight conditions permitted”. The recorded/indicated tailwind component was now slightly less at 58 knots.
The aircraft ground track during its descent after ‘MATEX’ (the numbers relate to spot height and airspeed at timed points taken from the FDR data). [Reproduced from the Official Report]
Whilst descending through FL 180 two minutes after instructing the cabin crew to secure the cabin, an area of turbulence was entered. At this time, the engines were still at idle thrust and the MCP selected speed was 320 knots (the recorded speed was 325 KCAS). Over the next 29 seconds, the MCP-selected speed was reduced to 308 knots as the recorded speed fluctuated between 306 KCAS and 329 KCAS as vertical acceleration of between +0.50 g and +1.36 g was recorded changing at a rate of up to 0.7 g per second. After half a minute of this, as the aircraft descended through FL170, the recorded speed suddenly increased from 322 KCAS to 334 KCAS within one second.
The Captain subsequently explained that the PFD speed trend vector had started to show large oscillations and he considered that the turbulence had become severe and judged that the AP was no longer able to maintain the selected speed so he had disengaged it “to avoid exceeding the 340 knot VMO”. This action was immediately followed by a +48 lb (rearward pull) on the control column and in one second, the recorded pitch attitude increased from -1.8º to +6.2º, reaching a vertical acceleration of +3.09 g. This was immediately followed by a -51 lb (forward push) on the control column in the opposite direction and an immediate reduction in pitch attitude to -1.6º accompanied by a vertical acceleration of -0.18 g. The ‘AFT ENTRY DOOR’ EICAS caution was noted by the First Officer immediately following this violent pitch attitude fluctuation.
At the time the AP was disconnected, the cabin crew were in the process of securing the cabin. Three of them were in the rear galley where the SCCM was making a PA warning passengers of likelihood of continued turbulence and the fourth was at row 29, attending to a passenger request. All four were thrown against different parts of the cabin and fell to the floor. Two, including the SCCM were incapacitated and the No 2 assumed lead responsibilty.
Thereafter, the aircraft remained almost level at FL170 with normal forces on the control column recorded and positive pitch angles as the speed began to reduce. Descent was resumed at 283KCAS, the AP was re-engaged at FL 159 and the turbulence ceased altogether just over two minutes after it began. Re- clearance to 3000 feet was received and a speed of 250knots was requested until reaching ‘ASTEK’ as number 1 for the ILS Z to runway 07L.
It was noted that no report of a significant turbulence encounter was received by ATC which was contrary to the applicable regulations ((EU) 923/2012 of 26 September 2012 and the Air Traffic Regulation) which require such a report if a flight encounters moderate or severe turbulence.
Why It Happened
The moderate turbulence encountered was as expected based on forecast information provided prior to departure from Fez. Also, the OFP for the flight indicated that wind shear of between 3 and 4 units (equivalent to moderate turbulence) could be expected whilst flying the expected STAR.
The FCOM was noted as recommending leaving the AP and/or A/T engaged “unless their performance is unsatisfactory” and that temporary speed variations of between 10 and 15 knots may occur. It stated that in the event of severe turbulence, use of the AP is “optional” with a speed of 0.76 Mach/280 knots maintaining a constant pitch attitude with wings level thus permitting variations in altitude and warned explicitly that “flight control systems should not be subjected to sudden large inputs”.
The FCTM was found to recommend that in the event of unexpected overspeed, “the autopilot should remain engaged unless it is clear that it is not correcting the overspeed event”. It also warned that “disengaging the autopilot to prevent or reduce the severity of an accidental overspeed event can result in abrupt changes to the pitch angle”.
The operator’s procedures for cabin crew stated that they should not wait to receive a warning from the flight crew before ceasing their activities if they consider that the level of turbulence is such that they should cease their activities and secure themselves in their seats.
The Probable Cause of the accident was formally documented as "the series of actions taken by the PF to prevent the aircraft from exceeding the VMO while it was flying through an area of turbulence which resulted in vertical acceleration that caused severe injuries to a passenger and two cabin crew members”.
The Final Report of the Investigation was approved on 30 March and subsequently published simultaneously in the definitive Spanish language and in an English language translation on 22 June 2022. No Safety Recommendations were made.