Contained Failure / IFSD of a PW4056 on 2 October 2015
Contained Failure / IFSD of a PW4056 on 2 October 2015 On 2 October 2015, while in cruise at 33,000 feet, Delta Air Lines Boeing 747-451 N662US experienced a loss of power from its Number 3 engine, a Pratt & Whitney PW4056. The US National Transportation Safety Board (NTSB) say in their safety investigation report that the flight crew heard a loud “pop” that was followed by the engine’s N1 (fan speed) decreasing while the exhaust gas temperature (EGT) increased. The engine was shut down and the aircraft diverted safely. The aircraft suffered minor damage (nicks and dents to the underside of the right wing, inboard aileron and the leading edge of the right horizontal stabilizer). The NTSB say the engine failure was ‘contained’ i.e. no debris was released radially (only axially). The NTSB Safety Investigation The NTSB go on to explain: The disassembly of the engine revealed one 3rd stage turbine vane cluster, No. 29, was missing although the cluster’s bolt hole tab with the retaining nut were still in place on the inner transition duct. The missing cluster’s inner shroud was found in the bottom of the engine in the path of the 3rd stage turbine blades. The metallurgical examination of the inner shroud revealed fatigue, however the full extent of the fatigue could not be determined because the end of the fracture surface was smeared. The examination of the remainder of the LPT revealed all the other turbine vane clusters were complete and in place or the inner and outer shrouds were in place with just the airfoils missing. All the LPT blades were fractured and the fracture surfaces were coarse and grainy indicating an overload fracture. The examination of the remainder of the engine between the fan and high-pressure turbine did not reveal any damage. The extensive damage to the LPT and the absence of damage throughout the remainder of the engine indicated that the damage to the engine originated within the LPT. The visual examination and a dimensional inspection of the LPT case revealed the 3rd stage turbine vane hooks had extensive wear that varied significantly between adjacent hooks. The dimensional inspection revealed the hook for 3rd stage turbine vane cluster No. 29, the missing vane cluster, had the most wear and that wear was tapered. The tapered wear on the hook and indicates that the vane cluster’s outer foot disengaged from the LPT case and tilted rearward. It was not possible to determine the cause of the tapered wear on the LPT case vane hook that led to the vane cluster disengaging. The finding of fatigue on the inner shroud further supports that the vane cluster’s outer foot disengaged from the LPT case initially and fatigue was caused by either the transfer of the loads to the inner shroud or from the cluster being strummed by the passing 3rd stage turbine blades. Safety Actions The NTSB Comment Because of previous contained and uncontained PW4000 LPT events, P&W has revised the engine manual to add extensive inspections and repairs to LPT components as well as limiting the number of strip and recoat repairs that can be done to PW4000 LPT airfoils. The inspections and repairs that were subsequently adopted into an airworthiness directive (AD) included a visual and dimensional inspection of the LPT case’s vane hooks. On 7 November...
read moreHigh G Drama at Cold Lake
High G Drama at Cold Lake On 20 June 2017 two Royal Canadian Air Force (RCAF) Boeing CF-188 Hornets, call signs Mig-1 and Mig-6, were returned to CFB Cold Lake, AB after Maple Flag 50 training missions. When overhead runway 13R at 1500 ft above ground level and 470 knots, the RCAF safety investigators report that: Mig-1 entered the overhead break in a right hand turn followed three seconds later by Mig-6 [CF-188 CF188796]. During the overhead break Mig-6 set the throttles to idle, initially set the bank angle to 81 degrees, and pulled up to 6.8g in order to slow the aircraft in preparation for turning final with gear down and locked. Mig-6 did not perform the anti-g straining maneuver, and was flying with a loose fitting g-suit with comfort zippers undone. Two seconds into the overhead break and at 6.8g, Mig 6 almost lost consciousness. Mig-6 experienced short term (approximately 5 seconds) impairment of cognitive and motor functions, and the aircraft began descending towards the ground. Mig-6 heard the audible warning from the Terrain Alert Warning System, and with improved cognitive and motor functions, Mig-6 pulled 7.0g and avoided the ground by 270 ft. Both aircraft landed safely. The RCAF investigation is examining further human factors and technical aspects. UPDATE 23 July 2018: The RCAF say: The evidence demonstrated no aircraft or aviation life support equipment (ALSE) malfunction. The incident occurred due to human factors. The pilot was knowingly flying with a loose fitting g-suit. The g-suit was loose fitting due to pilot weight loss, and lack of adherence to an ALSE – Canadian Forces Technical Orders requiring a g-suit on body fit check to be completed every six months. The safety recommendation is to incorporate the g-suit inspection requirements and pilot responsibilities regarding g-suit fitting into an appropriate aircrew publication to provide lasting education/awareness for CF188 pilots. Other Safety Resources Crew Bag FOD Shatters Hawk Canopy RCAF Production Pressures Compromised Culture UPDATE 9 April 2022: SAR Seat Slip Smash (RCAF CH149 Leonardo Cormorant LOC-I) Aerossurance will be presenting at the Royal Aeronautical Society (RAeS) Human Factors in Aircraft Maintenance conference on 2 November 2017 in London. Our topic is: Helicopter Flying Control Maintenance HF Accidents: A Human Centred Design Opportunity Aerossurance is also pleased to be supporting the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference for the third year running. We will be presenting for the second year running too, this time on the subject of the FSF‘s Maintenance Observation Programme concept. This year the conference takes place 13 to 14 November 2017 at the Hilton London Gatwick Airport, UK with the theme: How do we improve human performance in today’s aviation business? Aerossurance has extensive air safety, operations, airworthiness, human factors, aviation regulation and safety analysis experience. For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com Follow us on LinkedIn and on Twitter @Aerossurance for our latest...
read moreDMAIB: Risk Assessment Facilitated Taking Risks
DMAIB: Risk Assessment that Facilitated Taking Risks (MÆRSK SEARCHER and MÆRSK SHIPPER Sank Undertow) The Danish MAIB have published their report on the loss of two Maersk Supply vessels being towed for breaking. Key staff being made redundant, a risk management system that facilitated risk prone operations and a perception of being “industry frontrunners, capable of solving difficult and novel tasks” all feature in this accident. The DMAIB say: The DMAIB regards the foundering…as a systemic accident. This means that local and technical circumstances unfolding on board MÆRSK BATTLER during the voyage cannot be isolated from the preceding organisational events and circumstances taking place months earlier. The Accident Voyage DMAIB explain: On the night between 21 and 22 December 2016, the Danish offshore supply ships MÆRSK SEARCHER and MÆRSK SHIPPER capsized and sank in the Bay of Biscay approximately 65 nm off the French coast while being towed by another offshore supply ship, MÆRSK BATTLER, en route to Aliaga, Turkey [to be broken up]. MÆRSK SEARCHER and MÆRSK SHIPPER were configured in a side-by-side towing setup during the voyage. During the passage of the English Channel, the fenders between the ships on tow failed, and the ships started to interact. This caused damage to the ships’ superstructure, which eventually compromised MÆRSK SEARCHER’s watertight integrity and led to water ingress. The crew on MÆRSK BATTLER did not realise that the ships were at risk of being lost due to loss of stability until 10 minutes before the capsizing of MÆRSK SEARCHER, though both ships had suffered significant structural damage MÆRSK SEARCHER capsized and sank, and subsequently MÆRSK SHIPPER was pulled under by MÆRSK SEARCHER. The crew on MÆRSK BATTLER carried out a controlled breakage of the towing wire and came loose of the foundered towage. DMAIB Analysis: The Towing Plan and Redundancies In early September 2016, a Management of Change (MoC) meeting was arranged as a kick-off for the planning of the MÆRSK SEARCHER and MÆRSK SHIPPER’s voyage to Aliağa. Significantly the tow was originally planned with a less well-equipped ship, with only one winch (and so incapable of a double tow). This necessitating a higher risk side-by-side tow configuration to avoid the cost of crewing one of the towed vessels in a serial tow configuration. In October 2016 the superintendent who started the towing plan was laid off in a round of dismissals. He was not given time to hand over his work as the redundant employees were all required to leave work on the day the redundancies were announced. Most of the work documents were saved on the individual employee’s personal work computer, and the company did not have a system in place for securing such documents during dismissals. Therefore, the knowledge of the ongoing operations possessed by the individual employee was lost. The DMAIB make no mention of any MoC or risk assessment of the staff changes even though… According to the company’s own definition, the Management of Change meeting has the purpose of ensuring that “changes are managed and that the risk of failure is minimised without jeopardising people, the environment, our property and our organisation.” Furthermore: Later in October, the company underwent a second organisational change, which imposed a large-scale rotation among the employees as a result of the merging fleet teams. As a result of the merging, the employees in the operations...
read moreWhen Green is for Stop: Poor Design HF
When Green is for Stop: Poor Design HF On 28 April 2017 Cessna 310R VH‑COQ was on approach to Tindal Airport, NT. Tindal is used by the Royal Australian Air Force (RAAF) as well as being a civil airport. The airfield is one of around 2000 world wide fitted with a Runway Arrestor Gear (RAG) or Aircraft Arrestor System (AAS) for use by military fast jets. This uses cables across the runway that can be raised 100 mm off the runway surface to allow a fast jet hook the cable and be decelerated. When lowered the cable is held just off the surface by rubber ‘donuts’. According to an Australian Transport Safety Bureau (ATSB) investigation report: The air traffic control tower had opened for a scheduled military jet departure and was therefore active when VH-COQ made its approach to land. During the tower opening checklist procedure, the tower controller annotated the ‘cables’ check was completed. About 21 minutes after the tower opened, VH-COQ requested a clearance…. The tower controller scanned the control console, noted that both hookcable pushbutton lights were green, and cleared VH-COQ to land… When… on short final approach to land…the pilot noticed the approach end hookcable was raised. The pilot adjusted their aim point beyond the hookcable and landed without incident. The pilot of COQ reported the position of the hookcable to the tower controller, who then rectified the situation. This ATSB say this incident “highlights the risks of expectation bias” with a strong anticipation of a particular outcome: The tower controller observed two green lights on the control console, but did not recognise they were the UP indicators. There are four pushbutton selection/indicator lights for each hookcable. Two separate green UP and green DOWN pushbuttons are used to select, and then indicate, the desired position for each hookcable. The ATSB say: …the design of the indicators, where green lights can have two different meanings, removes the usefulness of the colour of the lights in determining whether the hookcable is up or down. The pilot detected the problem in time to avoid trampling the hookcable during the landing. However, pilots should take note that the hookcables will automatically raise in the event of a power failure. Safety Actions The air traffic service provider is taking the following safety actions: The possibility of changing the colour of the UP lights will be investigated through an engineering process to better differentiate between the UP and DOWN positions (this will be for all our sites that have arrestor systems). Furthermore, due to the relative low number of civilian aircraft operating at Tindal Airport, Tindal air traffic control will be advising the position of the cable with every landing and take-off clearance given to civilian aircraft. This will help force the controller to verify the position of the cable in addition to the conduct of the instrument scan. Previous Incident The Tindal RAG has been troublesome before. Eight months before on 9 August 2016 Beechcraft King Air B200 air ambulance VH‑ZCJ was ready to depart from Tindal to transfer a patient to Darwin, NT. On take off, according to the ATSB, the nose wheel struck a raise cable at about 86 knots, fortunately without incurring damage. Previously the RAG/AAS was used to stop a military fast jet… The barrier crew then entered the runway and performed the servicing...
read moreIrish Coast Guard Select Aerossurance as SAR Aviation Consultancy
Irish Coast Guard (IRCG) Select Aerossurance as SAR Aviation Consultancy The IRCG announced on the 26 September 2017 that they have selected Aberdeen based aviation consultancy Aerossurance to be their new aviation consultants after a competitive tender with 7 bidders. Aerossurance already provide a range of safety, technical, requirements management, tender development and contract assurance services to other diverse customers for aviation internationally. The Tender Trap: SAR and Medevac Contract Design: Aerossurance’s Andy Evans discusses how to set up clear and robust contracts for effective contracted SAR and HEMS operations. UPDATE 1 July 2018: Aerossurance has started work supporting a second European Coast Guard with a procurement project. UPDATE 21 September 2018: Aerossurance has started a project to develop the technical specification for a European agency wishing to place an air service contract for a fleet of special mission aircraft. UPDATE 30 December 2018: Emergencytimes.com publish The Irish Coast Guard has saved over 400 lives and carried out over 1100 missions in 2018: By year end Coast Guard helicopters will have flown in excess of 670 missions, of which 119 were conducted on behalf of the Health Service Executive. Coast Guard helicopter services also include inland searches for missing persons and medical evacuations in support of An Garda Síochána and Mountain Rescue Teams. The Coast Guard attaches particular attention to what is categorises as Lives Saved. In 2018 the Coast Guard has recorded that in excess of 400 people were categorised as Lives Saved in comparison with 340 in 2017. Director Chris Reynolds reiterated a core message of raising the alarm in time. “If you can raise the alarm and you can stay afloat then you have an outstanding chance of being rescued by our world class rescue service” If you see somebody in trouble or if you think they are in trouble at sea, on the water or along the coast Dial 112 and ask for the Coast Guard. UPDATE 1 May 2019: Aerossurance has started work supporting a UK air ambulance charity. UPDATE 19 July 2019: Aerossurance has commenced multiple projects supporting an overseas air force with an independent SMS review, training and consultancy. UPDATE 3 February 2020: Aerossurance commenced providing an independent quality system review service to an international humanitarian air service. UPDATE 6 May 2020: IRCG has issued a Prior Information Notice: The Department of Transport, Tourism and Sport/Irish Coast Guard is undertaking a procurement process to provide an updated, long-term SAR Aviation Service to replace the current SAR helicopter contract. This is a prior information notice to alert the market. A market engagement process will be announced in due course. Indicative timeframes are: Publication of the OJEU Notice Quarter 4 2020, contract award quarter 4 2021 and service commencement quarter 3 2023. Precise procurement procedure has yet to be decided. UPDATE 11 May 2020: European Search and Rescue (SAR) Competition Bonanza Coincidentally, Aerossurance will be presenting at the Flight Safety Foundation International Air Safety Summit in Dublin 23-35 October 2017 on the topic: Achieving peak safety performance: listening and learning Aerossurance is also pleased to be supporting the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference for the third year running. This year the conference takes place 13 to 14 November 2017 at the Hilton London Gatwick Airport, UK with the theme: How do we improve human performance in today’s aviation business? We will be presenting for the second year running too, this time on the...
read moreProduction Errors on a SAR AW139 Helicopter Full Ice Protection System
Human Factors: Production Errors on a SAR AW139 Helicopter Full Ice Protection System (Swedish Maritime Administration SE-JRH) Swedish Maritime Administration (SMA) Leonardo AW139 SE-JRH was returning to its base in Umeå from a Search and Rescue (SAR) training mission on 9 November 2014. About 10 minutes from landing a caution was displayed for a tail rotor Full Ice Protection System (FIPS) failure. The crew asked for a priority landing and landed safely shortly after. The Swedish Accident Investigation Board (the Statens Haverikommission [SHK]) explain in their report: After engine shut down…it was discovered that the slip ring, which in its entirety weighs 5.9 kg and which was mounted adjacent to the tail rotor, had come loose…The slip ring was prevented by the electrical harness from coming into contact with the moveable parts of the tail rotor. A seriously damaged tail rotor…can result in a catastrophic condition and this must therefore be considered a serious incident. The SHK Investigation The slip ring is produced in the USA under a Federal Aviation Administration (FAA) Part Manufacturer Approval (PMA). The slip ring constitutes…is intended to transfer electrical current from the helicopter’s solid structure to the heated up parts of the rotating tail rotor blades. FIPS allows the pilot to fly into weather which entails a risk of ice formation. The slip ring consists of the following parts: mounting plate, outer ring with electrical connections, carbon brush holder and inner ring driven by the tail rotor hub. The mounting plate is fixed with four screws to the tail rotor gearbox and to the outer ring with eight smaller screws. From the inner ring, separate electrical cables are led to the respective rotor blade’s anti-icing coating. The mounting plate and the rest of the slip ring are made of an aluminum alloy and the screws of a steel alloy with a high tensile strength. On examination they found that the screws had all failed in fatigue: The formation of the fracture began in the thread bottom, roughly 16 mm from the screw head of all screws, which also show signs of final fracture. According to the Exova [laboratory] examination, the likely cause of the fatigue fracture is excessively low prestressing of the screws. During the dismantling and subsequent examination, it was established that the wrong type of screws were used when attaching the mounting plate to the slip ring. These screws were 4 mm longer than those intended for the position, which meant that they bottomed out in the screw holes. Apart from the length, these screws were identical to the correct ones. The incorrectly mounted screws were of the type used for the lid on the power connection (see figure 2). The use of incorrect screws was a feature of one infamous accident to a British Airways BAC One-Eleven, G-BJRT, over Didcot, on 10 June 1990 (re-analysed in Beyond Aviation Human Factors: Safety in High Technology Systems). A panel of the windscreen failed at 17,400 feet sucking the aircraft’s captain, halfway out of the aircraft. It was also noticed that a significantly lower tightening torque than that prescribed was required to loosen all eight screws on one of the reference slip rings. SHK has taken into account the great difficulty of determining the amount of tightening torque that was actually applied. Furthermore, the majority of lock-wires for the screws on both reference slip...
read moreOffshore Helicopter Emergency Response
Offshore Helicopter Emergency Response Maintaining an effective emergency response capability is critical to any organisation contracting for helicopter operations in a hostile environment. Among the key questions they need to ask are: How will we be alerted? What assets are available for search and rescue? Have we documented, but more importantly TESTED, our Emergency Response Plan (ERP)? Alerting While aircraft carry Emergency Locator Transmitters (ELTs), in sudden accidents they have historically proven unreliable. An Aerossurance study of helicopter accidents in the Gulf of Mexico showed that most fixed ELTs failed to activate automatically. Controlled ditchings turned out to be too ‘soft’ to activate the ‘g’ (or ‘crash’) switch. Often the ELT transmitter is mounted forwards but the antenna in on the tail boom, and so the cabling does not survive a ‘more dynamic’ accident. Even if activated (automatically or manually) ELT’s do not work when they are submerged. So if the emergency flotation system does not fully deploy then even a perfectly functioning ELT will be lost below the waves before a satellite fix can be obtained, putting the emphasis on Survival ELTs carried by the crew or in life rafts. While Automatically Deployable ELTs (ADELTs) have been common in Europe and on large offshore helicopters, their performance has been patchy as described by UK CAA in CAP1144. The European Aviation Safety Agency (EASA) have a Rule Making Team (RMT.0274) looking ‘ELT installation, location and activation’ generally. A reliable method of tracking is to use a satellite tracking system. Although there was much debate after the MH370 accident of making satellite tracking mandatory for airliners, extremely cost effective technology is available today and widely used in the oil & gas and resource sectors on helicopters and fixed wing aircraft. Using sat tracking means that no matter what happens to the aircraft when an accident occurs, the track up to that point is captured back at a monitoring centre, greatly aiding the search and rescue effort. Simply fitting the kit is only part of the story however. One choice is the frequency of position updates. Most sat tracking providers charge for each position transmission. While reputable air operators recognise the benefit of regular position reports it has been know for some operators to pay for the minimum (the difference between a 2 minute and a 15 minute interval could be 35nm for a modern offshore helicopter). As a customer for air services, not only is it wise to specify sat tracking but also agree a transmission frequency. Another factor is who is going to monitor the progress of a flight and what do they do if an anomaly occurs? Accidents have occurred where the system was unmonitored and no automated alarms were set up. One example is a helicopter accident in Canada in 2011 during an onshore geological survey. In a similar case that Aerossurance investigated, while a dedicated flight following centre was monitoring a flight, when it crashed alongside is destination, an emergency response was delayed on the assumption the aircraft had landed as intended. Search and Rescue (SAR) SAR assets around the world vary from dedicated, regularly exercised All Weather SAR (AWSAR) helicopters with paramedic trained winchmen, four axis autopilot, dual hoists, FLIR, direction finding equipment etc on constant short notice stand-by (as illustrated above) to…well…next to nothing. Relying on a local military that flies a few hundred...
read moreSwedish Stick Shake: Dormant Sensor Defect not Detected on Installation
Swedish Stick Shake: Dormant Sensor Defect not Detected on Installation A serious incident occurred on 29 September 2016 to BAE Systems 146 / Avro RJ100 SE-DSP operated by Braathens Regional Aviation on departure from Malmö Airport in Sweden, when the stick shaker and the stick pusher were erroneously activated. The Swedish Accident Investigation Board (the Statens Haverikommission [SHK]) explain in their final report: At the pre-flight inspection the operator’s engineer detected damage on the left airflow sensor. The sensor was replaced, which caused a delay of more than 30 minutes. The take-off was normal until lift off, when the stick shaker was activated. However, the flight crew quickly identified the warning as false. A warning was indicated on the instrument panel, (IDNT 1). The commander pressed the IDNT/INHIB 1 button and the INHIB part of the button lit up, but felt that nothing happened. [The] memory action to inhibit stick push was not performed. Later during the climb, when they got into clouds at 660 feet above the ground the stick push was activated, which means that the control column is pushed forward. By following the emergency checklists, the systems could be shut down which solved the problems. Thereafter a normal landing was performed. Safety Investigation: Failure Mode The SHK say: An examination of the left hand airflow sensor showed that the unit was incorrectly assembled and that it was 45–50 degrees out of the specification for all angle readings. To get a stick shake it is sufficient for one sensor to indicate a high angle of attack. In order for the stick push to be activated, one sensor must have a high angle of attack and the other must have a high angle or a high rate of change. The most likely explanation for stick push activation is that the turbulence caused the change rate of the serviceable airflow sensor to become large enough. Safety Investigation: Maintenance The SHK examined the maintenance: Initially the idea was to just replace the damaged vane assembly. However, there was no vane in the store, but only a complete airflow sensor. The engineer printed out instructions for removal and installation of airflow sensor from the aircraft maintenance manual [AMM] and commenced the removal. At the same time as the new airflow sensor was delivered, the crew also arrived and received information that the flight would be delayed. The airflow sensor [authorised release] certificates were reviewed and accepted, and the installation commenced. A frequent feature in such investigations: During the installation, the engineers were interrupted several times by different people who wondered when they should be ready. This was perceived as stressful by the engineers. After completing the installation, the engineers: …browsed through to the last page of text in the manual, where the tests to be carried out after installation usually is written. The engineers read the test instructions for the “vane assembly”, i.e. installation of the vane, and not for replacement of the airflow sensor. After installation of the vane, only one test of the heating should be performed, which was also done with approved results. However, the engineers thought that this test seemed insufficient, why they decided to also perform a return to service test (RTS) for the airflow sensor and a sensor screen test. Both of these additional tests were performed with approved results....
read moreLoss of AS332L1 LN-OPG off Brønnøysund, Norway, 8 September 1997
Loss of AS332L1 LN-OPG off Brønnøysund, Norway, 8 September 1997 This accident and the loss of 12 lives off the coast of Norway in 1997 had far reaching consequences for the use and regulation of helicopter Health and Usage Monitoring Systems (HUMS). The Accident On 8 September 1997 at 06:00 local time, a Norwegian AS332L1, registered LN-OPG, departed from Brønnøysund, just south of the Arctic Circle, for the Norne oil production vessel in the Norwegian Sea. Two pilots and 10 passengers were on-board (AAIB/N 2001: 4). The operator had acquired this aircraft when it absorbed a Norwegian competitor (AAIB/N 2001: 11). It was equipped with IHUMS (AAIB/N 2001: 14). At 06:50 the crew observed a short illumination from the overspeed (or ‘OVSP’) light (AAIB/N 2001: 7). This light is primarily intended to indicate activation of an engine overspeed protection system (AAIB/N 2001: 13 and Padfield R.R. ‘Finger Trouble in the Super Puma’, Helicopter World, January-March 1990 pp 28-32) but both engines continued operating and they were not able to identify a problem (AAIB/N 2001: 7). The indication was however caused by an abnormal vibration causing a momentary loss of contact between the power turbine and its speed sensing probes (AAIB/N 2001: 98). The vibration was due to a failure developing at the gearbox end of the drive system connecting the right hand engine to the main gearbox (MGB). It is believed that two components at the MGB end of right hand high speed input shaft (known as the ‘Bendix shaft’), the splined sleeve and subsequently the splined flange (below), had become cracked to a considerable extent during previous flights. The thin walled Bendix shaft transmits (see below) power from the Makila engine (up to 1742 shp at 22850 rpm) and has flexible bellows (or diaphragms) at each end to allow relative movement between the engine and the MGB (AAIB/N 2001: 11-12). These bellows are also weak points, designed to fail first. In such an event the longer piece of the shaft is prevented from flailing by a small stub shaft (one protruding beyond the bellows at each end). About six minutes after the overspeed light first flickered the splined sleeve and flange deteriorated so as to release a lock washer and circlip (see AAIB/N 2001: Figure 6 above) into the Bendix shaft. The Bendix shaft severed (see below) under the one tonne static load imposed by the lock washer (AAIB/N 2001: 97-99). The break was between the bellows, meaning that the severed shaft flailed, generating strong vibrations. If the Bendix shaft fails the most serious hazard is that the power turbine (see below), no longer constrained by the load of the rotor system, could be accelerated by the gas generator exhaust until its turbine discs burst, releasing high-energy debris. The defence against this is an overspeed protection system designed to shut down the gas generator as the power turbine accelerates above 120% power turbine speed (Nf) (AAIB/N 2001: 13). There were no requirements for the further defence of debris containment when the AS332L was certified (AAIB/N 2001: 24). Unfortunately the progressive failure in the drive system also damaged the Nf sensors used by both the overspeed protection and Engine Electronic Control Unit (EECU). Due to a flaw in the design, not only did the loss of Nf signal cause the...
read moreMind the Handrail! – Walk-to-Work Helideck Hazard
Mind the Handrail! – Walk-to-Work Helideck Hazard A recent offshore helicopter incident has highlighted a new hazard as offshore industry practices change. A medium helicopter carried a team to a Normally Unattended Installation (NUI). After an uneventful landing, the disembarking team noticed that the folding handrail for the access steps behind the helicopter was in the up position. It was just 300mm from the tail rotor. A pitch angle of nine degrees during the landing would have been sufficient for a tail rotor strike. When a NUI is temporarily manned the Helicopter Landing Officer (HLO) must ensure these hand rails are folded down when an aircraft in on approach. When de-manning the NUI, the HLO will fold the handrail in use down as the passenger embark. So what went wrong when the last helicopter departed? Nothing! On investigation the last visit to the installation had been by a walk-to-work (or W2W) vessel, where workers transfer by stabilised gantry (VIDEO). The investigation found there was no helideck inspection as part of the W2W NUI de-manning checks. To prevent a reoccurrence Installation, Maintenance and Safety Managers need to ensure that NUI down-manning procedures include verification that the helideck is being left in a safe condition. The helicopter crew did not see the relatively small erect handrail. The UK Civil Aviation Authority (CAA) Standards for Offshore Helicopter Landing Areas (CAP437) says: Where handrails associated with helideck access/escape points exceed the height limitations given at paragraph 3.23 [shown graphically in an illustration below] they should be retractable, collapsible or removable. When retracted, collapsed or removed the rails should not impede access/egress or lead to gaps which could result in a potential fall from height. Handrails which are retractable, collapsible and removable should be painted in a contrasting colour scheme. Procedures should be in place to retract, collapse or remove them prior to helicopter arrival. Once the helicopter has landed, and the crew have indicated that passenger movement may commence…, the handrails may be raised and locked in position. The handrails should be retracted, collapsed or removed again prior to the helicopter taking off. The handrail in question appears to have been marked with faded yellow and blacks stripes, which may have been difficult to see against their background with adjacent black and dark coloured structure. There are high contrast GRP handrail options available: CAP437 chapter 3 paragraph 3.23 defines the clearance, based on the D value, required on a helideck as shown below: Note: More detailed information on this incident was shared in a safety alert by an industry association Aerossurance is a member of. Other Helideck Safety Resources Helideck Safety Alerts: Refuelling Hoses and Obstructions NTSB Recommendations on Offshore Gas Venting Passive Fire-Retarding Helideck Designs US BSEE Helideck A-NPR / Bell 430 Tail Strike Troublesome Tiedowns Helideck and Helicopter Egress Training Facilities Wrong Deck Landings FOD and an AS350B3 Accident Landing on a Yacht in Bergen UPDATE 16 April 2022: Helideck Heave Ho! Aerossurance regularly assists oil and gas companies and vessel operators review and update their helideck procedures and adverse weather policies, examine helideck structural integrity issues and provide independent assurance of helideck readiness. Aerossurance is pleased to be supporting the annual Chartered Institute of Ergonomics & Human Factors’ (CIEHF) Human Factors in Aviation Safety Conference for the third year running. We will be presenting for the second year running too. This year the conference takes...
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