No aircraft is so tolerant of neglect that it is safe in the absence of an effective inspection and maintenance programme. The processes that affect an aircraft are deterioration with age (e.g. fatigue, wear and corrosion) as well as chance failures (e.g. tyre burst, excess structural loads).
Aircraft maintenance can be defined in a number of ways and the following may help understand the different aspects:
“Those actions required for restoring or maintaining an item in a serviceable condition including servicing, repair, modification, overhaul, inspection and determination of condition”. [World Airlines Technical Operations Glossary]
“Maintenance is the action necessary to sustain or restore the integrity and performance of the airplane” [Hessburg, 2001]
“Maintenance is the process of ensuring that a system continually performs its intended function at its designed-in level of reliability and safety.” [Kinnison and Siddiqui, 2013]
Aircraft maintenance is that part of the process of aircraft technical activity which is conducted on aircraft whilst it remains in the line maintenance or base maintenance environment. Aircraft maintenance is intended to keep the aircraft in a state which will or has enabled a certificate of release to service to be issued. A hangar environment may be available but is often not necessary. The reasons for carrying out maintenance are neatly summarised by [Lam 2002]:
Maintenance will consist of a mixture of preventive and corrective work, including precautionary work to ensure that there have been no undetected chance failures. There will be inspection to monitor the progress of wear out processes, in addition to:
In general terms, for preventive work to be worthwhile, two conditions should be met:
This would typically include Pre-flight checks, daily checks (before first flight) fluids, failure rectification as well as minor, scheduled maintenance tasks as follows. According to EASA Part 145, AMC 145.A.10, line maintenance should be understood as “any maintenance that is carried out before flight to ensure that the aircraft is fit for the intended flight.” This may include:
EASA Part 145, AMC 145.A.10 also explains that “for temporary or occasional cases (ADs, SBs) the Quality Manager may accept base maintenance tasks to be performed by a line maintenance organisation provided all requirements are fulfilled as defined by the competent authority”. It is also noted that “Maintenance tasks falling outside these criteria are considered to be Base Maintenance”.
The third form of maintenance can be termed as “Workshop” or just Shop maintenance. This covers maintenance on components when removed from aircraft e.g. engines, APU, seats. Sometimes this is carried out within the same organisation as the Base Maintenance, but sometimes special companies carry out this work separately.
The intervals of maintenance are parameters set within the Approved Maintenance Schedule (AMS), which is in turn based on the Maintenance Planning Document (MPD). These will be set according to different criteria, mostly depending on how well damage can be detected and failure predicted [CAA, 2017]: