Maintenance-free operating periods – the designer's challenge

It is generally accepted that a major hurt for the aerospace industry is the need for unscheduled maintenance. For a military aircraft operator this can affect operational availability and degrade the ability of an airforce to carry out its function. Maintenance-free operating periods (MFOPs) are not a new concept to the aerospace industry – the goal of MFOP attainment has been with us for many years; what has changed is the drive from customers of our products to reduce life-cycle costs (LCCs). The military aircraft customer is expected in future to demand a period of operations during which the availability of the air system can be guaranteed. This represents a paradigm shift in the way in which system designers approach the issues of reliability and availability. Traditional thinking on reliability has been centred around the measurement unit of mean time between failures (MTBF). The use of MTBF ‘budgets’ confers an attitude of an acceptable level of failures. The designers will have to design their system from the outset to meet the MFOP requirement of the aircraft. The challenge lies in the methodology and tools required to produce a system architecture that will reach the target MFOP. The MFOP ‘options’ are currently perceived to be (a) inherent reliability, (b) redundancy, (c) reconfigurability, (d) prognostics (the prediction of failure) and diagnostics and (e) lifing policy. When examining the required functionality of the system, a designer would assess each part of the system which contributes to that functionality (be it a valve, pump or transducer). Then a determination must be made of which MFOP ‘option’ is the most suitable to maintain that functionality. This paper will examine the concept of MFOPs and how the system designer can respond to the challenge to achieve the MFOP aircraft required for the 21st century.