B214ST Tail Rotor Drive Shaft Coupling Misassembly (C-GDYZ, HTS)
On 7 June 2021 Bell 214ST C-GDYZ of Helicopter Transport Services (HTS) made a forced landing 14 nm NE of Nipigon, Ontario while returning from a firefighting tasking. The pilot was seriously injured.
Wreckage of Bell 214ST C-GDYZ of Helicopter Transport Services (Credit: TSB)
Transportation Safety Board of Canada (TSB) released their safety investigation report on 8 February 2022.
The Accident Flight
The helicopter was being flown by a single pilot (8400 hours total time, 1940 on type).
The aircraft was equipped with a 550-gallon collapsible water bucket on a 150-foot long line. The pilot conducted approximately 45 drops before informing the fire boss on board the bird dog/spotter aircraft that the helicopter was low on fuel and that the end of his duty day was nearing.
The return trip…to the Nipigon fire base was flown at 3000 feet above sea level, about 1600 feet above ground level. The helicopter was travelling at 70 to 74 knots in a nose-down attitude of roughly 7° due to the empty water bucket when the pilot was alerted by a vibration in the tail rotor pedals and a grinding noise.
Moments later, the 42° BOX OIL PRESS and 90° BOX OIL PRESS annunciators for the 2 tail rotor gearboxes illuminated, and there was an audible engine overspeed noise. The helicopter yawed to the right and the nose began to pitch down.
The pilot lowered the collective control and moved the cyclic control aft to counteract the nose-down tendency, increase the main rotor rpm, and enter autorotative flight. As the helicopter began to spin, the pilot released the long line and water bucket by kicking the manual cargo release pedal.
The pilot transmitted a Mayday call stating that he had lost tail rotor control.
While the helicopter was descending at approximately 1000 to 1500 fpm, the pilot made 3 attempts to use some engine power to fly the helicopter to a suitable landing area near a small lake. He was able to regain some control over the adverse yaw via airflow acting on the vertical stabilizer.
As the helicopter descended below treetop height, he raised the collective control to cushion the landing, at which point the low rotor rpm horn activated.
At 1924, the helicopter landed on its left skid gear with almost no forward speed…and came to rest leaning to the left on soft, boggy terrain.
Both pilot seats were equipped with a shoulder harness, but the occurrence pilot felt that it restricted his ability to use the vertical reference bubble window. Therefore, the pilot did not utilize the shoulder harness during long line or slinging operations.
The TSB Safety Investigation
Investigators identified the tail rotor was not rotating at the time of impact, indicating a loss of tail rotor drive. The Bell Model 214ST Rotorcraft Flight Manual states:
A failure of this type, in powered flight, will result in the nose of the helicopter swinging to the right (left side slip) and usually a roll of the fuselage. Nose down tucking will also be present. The severity of the ships [sic] initial reaction will be affected by airspeed, cabin-loading, center of gravity, power being used, and density altitude.
TSB comment that:
On the occurrence flight, at approximately 1600 feet above ground level, the pilot completed [the Flight Manual] actions when he recognized the loss of tail rotor thrust and was able to slow the spin rate. However, he was committed to an autorotative descent onto the available terrain, which did not lend itself to a run-on type of landing.
TSB explain that:
The tail rotor drivetrain consists of 6 driveshaft segments, 4 hanger assemblies, 3 coupling assemblies, 5 disc assemblies, and 2 gearboxes (42° [IGB] and 90° [TGB]) at the base and tip, respectively, of the vertical stabilizer.
Intermediate & Tail Gearboxes are…
…self-lubricated by an internal oil pump and is monitored by a chip detector, temperature switch, and low oil-pressure switch. A separation of the tail rotor drivetrain…will cause an immediate loss of oil pressure in both gearboxes. As a result, the 42° BOX OIL PRESS and 90° BOX OIL PRESS annunciators on the main warning and caution panel will illuminate.
Investigators found the splined coupling (item 15 below) was found disconnected from the crowned gear coupling (item 16) of the No 2 Coupling Assembly.
The first driveshaft segment exhibited rotational scoring as a result of flailing and consequent contact with components in the fuselage compartment.
A retainer ring ( item 11), which should normally secure the seal holder ( item 12) and crowned gear coupling within the splined coupling, was found lying loose in the fuselage compartment. The seal holder had exited the splined coupling and was loose on the shaft between the crowned gear coupling and the bearing hanger (item 10).
The investigators then turned their attention to recent maintenance of the Tail Rotor Drive Shaft (TRDS) system:
At the time of the occurrence, the No. 2 coupling assembly had accumulated 1250.1 hours since new [installed in November 2015] and approximately 10 hours since the last servicing.
During the reassembly of the aircraft in early 2021 [on arrival in Canada], the 3 coupling assemblies were serviced in accordance with the 500-hour/12-month inspection. They were removed, disassembled, cleaned, inspected, lubricated, and reinstalled on 27 and 28 April 2021. At that time, the occurrence aircraft had accumulated 20,206.1 airframe hours.
All 3 coupling assemblies were serviced by the same aircraft maintenance engineer (AME).
Sadly, the TSB report does not further examine the circumstances of the misassembly error. The late Professor Trevor Kletz famously commented that attributing a cause to human error is about as useful as saying that falls are due to gravity. Identifying a human error should be the starting point of further enquiry not the end.
The TSB do explain a second AME carried out what was termed as a dual control check (DCC) (i.e. an independent inspection) during the completion of the work.
Rationale and guidance on conducting the inspection is contained in a Transport Canada Airworthiness Notice [C010 Inspection of Control Systems].The fundamental component of the procedure is that a “second set of eyes” carries out a careful inspection of the completed work.
AN C010 concludes with the following summary:
While aircraft control systems themselves are often extremely complicated, the kinds of errors in the assembly of these controls that lead to accidents are often extremely simple, so much so that, with hindsight, it can be difficult to see just how the oversight could have occurred. These are simple human errors of the most basic kind, involving poor communication, inattention, distraction, faulty assumptions, and overlooking the obvious. Of all the problems encountered in aviation maintenance, these are among the most avoidable.
It also makes the idealistic exhortation (emphasis added) that:
If all of us involved in the maintenance of control systems were to simply resolve to treat the task with the attention it deserves, regardless of how simple it may appear, control-rigging accidents could be completely eliminated.
This unrealistically assumes that all inspection errors are due to a lack of attention.
TSB note that:
Several daily inspections had been carried out before the occurrence flight. However, the Daily Inspection Check Sheet in use at that time by HTS did not call for an inspection of tail rotor driveshaft coupling assemblies.
TSB Conclusion
It was determined that an incompletely seated retainer ring in the No 2 Coupling Assembly likely led to a failure of the TRDS and loss of tail rotor drive. TSB also observe, countering the regulator’s generic ‘lack of attention’ assumption that:
..since some components, such as a seated retainer ring, may be difficult to view, the use of visual inspection aids and measuring tools may be warranted during installation and subsequent DCC [independent] inspection.
No comment is made however on what the Bell Maintenance Manual actually requires.
Safety Actions
The operator:
- Immediately carried on a fleet check on the other three Bell 214STs in their fleet to verify correct installation of the TRDS.
- Revised their daily inspection to require the opening of an additional access panel to facilitate the inspection of the engine-deck-mounted tail rotor driveshaft hanger and the No. 2 coupling assembly.
- Added a 5-page handout to their Bell 214ST aircraft certification authority course. The handout emphasized inspection areas and reiterated the requirement for an independent inspection after 500-hour/12-month servicing of the 3 coupling assemblies.
- Distributed a memo to pilots reminding them to ensure “sufficient airspeed to maintain helicopter control during an emergency involving the tail rotor or a total loss of tail rotor thrust in cruise flight”.
- Revised their Emergency Equipment and Procedures Ground Training curriculum to include training in loss of tail rotor effectiveness and total loss of tail rotor thrust (which suggests both were absent previously).
Of note is that none address the TSB observation above about visual inspection aids and measuring tools.
Prologue: C-GDYZ’s Prior North Sea Service as G-BKFN & Three UK AAIB Investigations
This B214ST had been the last Bell 214ST offshore operating in the UK sector as G-BKFN.
On 14 August 1985, while operated by British Caledonian Helicopters, G-BKFN made an emergency landing on the beach at Balmedie, Aberdeenshire after the onset of severe vibration. This was trace to the failure of a main rotor blade drag brace.
According to the UK AAIB safety investigation report due to fatigue likely originating from corrosion pitting on the threads.
Another B214ST, VH-LAO of Lloyd Helicopters (now CHC Australia) suffered a drag brace failure on 28 Mar 1988. VH-LAO had the improved drag brace introduce after the G-BKFN accident but investigators suspected insufficient torque on the nut.
Then on 15 May 1986, while still operated by British Caledonian, G-BKFN ditched 14 miles North East of Fraserburgh, Aberdeenshire. There was difficulty deploying the liferafts, the right hand cockpit door would not jettison and the forward floats suffered the deflation of one compartment on each side (one due to damage by a frangible GRP fairing, the other for reasons undetermined).
BCal Bell 214ST G-BKFN Shortly After Ditching NE of Fraserburgh 15 May 1986 (Credit: RAF Shackleton via AAIB)
All 16 occupants were rescued by the Fishing Vessel Constant Friend.
The accident was caused by the partial loss of collective pitch control following the disengagement of the collective hub retaining nut in flight. A contributory factor was the unsatisfactory design of the collective hub nut locking plate assembly.
The AAIB made 4 safety recommendations, including one on demonstrating ditching, life raft deployment and evacuation during certification that was rejected by the CAA. Unusually G-BKFN was refurbished and reentered service.
British Caledonian Helicopters was subsequently bought by Bristow Helicopters and G-BKFN resulted in one further AAIB investigation when on 24 December 1990 it diverted to Safe Gothia while en route in the East Shetland Basin after vibrations. On inspection, cracking of tailboom/fin was found in an area that had been subject to repair after the 1986 accident by a third-party maintenance organisation in consultation with the manufacturer’s technical representative. This cracking had initiated at a “poorly finished” rivet holes.
Alignment of the new holes in the doubler with the existing holes in the spar flange was apparently difficult since the spar side flange twists and curves at this location, making the direction of the rivet holes difficult to judge. The photographs show the resultant damage to the original rivet hole surfaces caused by the drill-bit whilst hole alignment was being attempted. This damage is typical of that expected when rivets are removed and replaced without drilling-out to an oversize diameter and fitting appropriate oversize rivets. However, such an increase in rivet and hole size should not be undertaken without the approval and consent of the design authority and manufacturer.
Safety Resources
The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn. You may also find these Aerossurance articles of interest:
- B1900D Emergency Landing: Maintenance Standards & Practices The TSB report posses many questions on the management and oversight of aircraft maintenance, competency and maintenance standards & practices. We look at opportunities for forward thinking MROs to improve their maintenance standards and practices.
- CHC Sikorsky S-92A Seat Slide Surprise(s)
- SAR AS365N3 Flying Control Disconnect: BFU Investigation
- In-Flight Flying Control Failure: Indonesian Sikorsky S-76C+ PK-FUP
- AAR Bell 214ST Accident in Afghanistan in 2012: NTSB Report
- Misassembled Anti-Torque Pedals Cause EC135 Accident
- EC130B4 Accident: Incorrect TRDS Bearing Installation
- Ungreased Japanese AS332L Tail Rotor Fatally Failed
- R44 Ditched After Loss of TGB & TR: Improper Maintenance
- Missing Cotter Pin Causes Fatal S-61N Accident
- Emergency S-76D Landing Due to Fumes
- Engine & Emergency Flotation Failures – Greenland B206L4 Ditching
- BA B747 Landing Gear Failure Due to Omission of Rig Pin During Maintenance
- When Down Is Up: 747 Actuator Installation Incident
- Maintenance Human Factors in Finnish F406 Landing Gear Collapse
- Lost in Translation: Misrigged Main Landing Gear
- Crossed Cables: Colgan Air B1900D N240CJ Maintenance Error
- Frozen Dash 8-100 Landing Gear After ‘Improper Maintenance Practices’ Say NTSB
- ATR 72 Rudder Travel Limitation Unit Incident: Latent Potential for Misassembly Meets Commercial Pressure
- Loose B-Nut: Accident During Helicopter Maintenance Check Flight
- USAF RC-135V Rivet Joint Oxygen Fire
- The Missing Igniters: Fatigue & Management of Change Shortcomings
- FAA Rules Applied: So Misrigged Flying Controls Undetected
- BEA Point to Inadequate Maintenance Data and Possible Non-Conforming Fasteners in ATR 42 Door Loss
- BA A319 Double Cowling Loss and Fire – AAIB Report
- BA A319 Double Cowling Loss and Fire – AAIB Safety Recommendation Update
- ANSV Report on EasyJet A320 Fan Cowl Door Loss: Maintenance Human Factors
- Tiger A320 Fan Cowl Door Loss & Human Factors: Singapore TSIB Report
- Human Factors of Dash 8 Panel Loss
- Fuel Tube Installation Trouble
- How One Missing Washer Burnt Out a Boeing 737
- Flying Control FOD: Screwdriver Found in C208 Controls
- Cessna 208 Forced Landing: Engine Failure Due To Re-Assembly Error
- AAIB Report on the Ditchings of EC225 G-REDW 10 May 2012 & G-CHCN 22 Oct 2012
- EC225 LN-OJF Norway Accident Investigation Timeline
- Meeting Your Waterloo: Competence Assessment and Remembering the Lessons of Past Accidents
You might find these safety / human factors resources of interest:
- James Reason’s 12 Principles of Error Management
- Back to the Future: Error Management
- This 2006 review of the book Resilience Engineering by Hollnagel, Woods and Leveson, presented to the RAeS by Aerossurance’s Andy Evans: Resilience Engineering – A Review and this book review of Dekker’s The Field Guide to Understanding Human Error: The Field Guide to Understanding Human Error – A Review
FSF Maintenance Observation Programme (MOP)
Aerossurance worked with the Flight Safety Foundation (FSF) to create a Maintenance Observation Program (MOP) requirement for their contractible BARSOHO offshore helicopter Safety Performance Requirements to help learning about routine maintenance and then to initiate safety improvements:
Aerossurance can provide practice guidance and specialist support to successfully implement a MOP.
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