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Sunday, July 24, 2011

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

Previous: Part 01 - Part 02 - Part 03 - Acronyms
How to determine the point of replacement of component that is under On-Condition Maintenance?
The concept of P-F curve or degradation curve is used to determine the point of failure and thus before it, the point at which the component must be replaced as part of the On-Condition Maintenance process. The P-F portion of the curve indicates when a potential failure is first visible to us [potential failure point 'P'] and the operating condition where the equipment is rendered unusable due to its performance having degraded to unacceptable levels [functional failure point 'F'].

Let us take an example of a pump designed for special duty. The pump may be used until its impeller wears to such an extent that it fails to deliver the required minimum flow it had been designed for.  The first indication of lowered flow is denoted by the point 'P' while the state of the impeller at which it fails completely to deliver required flow of fluid is therefore denoted by point 'F'. Although the pump may still be operating subsequent to point 'F', it is no longer fulfilling its minimum functional duty.
The concept of the P-F curve is applicable to every component and system in the aircraft. If a component has, say 20 working element, each element will have its own P-F curve. Condition monitoring equipments are used to indicate the point 'P' of the element to be forewarned of any impending failure. For practical & economic reasons only critical elements are placed under observations.
The P-F interval selected is typically based on the worst failure suffered during operation of the equipments. The history of failure of other comparable equipment could also be used for determining the P-F interval. Also, performing a  reliability failure assessment of the item could be used to find out the corresponding P-F curve.
The P-F curve is probabilistic & its variation depends on stresses acting on the component & number of occasions when the part has been over-stressed. If the component is subjected to frequent high stresses, including beyond that specified by the Factor of Safety, chances of its failure correspondingly increase.
Taking the previous example of component for consideration, if it is properly setup & operated following all laid down guidelines, then the stress causing wear accumulates slowly and the P-F curve of that element could be decades. However, if operating conditions are extreme & guidelines not strictly adhered to then the time interval denoted by P-F curve could even be months. In the earlier operating condition, the element may have long intervals between condition monitoring while in the latter case condition monitoring may be need in much smaller time periods.
Benefits of implementing On-Condition Maintenance
It is estimated for the Phenom 100 aircraft, implementation of OCM would push inspection & removal & overhaul limit of the hot-section of the engine from 1750h & 3500h respectively to 5000h for inspection & 10000h for removal & overhaul.
Proper implementation of an OCM policy would help bring down expenditure incurred due to replacement & overhauling and thereby bring down running costs, while maintaining the same standards of safety & reliability, or in many cases even enhancing it.
Another benefit of reduced maintenance requirements of the aircraft is the fact that interaction of human operators [maintenance personal] with the aircraft is reduced. As a result opportunities for the operators to commit a mistake while working on the aircraft too correspondingly reduces.
Challenges and drawback in implementing On-Condition Maintenance
OCM as a policy is not without its drawbacks & challenges. A common misconception often attached with On-Condition Maintenance is that it is a fit & forget policy. Also, many AM's are unwilling to service an aircraft in accordance with OCM philosophy. This may arise due to old stereotypes that the AMs may hold on to & also since an OCM, if improperly implemented, could even cause equipment failure during operation, adding to AM liability. The margin of error offered to those responsible for an aircraft in OCM is smaller than for an aircraft following traditional maintenance standards. AMs, have, all these years been familiar with maintaining aircraft following the hard-time overhaul philosophy, creating a certain level of comfort with this philosophy. So suggesting any change in their practices is bound to be met with reluctance & possibly resentment on their part.
These reservations on part of the AMs & TCs could be mitigated if, say the Civil Aviation Authority or any other relevant body of the country undertakes a campaign to raise awareness & dispel myths & wrong notions about the practise of On-Condition Maintenance & conduct workshops & seminars for them to bring them up to the challenges of performing OCM.
Due to increased monitoring and inspection requirement of an On-Condition Maintenance programme, use of condition & trend monitoring equipment is highly desired for safe operation. However adding these equipment in the scheme of things of the aircraft also leads to cost increase. This adds both to the complexity and cost of operation of the aircraft. Therefore a careful evaluation must be made before any decision can be taken with regards to adoption of On-Condition Maintenance programme.
As an aircraft being maintained by OCM does not undergo replacement and overhaul in any fixed time duration, cost incurred in maintaining the aircraft is distributed unequally over its life, thereby posing challenges with estimating expenditure.
The need to undertake the most cost-effective operation has been the driving force for organisations to strive towards improving the reliability of equipment in order to improve overall performance of the system. Successful efforts to increase reliability growth and improve performance is about 'doing the right maintenance on the equipment'. In view of this sentiment, On-Condition Maintenance fits in quite perfectly as a philosophy. A properly designed OCM would play a significant role in achieving the end.
The wide range of mindsets, myth, mistakes & misconceptions are prevalent in the maintenance arena which often hampers adoption of new processes & even if adopted, are wrongly implemented, causing grievances & forcing a re-look at a potentially beneficial solution. Achieving reliability growth or performance improvement is neither quick nor easy.
On-Condition Maintenance has been proven on multiple occasions to be the right maintenance policy for plants and equipment in its operating context. With its implementation in the demanding aviation industry, both civil & defence, OCM is not a 'quick fix' but when applied correctly, it can transform an organisation's outlook towards maintenance and lead to tangible improvements in equipment reliability, cost-effectiveness and overall performance. OCM optimises the maintenance for the on-site equipment and in so doing ensures that money spent on maintenance is utilised in an optimum manner, reducing wasteful expenditure, like replacing components still performing as required, owing to demands of hard-time overhaul.
Applying On-Condition Maintenance correctly requires both time and resources. However, once implemented, for an organisation that possess high value assets, the investment required to implement it correctly would be negligible & is bound to give assured  cost returns.
In conclusion, it must be said that RoI for OCM is significant provided that the implementation of the programme is carried out with due diligence & preconceived notions & prejudices  are dispelled and the right maintenance is implemented.