Due to their design, helicopters are potentially vulnerable to catastrophic mechanical failures because of the number of single-load-path critical parts within the rotor and rotor drive systems and the reduced redundancy within their design. This vulnerability and high accident rates experienced in the 1970s and 1980s led to the development of systems able to monitor the health and integrity of rotors and rotor drive systems. (EASA RMT)
Health and Usage Monitoring Systems (HUMS) are designed to automatically monitor the health of mechanical components in a helicopter, as well as usage of the airframe and its dynamic components. HUMS enable aircraft to record structural and transmission usage, transmission vibrations, rotor track and balance information, and engine power assurance data. HUMS not only monitor the health of rotating components such as gearboxes, bearings, shafts, engines and rotors through vibration but also can record parametric data from the aircraft’s bus for usage and event analysis. Subtle changes in vibration are recorded in flight, visualized on the HUMS ground station compute, and evaluated by technicians. The intelligence gained from the use of HUMS allows aircraft maintainers and fleet operators to make informed decisions about flying and maintaining their aircraft. (IHST)
Vibration Health Monitoring (VHM) systems are an element of HUMS. Research and statistics show that HUMS and VHM have significantly reduced the accident rate in rotorcraft since their introduction into North Sea operations in the early 1990s. As a result of this success, the U.K. Civil Aviation Authority mandated HUMS (CAP 753) in aircraft that carry nine or more passengers. Studies have also concluded that these systems were capable of successfully detecting approximately 70% of the failure modes that occurred on components that the system was designed to monitor. (IHST)
- Vibration — Mechanical oscillation or motion about a reference point of equilibrium.
- Vibration Health Monitoring — Use of data generated by processing vibration signals to detect incipient failure or degradation of mechanical integrity.
- VHM Indicator — The result of processing sampled data by applying an algorithm to achieve a single value that relates to the health of particular component failure modes. Primary VHM indicators will be those that can be monitored directly for the purposes of generating alerts. Secondary VHM indicators are those that can be used in the diagnostic process after an alert is generated but are unsuitable for direct alert generation.
A VHM system typically comprises vibration sensors and associated wiring, data acquisition and processing hardware, the means of downloading data from the helicopter, a ground station and associated instructions for the operation of the system.
The VHM system should measure vibration characteristics of rotating critical components during flight utilising suitable vibration sensors and recording equipment. The vibration measurements being processed, in order to generate VHM indicators, should normally relate to specific component failure modes or be capable of detecting changes that affect safe operation.
A VHM system typically monitors a number of assemblies and component types, including the engine; gearboxes and associated shafts, gears and bearings; tail rotor drive shaft; engine to main gearbox input drive shafts; the oil cooler; and the main and tail rotors.
Accident & Incidents
On 28 December 2016, yaw control was lost during touchdown of a Sikorsky S92A landing on a North Sea offshore platform and it almost fell into the sea. The Investigation found that the loss of control was attributable to the failure of the Tail Rotor Pitch Change Shaft bearing which precipitated damage to the associated control servo. It was also found that despite HUMS monitoring being in place, it had been ineffective in proactively alerting the operator to the earlier stages of progressive bearing deterioration which could have ensured the helicopter was grounded for rectification before the accident occurred.
On 29 April 2016, an Airbus EC225 Super Puma main rotor detached without warning en-route to Bergen. Control was lost and it crashed and was destroyed. Rotor detachment was attributed to undetected development of metal fatigue in the same gearbox component which caused an identical 2009 accident to a variant of the same helicopter type. Despite this previous accident, the failure mode involved had not been properly understood or anticipated. The investigation identifies significant lessons to be learned related to gearbox design, risk assessment, fatigue evaluation, gearbox condition monitoring, type certification and continued airworthiness, which may also be valid for other helicopter types.
On 3 July 2010, an AW139 helicopter was climbing through 350 feet over Victoria Harbour Hong Kong just after takeoff when the tail rotor detached. A transition to autorotation was accomplished and a controlled ditching followed. All occupants were rescued but some sustained minor injuries. The failure was attributed entirely to manufacturing defects but no corrective manufacturer or regulatory action was taken until two similar accidents had occurred in Qatar (non-fatal) and Brazil (fatal) the following year and two interim Safety Recommendations were issued from this Investigation after which a comprehensive review of the manufacturing process led to numerous changes.
On 1 April 2009, the flight crew of a Bond Helicopters Eurocopter AS332 L2 Super Puma en route from the Miller Offshore Platform to Aberdeen at an altitude of 2000 feet lost control of their helicopter when a sudden and catastrophic failure of the main rotor gearbox occurred and, within less than 20 seconds, the hub with the main rotor blades attached separated from the helicopter causing it to fall into the sea at a high vertical speed The impact destroyed the helicopter and all 16 occupants were killed. Seventeen Safety Recommendations were made as a result of the investigation.