However, as stated earlier, maintenance of not all systems and components can be transitioned to the On-Condition Maintenance standard. According to the Maintenance Directive NL-2010-001 issued by the Civil Aviation Authority of the Netherlands [CAA-NL] some components that may not be maintained by OCM are,
|1||Fuel injection system||No reliable condition determination is known to CAA-NL.|
|2||Constant-speed (variable pitch) propellers and constant-speed propeller parts and systems.||Critical failures can develop from the inside. No reliable condition determination is known to CAA-NL|
|3||Fixed pitch metal blade propellers||All O/C programmes known to CAA-NL basically contain the same instructions as applicable to overhaul, therefore no added value|
|4||Flexible hoses carrying flammable liquids||Critical failures can develop from the inside. No reliable condition determination is known to CAA-NL|
|5||Pitot Static System (incl. flexible hoses)||No reliable condition determination is known to CAA-NL|
How is on-condition maintenance procedure implemented?
It must be kept in mind that On-Condition Maintenance by no means advocates a policy of fit & forget or fit until failure. On the contrary, an average OCM programme demands increased inspection & monitoring of systems, to watch out for any signs indicating initiation of failure, & in such an eventuality, take the required corrective actions. By doing so, one can ensure safe operations, while at the same time maximising the operational life of components & work towards minimising expenditure incurred due to overhauling & replacing, without endangering operation.
Every system requiring periodic maintenance & over-haul requires an on-condition maintenance process compatible with it. Also authorized aviation authorities of countries lay down guidelines to be followed by Aircraft Maintenance Organisation [AMO] who intend to use the On-Condition Maintenance standard for aircraft's maintenance.
The broad, basic framework of the process can, however, be said to common across the board.
For example, prior to transitioning an aircraft's engine to On-Condition Maintenance, it is necessary to asses & evaluate its baseline conditions. Data obtained from this assessment would then serve as datum for comparing future condition of the aircraft systems once transition to OCM has been completed. However, it is pertinent to note that the assessment must be made prior to the system reaching manufacturer recommend Time Between Overhaul [TBO]. Thus it is essential to schedule such an assessment accordingly. Once assessed, accurate records need to be kept.
Some general guidelines that is to be followed while transitioning from the hard time standard of maintenance to an On-Condition Maintenance standard,
- Seek information and data obtained from a similar transition to On-Condition Maintenance standard by a related organisation.
- Define the required type and content of inspection and equipments needed to carry it out.
- Analyse own earlier efforts, if any, in extending life of aircraft components & reducing the need to carry out replacements & see if the results of the project can benefit existing transition programme.
- Define relation between limitations imposed by manufacturer and Total Accumulated Cycle [TAC]
- Compare structural concept of aircrafts in operation with different operators & different geographical locations to evaluate if the policy to be implemented can be replicated elsewhere by different operators too or it is specific to the organisation taking up this transition.
- Establish a time interval between inspection & create appropriate technical documentation.
- Analyse all earlier failures & outcome of subsequent repairs.
- Determine parts critical to aircraft operation & also the most critical automation elements.
- Calculate the aircraft engine's TAC based on annual running time & kind of assignments performed by the aircraft.
- A competent level of decision making authority must be presented with the concept for their approval prior to its application in the real-world.
- The programme be kept in continuous control & refinements be incorporated in it with increased availability of data & experience with the program.
According to the Civil Aviation Authority of New Zealand, On-Condition Maintenance should also include the assessment of pilot monitored performance, functional checks, and scheduled maintenance, and use circumstantial servicings to carry out opportunity assessments of components.
In order to ensure accurate data collection involving the state of the components, on-line & on-board trend monitoring equipments are also commonly utilized as part of the OCM programme. It must be ensured that trend monitoring & analysis is carried out by a competent authority.
For example, the Embraer Aviation company, in collaboration with engine-manufacturer Pratt & Whitney [P&W] Canada are working towards developing such an automated monitoring system for the Embraer's Phenom-100 Executive Jets. It is being designed to send the data recorded in the aircraft's on-board maintenance computer to ground receiver stations. On ground, Embraer will analyse the engines & other sub-systems' performance to look out for anomalies & other trends. If deviation from expected behaviour are observed, then the owner of the aircraft would be informed, if required & the information logged for record as is expected in On-Condition Maintenance. As described, the monitoring system would transmit fault data of the engine & aircraft as a whole to the ground for analysis. If required, the system would also permit the owner of the aircraft to download the data and send it to Embraer for evaluation, if he so feels.
Any Maintenance process, in general, must prove to be capable of monitoring these following conditions:
- Material degradation [corrosion, drying, hardening]
Of these, Fatigue issues are generally addressed by means of Airworthiness Limitations guidelines.