Aircraft maintenance requirements fall into two main categories:
- Scheduled – where we know, in advance, when and what will cause the maintenance to be due.
- Unscheduled – where we do not know when the maintenance will be due although we may be able to anticipate a certain number of unscheduled maintenance events over a period of time from the analysis of reliability records.
In both of the above cases certain factors or events trigger maintenance as illustrated.
Consider, first, the factors which trigger scheduled maintenance. The primary trigger is the MAINTENANCE SCHEDULE itself, this is usually part of the MAINTENANCE PROGRAMME. The schedule must be approved by the Airworthiness Authority.
The maintenance schedule or programme specifies:
- which maintenance tasks are to be carried out and.
- when they are to be carried out.
The specification of WHEN is stated in periods between maintenance events measured in:
- days, weeks, months or years (calendar time).
- aircraft flying hours.
- engine or propeller hours.
- number of landings.
- number of cycles (eg engine start/stop cycle or pressurisation cycle).
The time related intervals may be combined so, for example, a maintenance task may have to be carried out every 50 flying hours or 62 days whichever is the sooner.
