Ensuring that engines achieve their predicted reliability is governed by several factors - e.g. continuing airworthiness using condition monitoring and power assurance. This sometimes results in engines being replaced before their scheduled in-service life time. Here is a copy of some guidance on engine reliability / monitoring:
Engine Reliability / Monitoring
The reason for monitoring engine reliability is to eliminate ‘abuse’ or to spot a ‘trend’ that will eventually lead to failure if no action is taken; this is particularly important when the helicopter is used in a dual role – i.e. aerial work as well as commercial air transport. A reduction in power revealed through a scheduled ‘power assurance’ check is a useful indicator, providing it is used in accordance with manufacturers procedures for rectification or removal. No over-temp or over-torque is likely to result in immediately failure, but they can be precursors to eventual failure.
It is unlikely that any current system (apart from chip warning) will provide an ‘immediate’ warning of impending failure; in any case, if operating in PC3 over a hostile environment an indication of failure could not be acted upon without consequence.
An operational approval system should require the engine/type to be assessed for reliability – and specify a means for achieving that. This requires the reliability analysis and reports, produced by manufacturers, to be available to States on request. The measure of reliability is a principal element (power unit failure rate per 100,000 engine hours). Generally, for turbine engines, reliability for the helicopter/engine combination is established by major manufacturers.
Flagging exceedances, eliminating abuse and spotting trends, is a way of ensuring that the ‘reliable’ engine/type, used for operations with exposure, stays within its reliability target. Monitoring systems may be used to encourage pilots not to abuse engines, and operators to control trends and report failure. Thus, instilling a culture of caring for propulsion units.
With modern control of engines using Full Authority Digital Engine Control (FADEC), and other systems, there is often built-in control and potential for monitoring. Abuse of engines is prevented by FADEC or shown as an ‘indelible’ flag that must be recorded and investigated.
Only with legacy helicopters should it be necessary to ‘bolt-on’ an engine usage monitoring system. Such systems are now relatively low cost and can pay for themselves just by eliminating hot starts.
Statistically, most engines can be shown to be within the 1 x 10-5 to 1 x 10-6 reliability range. Abuse control is aimed at ensuring they stay within that range. Further improvement in engine reliability by monitoring is unlikely to change the risk profile to any discernible degree.
In assessing how much weight to give to engine monitoring systems in the risk assessment, it is necessary to be realistic about the effect of reliability on the risk profile of the exposed operation. It is likely to be less than one order of magnitude. Potential gain may have already been realised by manufacturers installing the latest engine control and monitoring equipment as standard.
Considering the statistical reliability of engines is one way to assess risk. Not all aircraft engines have the same reliability and it is reasonable to require a higher level of dependability for particular ‘types of operations’ or flight over specific ‘geographical areas’. Certain aircraft/engine types might, appropriately, be deemed unacceptable for some high-risk operations.
Assessment of reliability is a complex task because it relies upon the reporting of failure, regardless of outcome, and an accepted method of collecting and collating flying hours for the helicopter/engine type. Without these, it is extremely difficult to establish failure rates on which any measure of reliability can be based but neither failure nor flying hours are required to be reported under the existing regulations of some ICAO Member States.
A comprehensive usage monitoring system, as well as the provision of failure and usage reports to the manufacturer, can provide data for trend monitoring and local analysis of health as well as reports of exceedances. These are likely to have a beneficial effect on local culture and reliability statistics as well as providing evidence of the precursors of failure.
Maintenance
It is essential that when operating with exposure, the maintenance programme includes the helicopter/engine modification standard using the preventive maintenance actions recommended by the helicopter and/or engine manufacturer. This representative guidance has been useful in establishing the categorisation of failure – with respect to exposure – and providing a method of establishing and promulgating reliability rates.
Maintenance is often overlooked as a risk mitigation strategy since it is considered a normal aspect of any air operation. If a quality maintenance program is in place, an additional preventative maintenance and trend monitoring regime can be considered as risk mitigation for operations with exposure. This must be part of a controlled system to avoid Human Factors events in unnecessary intervention procedures.
The modification standard that is applied to any helicopter/engine combination is dependent upon the assessment by the State of the susceptibility to known failure(s) of a component, and the subsequent risk to operations. For example, where a component is an element of a redundant system, rectification might be contained in a ‘bulletin’ rather than a ‘directive’, or time to rectification might be less stringent.
In cases where redundancy provides no protection: for PC2, exposure in the take-off and landing phases; or, for PC3, exposure in any phase, the State should ensure that operators are made aware of, and apply, the most stringent of modification standards with respect to any known susceptibilities.
In addition, the State should ensure that a system of preventative maintenance and trend monitoring is used to spot precursors ahead of failure as an element of the maintenance system and the overall operator Safety Management System.