On 7 September 2012, an Airbus A320-200 (EI-CVA) being operated by Aer Lingus on a scheduled passenger flight from Milan Malpensa to Dublin under R/T callsign Shamrock 43P with 68 occupants, had just begun descent from the cruise in smooth flying conditions when there was a sudden and unexpected upset which led to one of the cabin crew sustaining a serious injury. Once the flight crew had been advised of this, a ‘PAN’ call was made to facilitate expedited completion of the flight and a medical doctor travelling as a passenger rendered interim assistance.
When notified of the occurrence, the UK Air Accidents Investigation Branch (AAIB) delegated the Investigation to the Irish AAIU. The DFDR was obtained from the Operator and successfully downloaded. The Cockpit Voice Recorder (CVR) was also downloaded and included the relevant period.
It was established that the aircraft commander had been designated as PF and noted that both pilots were experienced generally and on aircraft type and had been employed by Aer Lingus for “several years”. The Investigation noted that the CVR record showed that “prior to the occurrence, communications between the flight crew were professional and relaxed”.
It was established that the flight had been given initial descent by London ACC from FL380 to FL340 to expect FL200 by a given position which had been correctly read back by the PM. However, the PF had set FL240 on the FCU and this had not been picked up by the required cross check. With the aircraft descending in OPS DES mode, the PM stated that he was about to make the standard call ‘2000 to go’ as the aircraft passed FL360 when he noticed that FL240 was set on the FCU and that he had written FL240 on the flight log. He attributed the latter to having not written down the clearance concurrently with the read back to ATC but instead subsequently copying it down from the FCU.
By the time clarification had been sought and received from ATC that the cleared level was FL340, the aircraft was passing FL 345. In response, having not perceived a quick enough change in rate of descent as a result of re-selecting the FCU altitude to FL340, the PF disconnected the AP and pitched up manually with the result that vertical acceleration suddenly “increased sharply from 1g to 1.7g and then decreased to approximately 0.8g over a period of 3.4 seconds” before oscillating slightly and returning to 1g. It was noted that the vertical speed of the interrupted descent had initially been 4000 fpm but that this had reduced to 2800 fpm by the time the occurrence commenced. Once level at FL340, the AP was re-engaged.
The seat belt signs were off and within a minute of the ‘upset’ the senior cabin crew member contacted the flight deck to advise of the injury to the No 3 Cabin Crew who had been working in the rear galley area. No other occupant was affected.
The Investigation noted that at higher altitudes “flight handling characteristics change significantly with reduced aerodynamic damping affecting the overall stability and controllability of the aircraft” and that “the reduced density of air flowing over control surfaces at high altitudes also decreases their effectiveness”. It was noted that in the A320 this effect is masked by Fly-By-Wire control system in Normal Law which modifies control surface displacement so that the aircraft responds consistently to given side stick inputs.
It was also noted that the recommended way of using the engaged automation to command an immediate level off “pressing the V/S-FPA selector knob (on the FCU Control Panel) will command the aircraft to conduct a smooth level off to zero vertical speed”. It was considered that “it is possible that the reason the PF did not push the V/S-FPA knob is that this action is rarely required in normal operations whereas reselecting a flight level is a routine activity”.
In respect of resolving discrepancies of view on clearances, it was noted that the Operations Manual Part ‘A’ required that, as had happened in this case, they should be resolved “with ATC and not between crew members” -, although in this instance, ATC clarification was not sought until after the PM had initially asked the PF to “level off” without giving a reason, an intervention which it was considered had temporarily confused the PF “as he was evidently unsure why it was made”.
It was concluded that at the time of the occurrence, “the aircraft was still at an altitude where careful consideration should have been given to the aerodynamic handling aspects of high altitude operations” and that “industry recommendations are that consideration should always be given to aircraft energy management and the careful manipulation of flight controls (with) large and/or abrupt manual flight control inputs … avoided”. It was also concluded from the available CVR evidence that “the Flight Crew did not appreciate the effects of what seemed to them (even afterwards), a reasonable control input” when attempting to comply with their ATC clearance.
It was determined that the Probable Cause of the incident was that “an abrupt manual pitch input resulted in higher than usual g forces being experienced by the Cabin Crew Members”.
It was further determined that there were two Contributory Causes:
- The ATC Flight Level clearance was not immediately recorded when received.
- Unclear communication between the Flight Crew when confusion arose over the cleared flight level.
One Safety Recommendation was made as a result of the Investigation as follows:
- that Aer Lingus should review its training programs and procedures to ensure that its flight crew are familiar with the use of levelling off procedures and the risks associated with manual flight control input during high altitude operations. [IRLD2014012]
The Final Report was published on 27 May 2014.