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Helicopter Safety
The large number of flight safety critical rotating components on a helicopter means that the problem of maintaining high standards of airworthiness relates directly to preventing catastrophic mechanical failures. An engine failure on a multi-engined aeroplane is a hazardous event, however it should not lead to loss of life. By comparison, failure of the main rotor, main rotor gearbox and, on some helicopters, the tail rotor or tail drive train, will often lead to loss of life.
The safety of rotating components is achieved by three complementary techniques. For each of these, there are corresponding monitoring techniques. Historically the only monitoring techniques available were based upon manual procedures but the advent of accurate flight data and analysis techniques has meant that the manual procedures can now be augmented or even replaced by flight data-based procedures.
Limiting the stresses imposed on the components
The oldest form of monitoring is to apply limits and require the pilot to restrict operation to these limits. Thus, for example, a rotor speed limitation of 105% might be imposed, and the pilot would be trained to fly keeping the rotor speed normally at 100% and never exceeding 105%. In the unlikely event that he did exceed that limit, it would be the pilot's responsibility to report the exceedance to a maintainer who would then inspect and possibly replace the overstressed components.
Helicopters often carry warning indicators for the most critical limitations, however these are seldom exhaustive and so assessment of flight data recordings can provide valuable information. The first stage is to analyse the data for all the flight manual limitations, and to alert the maintainers if any limit has been exceeded. Some limits can be difficult to interpret, particularly in moments of excitement. When the pilot is in difficulties and trying to avoid an accident, they should should be concentrating on flying the aircraft and not be monitoring all of the instruments for exceedances. After the flight,a readout of the data can allow any unnoticed exceedances to be identified, quantified and where necessary corrective maintenance can be undertaken.
Restricting use of the components to a safe life
Components that undergo high levels of fatigue stress can be given a life that ensures that any fatigue damage does not propagate to cause component failure. This process makes assumptions about the usage of a component and, in turn, the way in which the aircraft is to be flown and operated.
Flight data analysis can aid this process in tow ways. Firstly, for aircraft with an assumed operating spectrum, analysis of the data can show how well the assumption fits the actual operation of the aircraft. If there are significant discrepancies then the life calculations can be revised - up or down - to accommodate the actual pattern of use. Secondly, for more accurate lifing, flight data can be used to calculate the actual fatigue life consumed on each flight. This is then specific to the individual aircraft components and so means have to put in place to track the accumulated life consumed as components are replaced and returned for overhaul.
Identifying wear or damage to components while in service
Inevitably components wear out or fail unexpectedly. The traditional monitoring techniques such as oil debris, SOAP and engine performance trending are aimed at identifying these problems before the become hazardous.
Techniques using conventional flight data can be used here, for example, engine performance trends can be calculated from engine parameters gathered in flight. Unfortunately, traditional "Accident Data" is usually inadequate for monitoring wear in rotating components and so a host of specialised techniques have been developed to measure the "health" of the rotors and transmission. These rely heavily upon vibration measurements and require special instrumentation and data acquisition systems.
The impact of helicopter health monitoring has been remarkable. Up to 1992, when these systems were introduced to large helicopters flying to the oil fields of the North Sea, vibration related helicopter accidents or incidents occurred at the rate of one or two per year. In the following ten years, only one vibration related incident was reported.
HUMS and HOMP
Advanced helicopter monitoring systems were implement in the early
1990's following concern over the airworthiness of helicopters and, at the time, technical defects were the main issue. The acronym HUMS, Health and Usage Monitoring Systems, was introduced for these systems.
With the introduction of HUM systems, the number of incidents relating to technical malfunctions decreased and as a consequence the proportion of incidents relating to aircrew error increased. Interest therefore turned to implementing an operations-based monitoring programme, like the fixed wing FDM, but tailored for helicopter operations. As a result, the concept of a helicopter Operations Monitoring Programme, HOMP, was developed and trials, sponsored the the UK CAA and Shell Aircraft, started on the North Sea in 2001. |
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