Norwegian investigators have called for a European ban on commercial flights of helicopters not equipped with crash-resistant fuel systems in the wake of an accident in which six passengers and crew died due to a post-impact fire.
In its final report into the 31 August 2019 event, Norway’s SHT safety investigation authority says those on board ”could probably have survived had an intense fire not broken out immediately” after impact.
It says the crash of the Airbus Helicopters H125 (LN-OFU), which came down 5min after take-off near Alta in the far north of the country, “cannot be described as a high-energy accident”.
Even though the Helitrans-operated light-single was a new aircraft, built in 2019, it was not fitted with a crash-resistant fuel system (CRFS).
European regulations stipulate that such a system is not required in helicopters with type certificates issued prior to 1994; the H125, originally known as the AS350, was first approved by the French regulator DGAC in 1977, while the B3 variant involved in the crash was validated in 1997.
However, SHT points out the disparity between Europe and the USA, where “all new helicopters delivered after May 2020 should be equipped with crash-resistant fuel system.”
US regulations were changed in response to a series of accidents involving post-impact fires: the National Transportation Safety Board recorded a total 135 such incidents in the 1994-2013 period involving all helicopter types, of which only three were fitted with a CRFS.
Since October 2019, Airbus Helicopters has supplied the H125 with a CRFS as standard.
SHT is calling on the European Union Aviation Safety Agency (EASA) to require that “all helicopters, new and used, delivered or imported to Europe be equipped with a crash-resistant fuel system… regardless of their type certification date.”
Additionally, EASA should ban “commercial passenger flights with helicopters not equipped with crash-resistant fuel systems.”
The SHT believes the crash itself was caused by “servo transparency” – a phenomenon that occurs when aerodynamic loads, in conjunction with helicopter manouevres, exceed the hydraulic assistance of the servo actuators on the main rotor blades.
This can cause the controls to feel as though they are frozen or moving of their own accord.
“Servo transparency produces a right and aft cyclic load and a collective down displacement. Unless corrections are made by the pilot immediately, the helicopter’s altitude and heading will change,” the report says.
The SHT notes that the H125 does not have an indicator to warn the pilot that the helicopter is about to enter servo transparency. “There is no single limit that the pilot could observe to be sure to avoid the phenomenon,” it says.
Departing at 16.59 local, the pilot of the H125 planned to conduct a sight-seeing flight lasting about 10min with five passengers on board in support of the Hostsprell festival in Kvenvikmoen.
After taking off from Kvenvikmoen, the helicopter flew along the eastern side of the Kvenvika inlet towards Alta airport, before turning towards Elvestrand and heading southwest towards the Skoddevarre mountains and the festival area.
Meteorological data at the time showed scattered clouds, with very little wind.
Due to the post-crash fire, no data was able to be retrieved from the helicopter’s lightweight flight recorder or systems. Instead, investigators used FlightRadar24 and Helitrans’s GNSS system data to recreate the flightpath.
However, an “ADS-B shadow” caused by the region’s topography means that “data about the manoeuvres during the final minute prior to impact with the ground is missing”.
Due to that issue, the SHT cautions that while it cannot “draw an unequivocal conclusion” on the crash, it “has been able to identify one probable sequence of events.”
Around 400m (1,310ft) from the accident site, while at 485ft above the ground, the helicopter’s sink rate was increasing and had reached 1,728ft/min, according to tracking data. The sink rate increased, eventually reaching 3,072ft/min, with the rotorcraft also turning slightly to the right, and accelerating from 125kt (231km/h) to 135kt. Its last recorded altitude was below 300ft above the ground.
Investigations into similar accidents had previously noted that if servo transparency is encountered in a right turn “the associated uncommanded right roll and possible pitch-up have the potential to cause a significant deviation from the intended flightpath, which, if encountered in close proximity to terrain or obstacles, could be hazardous.”
Based on the weight of the passengers and pilot, plus the around 80% of fuel remaining – around 346kg (762lb) - investigators calculated the all-up weight of the H125 was 2,250kg, close to the type’s maximum mass.
Investigators believe “the helicopter may have entered servo transparency at that point in time. Given the high mass and speed the margins to servo transparency would be relatively small.” In addition, the proximity to the ground would have limited the chances of recovery.
But examination of the wreckage showed little damage related to a high-energy impact, leading the SHT to consider “a scenario where the pilot might have been about to regain control of the helicopter after a possible servo transparency.”
“The [SHT] considers that the forces involved in the accident probably did not inflict fatal injuries on those on board. The forces were sufficient, however, to rip open the fuel tank, separate the tail boom from the fuselage and misalign the engine and main gearbox.”
Although the wreckage was largely consumed by the post-crash blaze, examination of components led investigators believe there was no mechanical fault with the helicopter; any damage was consistent with the impact with terrain, the report says.
Following a 2011 accident in Norway where servo transparency was cited as a contributory factor, the SHT called for a permanent note to be added to the H125’s flight manual; that change was approved by the European Union Aviation Safety Agency (EASA) in December 2021.
In fact, Airbus Helicopters in 2018 began a feasibility study into possible technical solutions to the problem; these solutions and a roadmap for implementation were communicated to EASA in September last year, says the report.
As that has yet to be introduced, the SHT has restated its safety recommendation that “EASA, in consultation with Airbus Helicopters, establish a technical solution for helicopters that are prone to servo transparency, with a view to preventing or giving advance warning of the phenomenon.”