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Sunday, July 24, 2011

On-Condition Maintenance of Aircrafts: An overview 01 of 04

The Indian Air Force [IAF] is currently in the process of imparting a Mid Life Upgrade [MLU] to its entire fleet of the Russian origin MiG-29 multi-role combat aircraft. Included in the technology & capability upgrades being incorporated into the aircrafts, is the transition towards performing on-condition maintenance of the aircrafts from the traditional hard-time maintenance procedures currently being followed on the fleet.
Traditionally, life of aircraft components & their subsequent replacement have been specified in terms of Time Between Overhaul [TBO]. Such practice entails that the component in question be replaced or overhauled irrespective of their present state of performance & physical condition at that moment.

Considering the complexities of systems & assemblies that make up an aircraft &  available manufacturing technologies & materials used up until recently, such fixed-time based maintenance practices were essential in order to ensure safe operation & generate confidence amongst the stake-holders. This, in turn resulted in higher cost of operating the aircraft, owing to the rigid, non-negotiable need to replace or overhaul components as demanded by the TBO standard of  maintenance.
In recent times, the industry has seen sufficient advancements being made in manufacturing technologies which is enabling the manufacturer to produce even those components having the strictest & demanding product specifications of a very high quality. Also newer materials, possessing superior properties, have been developed & being used, giving better performance & reliability during operation.
These developments has helped instill confidence amongst Aircraft Maintainers [AM] & Type Certifiers [TC] to recommend & adopt newer practices of maintaining aircrafts that are adaptable & flexible, without compromising on the operational safety of the aircraft.
One such standard of practice that has emerged from the aviation industry & is steadily gaining acceptance amongst Licensed Aircraft Maintenance Engineers [LAME] & authorized national aviation authorities is that of On-Condition Maintenance [OCM].
History of On-Condition Maintenance
The concept of On-Condition Maintenance has emerged from the aviation industry as a means to reduce cost of operation without affecting performance & reliability.
It was first proposed in an United Airlines report prepared by Nolan & Heap – Reliability-Centred Maintenance [RCM]. They intended to develop scheduled maintenance programs for the aircraft based on safety and operational risk control reasons for a maintenance activity.
As stated in the report, the aim of the new process is,
“This volume provides the first discussion of reliability centred maintenance as a logical discipline for the development of scheduled-maintenance programs. The objective of such programs is to realise the inherent reliability capabilities of the equipment for which they are designed, and to do so at minimum cost. Each scheduled-maintenance task in an RCM program is generated for an identifiable and explicit reason. The consequences of each failure possibility are evaluated, and the failures are then classified according to the severity of their consequences. Then for all significant items – those whose failure involves operating safety or has major economic consequences – proposed tasks are evaluated according to specific criteria of applicability and effectiveness. The resulting scheduled-maintenance program thus includes all the tasks necessary to protect safety and operating reliability, and only the tasks that will accomplish this objective.”
Subsequently, the use of OCM as a policy has found application in other sectors and industries, and is more commonly called Condition-based Maintenance [CBM] in those industries. However, this write-up shall solely concentrate on its application & utilisation in the aviation industry for aircraft maintenance.
What is On-condition maintenance?
On-Condition Maintenance is a procedure standard that allows continued use of aircraft components & systems, even beyond the life as declared by the manufacturer, provided they meet quality & performance standards during monitoring and/or inspection. It involves implementing a preventive maintenance process whereby the  workpiece or system in operation is compared against an approved reference standard. If the condition of the inspected object is within acceptable limits, it continues to remain in operation. However, if not, it must be replaced or repaired in order to prevent any unsafe operating condition. In On-Condition Maintenance, the component in question is inspected as part of the schedule and, based on the results of the inspection, corrective actions may be initiated to ensure a functional & safe system.
This procedure standard does away with the mandatory time/cycle-based overhauling or replacement of a component. With the continued use of the system, longer than that specified by the manufacturer, significant cost savings can be achieved, without effecting the reliability & safety of the aircraft.
On-Condition Maintenance process are ideally implemented on components & systems whose testing, inspection, or examination can be carried out in-situ either visually or by means of some aid without the need to dis-assemble or dismantle it completely. OCM is carried out by monitoring the object under consideration either continuously or at pre-determined time schedule.
According to studies carried out in the aviation industry, it is estimated that maintenance of up to 89 per cent of components & systems can be satisfactorily handled through on-condition maintenance.
Is the practise of On-Condition Maintenance suited for implementation in a given setup?
Prior to implementation of the on-condition method of maintenance, it is important to evaluate the suitability, especially from the point of view of cost-effectiveness & effect on performance, of transitioning from current policy of maintenance to the on-condition standard. The effectiveness of the proposed OCM policy must be compared with that of current practice by using a suitable cost-comparison function model.
Effectiveness of an On-Condition Maintenance procedure depends primarily on how good the condition monitoring data is. It involves the quality of data collection, processing, and analysis. Accurate indication of the degradation process occurring in the system in operation is of prime importance in assuring success of the maintenance standard being proposed for adoption.
In order to evaluate the suitability of OCM, cost incurred using it could be compared with three other possible policies:
  1. Escalation: Increasing interval between Hard-time maintenance without changing the process & procedure followed.
  2. Change content of hard-time maintenance, leaving the time interval unchanged, to obtain equivalent or improved maintenance standard
  3. Reducing maintenance requirements due decreasing effect of possible failure by reducing utilisation, adding redundancies to the system etc.
Tests need to be performed and observations must be undertaken to understand the ageing & stress accumulation of aircraft components. They must be subjected to the scientific rigours of analysis and monitoring for failure.
After evaluating the results of these analyses, decision-makers would be in a position to,
  1. Judge possible benefits of the On-Condition Maintenance policy being proposed.
  2. Evaluate the economic viability of the new maintenance policy being proposed
  3. Instill a level of comfort amongst Reliability Engineers [RE] and Statisticians with regards to the performance of the policy, if implemented by virtue of having taken into consideration alternative solutions in the evaluation model.
Only when sufficient evidence is collected across the fleet, and after a thorough scientific assessment confirms that the scheduled replacement time can be extended, are the manuals updated with the new requirements of OCM.
If On-Condition Maintenance process is selected, then monitoring requirements of the components increases, leading to added expenditure of incorporating trend monitoring equipments. Careful study must be made to ensure that cost savings obtained by prolonging the working life of a component are not nullified by the added expenditure of more extensive monitoring of the component as required in an OCM policy. If this is the case, then On-Condition Maintenance process is not suitable for the given system.
Some pre-requisites that an On-Condition Maintenance process must satisfy are:
  1. Capability to detect failures
  2. Modular assembly design
  3. Engine Condition Monitoring System [ECMS]
Read subsequent portions: Part 02 - Part 03 - Part 04