On 18 May 2011, the crew of a Saab 340A (LV-CEJ) being operated by Sol Airlines on a scheduled domestic passenger flight from Neuquén to Comodoro Rivadavia in night Instrument Meteorological Conditions (IMC) encountered airframe icing of greater severity than expected whilst climbing to the planned cruise altitude of FL190. Despite initiating a descent, the aircraft stalled in continuing icing conditions as they were unable to recover control. The aircraft crashed into the terrain below, caught fire and was destroyed. All 22 occupants were killed.
An Investigation was carried out by the Argentinean Civil Aviation Accident Investigation Board (JIAAC). The FDR and CVR were both recovered and successfully downloaded and provided data essential to establishing the circumstances which led to the accident.
It was found that the 45 year-old Captain had accumulated over 2000 hours flying time on type and had previous experience on other twin turboprop aircraft and a total of 6902 flying hours. The 37 year-old Co-Pilot had accumulated 1340 hours in total including 285 hours on type, having recently become a pilot with the Operator after previous employment with them as a maintenance technician. He "had had only been approved to fly without an instructor one month prior to the accident". The effect of the obviously large difference in flying experience between the two pilots was noted as likely to have been operationally significant.
The Investigation found no evidence that any airworthiness deficiencies had any bearing on the accident, although it did find that the aircraft involved had not been in compliance with extant Airworthiness Directives unrelated to the accident. It was noted that the accident aircraft did not have the optional modification to install an 'Ice Speed System' - developed in Canada in recognition of the greater risk of exposure to significant airframe icing conditions - incorporated. It was found that after departure from Neuquén, the aircraft had never reached the intended and cleared cruising level, FL190, and noted that, with the AP engaged throughout and having reached a maximum of FL178 using the Operator's standard power management procedure - the 'Constant ITT' method, the crew had not then (and had not earlier) increased power to the maximum permitted level in order reach FL190. FDR data showed that there had been no significant changes in climb power prior to the aircraft reaching FL140. It was noted that "the crew were aware of the formation of ice on the aircraft from the beginning" and that "anti-ice protection systems were activated upon detection of icing conditions" and functioned normally. However, the selection of engine anti-ice to 'on' was not followed by a change in AP mode from CLIMB LOW' to 'IAS', the latter being the only mode which permitted AP use to continue, but the effect of this was that the IAS achieved as climb performance steadily reduced dropped below those specified in the AOM.
Some 24 minutes after take off it became clear that it was not practicable to continue climbing any further and, having levelled off at FL178, the crew "used V/S mode in order to increase speed" but this did not have the desired effect and the aircraft began to approach an aerodynamic stall, with activation of the stick shaker following. With the weather radar indicating that that the aircraft was about to exit cloud and a conversation with the pilot of another Company aircraft routing in the opposite direction at FL140 who reported having no icing problems, the crew response after remaining at FL178 for approximately 9 minutes, was to begin a descent to FL140. This was accomplished with the AP still engaged in V/S mode set to - 750fpm. Airspeed in this descent increased to "an average of 173 knots" with power set to 58%. The slightly positive vertical pitch during this 5 minute descent "led to ice forming on the bottom part of the airframe" which increased the weight and adversely affected the aerodynamic qualities of the aircraft. FDR data suggested that towards the end of the descent, "icing conditions rapidly worsened" and as a result, the aircraft "rapidly lost speed on levelling at FL140 despite power being increased". When buffeting began at an indicated speed of 145 knot indicating proximity to an aerodynamic stall, the crew attributed it to propeller ice accumulation and responded by increasing the propeller RPM to maximum. The aircraft remained in buffet for 13 seconds as the indicated speed slowly decreased and the stick shaker was activated at an airspeed of 137 knots.
Then, uncommanded disconnection of the AP occurred and "the aircraft commenced a series of uncommanded rolls with a nose down attitude". The crew were unable to level the wings or alter the pitch attitude even though they performed actions prescribed for recovery and it was concluded by the Investigation that the reason for this was "because the control surfaces (ailerons and elevators) were not sufficiently aerodynamic as a result of severe icing". It was then apparent from FDR data that the crew had then induced - and entered into - a secondary stall - which caused further stick shaker activations. A sustained and uncontrolled descent began with the aircraft remaining fully stalled. It was considered, "based on the aircraft's impact", that a sequence of uncontrolled rolls accompanied by a nose down attitude continued until impact.
The Investigation also found relevant deficiencies in VHF communications coverage along the route being flown and specifically in the area where the accident occurred. It also noted that according to the meteorological information provided prior to take off, only light icing was forecast. The lack of a current forecast in this particular case was considered to have been attributable to systemic weakness in the provision of aviation en route forecasts exacerbated by a lack of awareness of their potential significance by both dispatchers and pilots.
An infrared weather satellite picture of the area where the accident occurred at the time it occurred is reproduced below.
The false colour satellite image from GOES E at the time of the accident (reproduced from the Official Report)
The formal documentation of the Cause of the accident was as follows:
During a commercial, domestic passenger flight, while cruising, the crew lost control of the aircraft, which uncontrollably impacted the ground due to severe ice formation caused by the following factors:
- Entering an area with icing conditions without adequately monitoring the warning signals from the external environment (temperature, cloudiness, precipitation and ice accumulation) or the internal (speed, angle of attack), which allowed for prolonged operations in icing conditions to take place.
- Receiving a forecast for slight icing which - given that the aircraft encountered severe icing conditions - led to a lack of understanding regarding the specific meteorological danger.
- Inadequately evaluating the risks, which led to mitigating measures such as adequate briefing (distribution of tasks in the cockpit, review of the de-icing systems, limitations, use of power, use of autopilot, diversion strategy etc.) not being adopted.
- Levels of stress increasing, due to operations not having the expected effects, which led the crew to lose focus on other issues.
- Icing conditions that surpassed the aircraft's ice protection systems, which were certified for the aircraft (FAR 25 Appendix C).
- Inadequate use of speed, by maintaining the speed close to stall speed during flight in icing conditions.
- Inadequate use of the autopilot, by not selecting the IAS mode when flying in icing conditions.
- Partially carrying out the procedures established in the Flight Manual and the Operations Manual, when entering into areas with severe icing conditions.
- Realizing late that the aircraft had started to stall, because the buffeting that foretells a stall was confused with the vibrations that signify ice contamination on the propellers.
- Activation of the Stick Shaker and Stall Warning at a lower speed than expected in icing conditions.
- Using a stall recovery technique which prioritized the reduction of the angle of attack at the expense of altitude loss, and which was inappropriate for the flight conditions.
- The aileron flight controls reacting in an unusual manner when the aircraft lost control, probably due to the accumulation of ice on the surfaces of these, which made it impossible for the aircraft to recover.
- The increasingly stressful situation of the crew, which affected its operational decision-making.
A number of "Potentially Dangerous Conditions" that were not causal were also identified as follows:
- Pre-existing condition - The Operator's organisational and operational context
- Outdated operating specifications
- Only partial fulfilment of the Regulations established in section 121.133 (a) of the Argentine Civil Aviation Regulations (RAAC) with regard to the updating of Operator Operation Manuals (OOM)
- Crew scheduling being performed by an external management organisation
- Failure to keep aircraft manuals up to date
- Training Manuals which did not include the aircraft manufacturer's advice regarding stall recovery
- Only partial fulfilment of the Regulations established in section 121.407, Appendix G of the Argentine Civil Aviation Regulations (RAAC) with regard to the provision of Line-Oriented Simulator training for pilots
- Failure to act on Airworthiness Directives related to propeller maintenance.
- Systematic Context
- A lack of VHF coverage on the route which the aircraft was travelling on
- Meteorological services not being accessible during all of the company's operating hours at all of the (scheduled) stopover points.
A total of 18 specific and directed Safety Recommendations were made as a result of the Investigation as follows:
- that the National Civil Aviation Administration - National Directorate of Operational Safety should exercise effective control over updates in the Operator's Operations Manual (OOM), which all air operators are required to follow, in accordance with section 121.133 of the Argentine Civil Aviation Regulations (RAAC).
- that the National Civil Aviation Administration - National Directorate of Operational Safety should implement a system which tracks compliance with regulations in accordance with the growth and development of the air operators and the service providers.
- that the National Civil Aviation Administration - Directorate of Personnel Licensing - Department, Educational Control should make it a requirement that theoretical courses for operating personnel (pilots, dispatchers, etc.) which are taught at air operators' training centres, with regard to specific subjects such as meteorology, have to be taught (or supervised) by professionals with experience in the field of expertise so that they are properly prepared for assessing and managing the risks inherent in flight.
- that the National Civil Aviation Administration - Directorate of Personnel Licensing - Department, Educational Control should incorporate the requirements set forth in the previous recommendation into the applicable Argentine Civil Aviation Regulations (RAAC).
- that the National Civil Aviation Administration - Directorate of Personnel Licensing - Department, Educational Control should make UPSET RECOVERY a mandatory part of the flight simulator training curricula, which is to be practised both during the initial and the recurring courses and evaluated during the suitability inspection.
- that the National Civil Aviation Administration - Directorate of Personnel Licensing - Department, Educational Control should, with the aim of increasing the quality of the flight crew training, establish a workshop programme for instructors who teach initial and recurrent training for commercial airline operators, during which technical and didactic subjects are taught, that directly focus and relate to operational security.
- that the National Meteorological Service should consider the possibility of extending the service hours of the aerodrome's meteorological offices, to cover all the hours that these aerodromes operate, and to provide updated information that effectively contributes to Operational Safety.
- that the National Meteorological Service should consider modifying the aeronautical information system so that an aviation area forecast (ARFOR) which remains the same is not merely updated as NOSIG, but repeats the information of the previous ARFOR.
- that the Argentine Air Force - General Directorate of Air Traffic Control should, in order to establish an efficient network of aeronautical communications, implement a system which ensures that aeronautical communication traffic works both ways, in areas where this is deficient. In addition, the benefit of establishing, implementing and maintaining an effective VHF communication network, with permanent national coverage, should be evaluated.
- that the Argentine Air Force - General Directorate of Air Traffic Control should, in order to establish clear guidelines regarding the requirements of on-board communication equipment, conduct a proper study on aircraft traffic communications to determine the need for aircraft to be equipped with HF voice communication equipment.
- that the Swedish Accident Investigation Authority (SHK) should review the possibility of recommending that the aircraft manufacturer has Special Safety Bulletins, with the findings of this Investigation and those of three similar incidents that have previously occurred.
- that the Swedish Accident Investigation Authority (SHK) should evaluate the possibility of recommending that designers of flight simulator software and hardware, include simulations of various situations in icing conditions (general, disrupting aerodynamic functions, blocking control surfaces, affecting the flight attitude due to a shift in CG, etc.), in order to provide better training with regard to recognising these flight situations, as well as the techniques that need to be employed in these situations.
- that the Swedish Accident Investigation Authority (SHK) should, with the aim of increasing the aircraft's technological defences, evaluate the possibility of incorporating a warning system which inhibits the use of the climb mode in autopilot upon the activation of the de-icing system.
- that the Operator should, in order to increase the level of operational safety, include severe icing as a specific danger in the company's Safety Management System (SMS) programme so that the defences (technological, standards, procedures and training) can mitigate the risks.
- that the Operator should use the aircraft's original Flight Manual and avoid the use of adapted versions that do not contain all of the required information. In addition, compliance with the section 121.133 (a) of the Argentine Civil Aviation Regulations (RAAC) with regard to updating the Operator Operation Manuals (OOM).
- that the Operator should improve safety by establishing means, resources and procedures which will emphasise the theoretical and practical instruction/training of upset and stall recovery in accordance with the manufacturer's instructions so that new training practices are created, that involve the flight crew effectively identifying the signs and the aerodynamic phenomena that occur during flight prior to stalling (aeroelastic flutter, among others).
- that the Operator, with the aim of establishing better internal communication channels, should establish measures in order for management to implement an effective method of wide dissemination and strict compliance between the crews as established in AFM Supplement No. 1 "Operations in cold and icing conditions". An effective method should also be established to evaluate the results of training/instruction, in order to ensure that the pilots have adequate knowledge and practice, in accordance with the procedures established in the AOM and AFM, SOP (AFM Supplement No. 1 "Operations in cold and icing conditions", using the autopilot, etc.)
- that the Operator should consider the possibility of implementing modification No. 2650, "Ice Speed System", on all of its Saab 340 aircraft, as it is a tool that increases safety by alerting the crew of abnormal speed conditions during flight in icing areas.
The Safety Recommendations section of the Final Report also included a generic recommendation directed to the ICAO AIG (but proposing that they delegate implementation to Aviation Authorities, Aircraft Manufacturers, Pilot Training Centres and Air Operators) that enhanced training of pilots should be implemented in respect of "the recognition of and recovery from stalls and upsets ... alongside the ... systems of warning, protection and automation".
The Final Report in Spanish was published by the JIAAC on 10 March 2015 and an Unofficial English Translation was subsequently made available by the Swedish Accident Investigation Authority.