Description

On 31 August 2019, an Airbus AS350 B3 (LN-OFU) being operated by Helitrans on a local sightseeing flight which had departed a festival site at Kvenvikmoen in north Norway was manoeuvring in day VMC at low level five minutes into the flight when control was lost. Terrain impact followed soon afterwards and an intense fire destroyed the wreckage and neither the pilot nor any of the five passengers survived.    

Investigation

A comprehensive Investigation was carried out by the Norwegian Safety Investigation Authority (NSIA). It made extensive use of flight tracking data captured by the helicopter operator’s GNSS-based system and data commercially captured by a third party. Tracking data was not available for the minute immediately before the helicopter crashed because it entered airspace which was within an ADS-B shadow. Although the helicopter was fitted with an Appareo Vision 1000 recorder, its memory unit was not crash protected and was severely damaged by the impact and the fire which followed.

The Flight Crew

The 27 year-old Pilot had a total of 256 hours flying experience in helicopters of which 17 hours were on type, all obtained during the 2019 summer season. He had been issued with a Swedish CPL(H) in 2016 and had first been employed on a summer season contract by Helitrans the following year. In May 2018, he completed AS350 type rating training which included “a demonstration of and familiarisation with the servo-transparency phenomenon, a known manoeuvring limitation in the helicopter type”. This type rating was obtained without a Full Flight Simulator (FFS) being used as this “was not strictly required” although the Investigation did find an independent assessment from 2012 that concluded it was very realistic and of very definite value for pilot type training. In 2019, he was again employed by Helitrans on a summer season contract flying EC120 and AS350 helicopters and in the five months up to the time of the accident had flown 50 hours with passengers in either the EC120 or the AS350 after passing a Line Check valid for both types on 20 April 2019. Pilots at the operator characterised the accident pilot as “a meticulous, structured, calm and cautious person (who gave) no indication that he flew in an unsafe manner”. 

What Happened

In the early afternoon, the accident helicopter was positioned from the company base at Alta Airport to the festival site from where the sightseeing flights were going to be offered. During this 7 minute flight, the left hand front had been occupied by another more experienced company pilot who had earlier completed some sling load work on a different AS35 and was going to act as loadmaster for the sightseeing flights. The left seat in this particular helicopter had been configured specifically for sightseeing flights so the flight controls had been removed to allow a total of five passengers to be carried. 

The prevailing weather conditions including visibility were good. The first ten minute long sightseeing flight was subsequently completed uneventfully and when the second took off, again fully loaded with more fuel than necessary to safely complete the planned short flight which it was noted would have given a high (but within limits) load on the main rotor. The second flight followed a similar route (see the illustration below) and pattern of height variation as it continued on an elongated right hand circuit to the northeast of the festival site. During the flight, evidence that “the helicopter exceeded the limit value for pitch (nose down) given in the operator’s procedures for sightseeing flights” was subsequently found. 

The reconstructed tracks of the ferry flight and both the initial and accident sightseeing flights. [Reproduced from the Official Report]

The flight proceeded much as the previous one until it neared the festival site when the final recorded data available showed a significant change in the rate of descent and an increase in the ground speed. It appeared that the helicopter was descending in a slight right turn at a low altitude above the terrain immediately before the accident occurred with the last recorded data showing that it was below 300 feet. Terrain impact at what was assessed to be both a low ground speed and a low vertical speed occurred whilst the helicopter was heading towards upward sloping terrain north of the last documented flight path after it had changed course by almost 180°.

Whilst it was apparent that both the tail and main rotors had been rotating at impact, it was noted that despite the relatively low energy impact, it had been sufficient to “rip open the fuel tank, separate the tail boom from the fuselage and misalign the engine and main gearbox” and it was clear that a fierce fuel-fed fire had begun almost immediately and was likely to have been the primary cause of all the fatalities. 

A comparison of the reconstructed vertical profile of the final part of the accident flight and that from the corresponding part of the earlier flight is shown below. The total flight time of the accident flight was between six and seven minutes.

The final part of the accident flight (yellow) and the corresponding part of the first flight (blue). [Reproduced from the Official Report]

Why It Happened

An assessment of the wreckage and the accident site suggested that the pilot had been manoeuvring the helicopter as impact became unavoidable and had been conscious and close to regaining control when it occurred. This ruled out a high energy impact as the primary cause of the fatalities which were assessed to have been the consequence of the fire which followed. This highlighted the fact that although the helicopter involved was almost new, it was not required by the prevailing type certification requirements to have a Crash Resistant Fuel System (CRFS). Within the European regulatory system, these regulations only mandate a CRFS for helicopters whose initial type certificates were issued from October 1994 - after the one for the accident helicopter type - despite a considerable and worldwide history of otherwise avoidable fatalities in Airbus AS50 accidents in recent years.

Editors Note: Having finally begun delivering all new AS50 helicopters with a CRFS from October 2020, Airbus has also made a retrofit CRFS kit for in-service AS50 B3 helicopters available as an optional modification.

The assessment that terrain impact had been ‘low energy’ and that what appeared to be recovery action by the pilot was being attempted at impact led to a focus on what might have caused the apparent loss of control and whether more height above terrain when it occurred might have made recovery of control by the pilot possible. Significantly, no airworthiness issues were found that could have had any influence on the sequence of events prior to the accident.  

Attention turned to the known susceptibility of the A350 to a phenomenon known as servo transparency and whether it might have been the underlying reason for the loss of control. By elimination of other possible scenarios based on evidence collected, it was considered that “the margin to enter servo transparency was very small due to the high mass and speed” and that this may even already have occurred at the time of the last recorded data. Certainly, if the helicopter had not entered servo transparency at this time, “only small corrections to the flight controls” would probably have been enough to result in it, particularly since it was recognised that the recorded “slight right turn” would have increased the probability of servo transparency being entered. 

A detailed comparison of the final 20 seconds of recorded manoeuvres of the first and second flights whilst in similar positions/flight stages (see the illustration below) was made. On examining these data, it was noted that whilst the helicopter had been heavier during the first flight, its ground speed had been lower. However, on the first flight, it had levelled out after a similar descent sooner, which meant there had been not enough time for the speed to reach a critical level for servo transparency. It was therefore suspected “only small margins (had) prevented the helicopter from entering servo transparency during the last recorded seconds of the first sightseeing flight”. It was noted that other AS350 accidents triggered by servo transparency under similar load and forward speed conditions had required only “a slight correction with the cyclic”.

The final recorded ground and vertical speeds of both the first and second flights. [Reproduced from the Official Report]

It was noted that if servo transparency had indeed occurred at a low height, it would have been difficult to recover from it because to do so, it would have been necessary to “lower the collective pitch control to reduce the load on the main rotor” which would have brought the helicopter even closer to the ground.

Context

The Investigation considered that “the challenges that servo transparency entail appear to be under-communicated among AS 350 pilots in Norway” and noted that the AFM did not provide a means to determine under what conditions servo transparency is most likely to be encountered and noted that after an AS350 B3 fatal loss of control event in Norway in 2011 which was also likely to have been the result of servo transparency at too low a height to recover from, had noted that the helicopter was “not equipped with technical aids that give advance warning, or prevent reaching, the maximum load on the main rotor” which is a key facilitator of servo transparency. It was noted that in September 2021, Airbus Helicopters had presented the European Union Aviation Safety Agency with “a roadmap” for a technical solution intended to warn to the pilots prior to encountering servo transparency.    

Given that the accident scenario was considered to have originated in the handling of the aircraft, the training and experience of the pilot was examined further. Since qualifying as a professional pilot, his experience had been accumulated erratically through intermittent fixed term contracts to the extent that the Investigation considered him to have been “inexperienced”, particularly so in respect of the AS350 which, as already noted, he had flown for only 17 hours. However, it was also noted from comparison with other independently investigated AS350 servo transparency events that a pilot’s considerable type experience and currency alone were no defence against the scenario believed to have been key to this accident. 

More universally important was considered to be the effectiveness of training. It was found that the accident pilot’s type conversion training at Airbus Helicopters had been conducted only on the aircraft despite the availability of a full flight simulator on which it would have been possible to effectively demonstrate the servo transparency phenomenon associated specifically with this type. Instead he was only “briefed” on this subject. It was noted that after the previously referenced 2011 Norwegian AS 350 accident, the question of simulator training for AS 350 pilots was considered and its use during initial and recurrent training was identified as very effective and particularly so in terms of demonstrating servo transparency. Since then it was noted that an AS350 Level ‘D’ simulator had become available in Helsinki and the Norwegian CAA has advised its intention to ensure that Norwegian operators take advantage of this. 

However, even with clearly beneficial use of such a simulator, the Investigation was of the opinion that “in addition to practical simulator training, more knowledge and understanding is required in this area.....especially for pilots flying helicopters equipped with simple hydraulic systems”. It was considered that such pilots need to recognise that under certain circumstances, safety margins may shrink faster than expected. This view was informed by the fact that the accident pilot’s recent manufacturer type conversion training had meant that among the operator’s pilots, he had the most updated knowledge of servo transparency. Nevertheless, on both sightseeing flights, he had:

• exchanged altitude for speed when the helicopter was close to its maximum authorised mass.
• chosen to fly at an altitude above the terrain that reduced or eliminated the safety margins should anything unexpected occur.

Finally, in respect of the particular ‘demands’ of sightseeing flights, it was considered that one potential aspect of this accident was undoubtedly that such flights can result in inexperienced pilots being more easily influenced by their passengers and feeling greater strain than more experienced pilots might in similar circumstances.  

Conclusion as to Cause and Consequences 

On completion of the Investigation, the following “Main Conclusion” was formally documented:

The Investigation has not revealed technical faults or irregularities relating to the helicopter that could have impacted the sequence of events. At the last recorded position the helicopter was in descent and in a slight right turn. The helicopter was heavily loaded. The Norwegian Safety Investigation Agency (NSIA) believes the flight control hydraulic system may have reached its limit during the manoeuvring, and thereby triggered servo transparency. Based on the last recorded altitude data, the helicopter had insufficient altitude above the terrain for the pilot to regain control in time.  

The helicopter was almost new but was not fitted with a crash resistant fuel system (CRFT or CRFS). Since this can be excluded as a high-energy accident, the NSIA believes that such a protection system could have reduced the possibility of a fire where all the passengers died.  

A total of 12 Safety Recommendations were made as a result of the findings of the Investigation as follows:

• that the European Union Safety Investigation Agency requires that all helicopters, new and used, delivered or imported to Europe be equipped with crash resistant fuel systems in accordance with CS 27.952 or CS 29.952, regardless of their type certification date. [2022/01T]
• that the European Union Safety Investigation Agency does not permit commercial passenger flights by helicopters not equipped with crash resistant fuel systems in accordance with CS 27.952 or CS 29.952, regardless of their type certification date. [2022/02T]
• that the Norwegian Civil Aviation Authority ensures when approving procedures (SOP) for commercial air transport operations that the helicopter operator’s risk assessment involves aspects related to the use of helmets by pilots. [2022/03T]
• that the European Union Safety Investigation Agency requires the AS 350t Type Certificate Holder Airbus Helicopters to establish a technical solution preventing or giving advance warning of servo transparency, for helicopters that are sensitive to this phenomenon. [2022/04T]
• that the European Union Safety Investigation Agency review instruction and continuous training on the AS 350. This is to ensure that the training includes attention training that enables early recognition and recovery from a servo transparency situation based on the UPRT principles. [2022/05T]
• that the Norwegian Civil Aviation Authority review instruction and continuous training on the AS 350. This is to ensure that the training includes attention training that enables early recognition and recovery from a servo transparency situation based on the UPRT principles. [2022/06T]
• that Helitrans A/S revise its standard procedure for sightseeing flights in general, and for AS 350 flights in particular, to better reflect the challenges related to flight altitude, mass and speed. [2022/07T]
• that the Norwegian Civil Aviation Authority through its leadership of various safety forums for inland helicopters, raise awareness of the challenges related to flight altitude, mass and speed in general, and to the AS 350 helicopter type in particular. [2022/08T]
• that the Norwegian Civil Aviation Authority develop and publish a standard for commercial air transport operations with single engine helicopters. This should pay special attention to sightseeing flights. The standard should include guidance to develop the underlying risk assessment and a description of the operation for SOPs with accompanying requirements for (a) qualification and training program. The standard should be developed in collaboration with the two forums for inland helicopter operations and be promoted to the operators through these. [2022/09T]
• that the European Union Safety Investigation Agency revise Regulation (EU) 965/2012 for lightweight flight recorders, by including requirements for further registration of data, as well as for audio and video recordings to be made and stored. [2022/10T]
• that the European Union Safety Investigation Agency revise Regulation (EU) 965/2012 for lightweight flight recorders, by extending the scope to all types of light helicopters used for commercial air transport of persons. [2022/11T]
• that the European Union Safety Investigation Agency revise Regulation (EU) 965/2012 for lightweight flight recorders, by extending the scope to all helicopters used for commercial air transport of persons, regardless of their certificate of airworthiness date. [2022/12T]

The Final Report of the Investigation was published on 18 March 2022. 

Related Articles

• Loss of Control
• Servo Transparency
• Upset Prevention and Recovery Training (UPRT) - Updated Perspectives
• Safety Oversight
• Post Crash Fires
• AS50, Dalamot Norway, 2011