On 28 May 2006, an Airbus A320 being operated by Vueling on a scheduled domestic passenger flight from Barcelona to Santiago de Compostela in day Visual Meteorological Conditions (VMC) encountered sudden significant turbulence in the climb through FL325 to cleared level FL370 and was forced down to FL310 before the flight crew regained control. Seven of the 142 occupants, including three of the cabin crew, sustained minor injuries as a result and some superficial internal damage to the aircraft also occurred caused by the impact of an unrestrained cabin service cart.
An Investigation was carried out by the CIAAIC. It was noted that the prescribed aircraft inspections following an excessive turbulence encounter had found no damage but that when downloaded flight data sent to the manufacturer by the Operator revealed, nine days later, that limit loads on the airframe may have been exceeded during the event, the aircraft was grounded for more detailed review. Airbus found that in the case of lateral loads, the vertical stabiliser and rudder had been subjected to loads of 125% of the limit load value compared to the maximum theoretical resistance of the tail prior to failure is 150% of the limit load. It was found that the wing tips and the aft section of the fuselage had been subjected to loads below the limit load.
The Cockpit Voice Recorder (CVR) had not been isolated after the event and the incident recording had therefore been overwritten but the DFDR data was successfully downloaded and replayed. It showed that the aircraft had been at a speed of 0.78M with the AP and autothrottle (A/T) engaged when the turbulence encounter occurred and a sudden 40° right bank followed. In response to increase in bank angle, the AP had attempted to counteract it with deflection of the ailerons to their maximum value of 8° at 250 knots as the vertical load factor concurrently increased to 1.23g. Both pilots had responded by moving their sidesticks, which had led to the disconnection of the AP. The aircraft commander, who had been PM had reacted to the decreased vertical load factor by commanding a pitch up whilst almost simultaneously, the First Officer had input an opposite pitch down command to his sidestick. For a period of 21 seconds, both pilots continued to make simultaneous inputs to their sidesticks; initially this had included the pushing of their respective sidesticks to the maximum available in both lateral directions. An input to the left rudder pedal was also recorded and the A/T had been disconnected.
The culmination of the crew response was that over a short period of time, sharp lateral oscillation between maximum bank angles of 33° left and 49° right and sharp pitch variation between +8.7° and -0.4° had occurred. The effect of this had been longitudinal load factors between +1.69g and -0.45g and lateral load factors between 0.47g left and 0.32g right. It was noted that as the simultaneous inputs in pitch had been largely in opposing directions, the resulting movement had been smooth and thus had little actual effect on the pitch of the aircraft. After this, and as a result of the various pitch-down commands, the aircraft had made the descent to FL310 before control was regained and the AP and A/T re-engaged.
Having concluded that the initiation of the upset could be attributed to a wake vortex encounter, the Investigation noted that the strength of such vortices depends on a number of factors including aircraft weight, speed, configuration, wing span and wing angle of attack. It also noted that whilst the process of vortex dissipation is not fully understood, “studies so far indicate that both the mixing action of the eddy viscosity and the interaction between the vortices themselves have an effect” and that other factors that may have influence in vortex longevity include wind velocity and the extent to which atmospheric mechanical turbulence prevails. In the investigated event, it was found that there had been high atmospheric stability which would have probably contributed to increased time required for vortex dispersal.
The Cause of the upset was determined as:
“The turbulence initiated by the wake of an Airbus A340-300 that was on the same airway, 10.13 NM ahead of the Vueling Airbus A320-200, and on the same course, flying towards point “Kuman” at FL330 and a ground speed of 464 knots. The extent of the turbulence was such that it could not be counteracted by the autopilot.
The actions carried out subsequently by the crew, simultaneous sidestick inputs, retarding the throttles, disconnecting the autothrust and, especially, stepping on the left pedal, were not in accordance with the aircraft’s flight procedures and could contribute to exacerbating the effects of the external turbulence.”
It was noted that:
“When an abnormal or emergency situation occurs during a flight, the crew must take immediate actions to neutralize it by following the proper procedures. In order to execute these actions quickly and accurately, the crew must carry them out “automatically”. This is achieved through instruction and training. The investigation into this incident revealed that the crew did not properly adhere to the procedures required by the situation.”
Two Safety Recommendations were issued as a result of the Investigation:
- That the aircraft operator, Vueling, review and enhance its Airbus A-320 crew training programs so as to improve the crews’ knowledge and application of aircraft procedures, in particular of these applied to dual sidestick inputs, flying in severe turbulence and rudder use. [REC 03/11]
- That the operator of the aircraft, Vueling, review and enhance its training programs in Crew Resources Management CRM. [REC 04/11]
The Final Report was approved on 21 February 2011 and an English language version was subsequently made available.