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.
(Evangle Luo of TTFLY shared with you)
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