UK accident investigators have called on safety regulators to mandate real-time analysis of helicopter vibration monitoring data to allow pilots to be warned rapidly of impending problems.

In addition, the European Aviation Safety Agency should work to reduce the interval for capturing the data "thereby enhancing the usefulness of [the] data for the timely detection of an impending failure".

The recommendations come after the UK Air Accident Investigation Branch concluded that flawed analysis of recorded safety data led maintenance engineers to miss signs that a Sikorsky S-92 heavy helicopter was at risk from a potentially catastrophic tail rotor failure.

The aircraft was subsequently released to service, but shortly into its next flight, the crew lost tail rotor authority as the helicopter came into land on a North Sea oil platform, with only their swift reactions preventing a tragedy.

In fact, the CHC Scotia-operated S-92 came to rest just inches from the edge of the helideck on the West Franklin platform, east of Aberdeen, having turned through more than 180° as it yawed uncontrollably to the right, says the AAIB in its final report into the 28 December 2016 incident.

"If the loss of yaw control had occurred at an earlier stage of the flight, the helicopter would most likely have made an uncontrolled descent into the North Sea," says the AAIB.

The event was triggered by the failure of the tail rotor pitch change shaft (TRPCS) bearing, which then damaged a related component "resulting in uncommanded and uncontrolled inputs being made to the tail rotor".

Evidence of a looming problem had been recorded the previous day by the S-92's health and useage monitoring system (HUMS), which captured excessive vibration levels related to the TRPCS bearing.

But an engineer conducting routine maintenance on the helicopter on 27 December, which included analysis of the HUMS data, failed to spot the warning signs, in part due to limitations with the software.

"Whilst an anomaly for tail rotor gearbox (TGB) bearing energy was detected by the maintenance engineer, the exceedences were not identified, in part, due to the way they were presented in the analysis tool; the helicopter was released to service without further investigation," says the AAIB.

It says the otherwise "conscientious" employee failed to further investigate the anomaly; a recent break-in at his house could have been preying on his mind, it adds.

The two key licenced personnel at the operator's main Aberdeen base failed to zoom in on the relevant portions of the HUMS data, says the report; had they done so "the exceedences would have been clearly visible".

Instead, a process of "mutual reinforcement" led them to conclude that an "unknown" fault with the software was the underlying issue.

CHC's standard practice was to forward all data to its Norway-based HUMS global support team for further review. Although that team detected the problem, by the time they informed the Aberdeen site they were "told that the helicopter was currently on the West Franklin platform".

Although unaware of the exceedences, the flightcrew did receive what turned out to be a warning as they lifted from another platform for the 5min trip to West Franklin.

As the helicopter pulled away from the helideck it yawed unexpectedly through 45°, but having regained control, the pilots assumed it was due to a miscalculation of variable wind conditions

Following the incident, regulators mandated a worldwide check of TRPCS bearings on all S-92s: of the 253 helicopters examined, shafts from 19 were removed, variously due to HUMS data warnings, visual indications or poor bearing condition.

Although Sikorsky has since modified its HUMS diagnostic software, originally developed in 1991, to enable easier analysis of the data, and improve the human machine interface, the AAIB believes real-time analysis would be more useful to flightcrews.

This should available to pilots "at least before take-off and after landing", it says.