Serious AW139 tail rotor incident prompts AAIB to recommend additional safety measures

Leonardo Helicopters should better assess risk of duplex bearing failure in intermediate-twin’s pitch control assembly, probe states.

UK investigators have urged Leonardo Helicopters to tighten the inspection regime on the AW139 intermediate-twin in relation to a critical tail rotor bearing, the failure of which caused the fatal Leicester City AW169 crash in 2018.

They warn that mitigations put in place by the airframer to address such failures on the AW139 “are substantially less than those introduced in response to the AW169 accident”.

Detailed by the Air Accidents Investigation Branch (AAIB), the safety recommendations follow its probe into the discovery during a post-flight maintenance of a collapsed tail rotor duplex bearing fitted to a Bristow Helicopters-operated AW139 (G-CIMU) on 13 June 2022.

“The extent of the damage to the bearing confirmed that a loss of tail rotor control event would likely have occurred had the helicopter continued to operate with the bearing fitted,” says the agency.

Conducted at its Norwich airport operating base, the walk-round inspection identified the issue after noticing that a hole had been worn in a cover over the bearing and the end of the tail rotor control actuator.

That actuator mechanism helps the pilot to control the pitch of the blades on the anti-torque tail rotor, increasing or decreasing the amount of yaw and countering the main rotor torque.

In the Leicester city crash, a seized duplex bearing triggered a cascading sequence of failures that pushed the tail rotor blades to their physical limit of travel. This put the helicopter into an unrecoverable spin, killing all four occupants when it crashed.

Following removal of the cover, the maintenance crew observed that the actuator was protruding more than usual and the bearing race had collapsed, with the balls in the outer race visible.

“The maintenance team tried to remove the bearing, but the damage was so severe that the bearing disintegrated into its component parts during this process,” says the AAIB.

Due to heightened scrutiny in the wake of the Leicester crash the operator notified the investigation agency of the failure.

Although an AAIB team was despatched to Norwich and the components recovered for forensic analysis, the damage sustained meant that “the bearing was too badly damaged to determine the cause of failure”.

Subsequent analysis of the operator’s maintenance records revealed that the bearing, installed as part of a larger assembly, had been removed during a 2,400h required inspection of the aircraft.

But, the report states that “the duplex bearing had been refitted to the replacement spider/slider rather than being disposed of, which was contrary to the [approved maintenance manual] instructions.”

Having accumulated 1,007h between its initial installation and refitting, the bearing flew another 1,743h before the failure was identified.

During the flights preceding the discovery, the AW139’s crew did not raise any serious issue with the helicopter.

Although the pilots were captured on the helicopter’s cockpit-voice recorder commenting that the foot pedals “felt a bit strange”, the AAIB describes the exchange as a “passing, isolated comment” which “did not appear to be of great concern to the crew”.

Despite the visible damage to the bearing, no warning signs of its failure were flagged by the AW139’s Health and Usage Monitoring System (HUMS).

However, subsequent analysis of the HUMS data revealed a “changing relationship between airspeed and the average pedal position” beginning around 7 June – approximately 15 flight hours before the failure was discovered.

At the time of the Leicester crash, the duplex bearings used in the AW169 were identical to those in the larger AW189 and AW139, although the actuator system in the latter differed slightly.

Increased monitoring and other safety measures – temperature indicating strips and a specific HUMS sensor – were introduced for the AW169 and AW189, with the airframer ultimately introducing in 2020 a new design of tail rotor actuator and duplex bearing for those types.

But, as the AAIB points out: “None of these safety actions were applied to the AW139 fleet, as the helicopter manufacturer considered it was not affected by this issue.” In addition, the extra sensors could not be accommodated within the space available on the AW139.

A duplex bearing failure had occurred in an AW139 in 2012, with Leonardo Helicopters subsequently introducing a repetitive inspection process for the part. However, this was “specifically targeted at higher-life bearings above 1,200 hours”, says the AAIB.

This position was reinforced by a service bulletin introduced in 2022.

On top of which, the European Union Aviation Safety Agency “chose not to mandate this inspection requirement”, it adds, despite having done so for the AW169 and AW189 after the Leicester crash.

“While the failure mechanism on the AW139 wasn’t proven to be the same as the AW169, it has also not been proven to be a high-life failure mechanism, and the existing inspection requirement offers limited additional protection from a low-life failure below 1,200 hours,” the AAIB states.

“When considered collectively the mitigation actions put in place by the manufacturer and EASA to address duplex bearing failures on the AW139 are substantially less than those introduced in response to the AW169 accident.”

Although acknowledging that a design difference prevents the same scenario as seen on the AW169, it says: “There is still a high risk associated with a loss of tail rotor control in the hover.”

This is a particular risk for those conducting flights to oil rigs “where all offshore landings and take-offs transition to or from the hover”.

“The manufacturer and EASA were questioned by the investigation whether they considered the existing mitigation actions were sufficient. They stated that no additional actions were considered necessary at this time,” the UK agency adds.

Similarly, there is currently no requirement for the airframer to assess the condition of AW139 bearings that have been removed from service, in contrast to the AW169 and AW189.

In response, the AAIB recommends “that Leonardo Helicopters implement a comprehensive inspection and assessment programme for tail rotor duplex bearings fitted to the AW139”, covering a range of accumulated operating hours.

Findings from this process would then help to ensure the current mitigation actions are “adequate to address the airworthiness risk” from a bearing failure.

Two other recommendations seek changes to the AW139’s HUMS to provide early indication of the deterioration of the bearing.

However, the proposals have met with opposition from Italian accident investigation authority ANSV.

In a response appended to the AAIB report, it says that while noting the “positive intent” of the recommendations, “the report seems to not give enough credit to the already efficient inspection and assessment activities set in the normal airworthiness process.”

ANSV also argues against the classification of the bearing failure as a “serious incident” rather than the less severe “occurrence”.