In a normal scenario :
- The date of signing the certificate of release to service (CRS) should be considered the date of the accomplishment.
- The next due date should be calculated using this date.
However, there may be a lot of different considerations that change the normal scenario and make the statements above no longer valid. For example:
Case 1: The interval of the maintenance task has been ‘extended’ using a procedure included in the aircraft maintenance programme and approved by competent authority (refer to Appendix I to AMC M.A.302 point 4). Such procedure is often referred as permitted variation or ’tolerance’. In this case the next due date calculated using the original due date.
Case 2: The maintenance task refers to a component maintenance task, for example the landing gear overhaul. In this case the start of the interval would be the date of the release to service after the overhaul of the landing gear or in some particular cases when specified in the maintenance data the interval may start from the date of installation.
Case 3: The task is part of a maintenance check, where the duration of the check is significant compared to the interval of the task. For example, a check that lasts for 2 months and an inspection that has an interval of 3 months. In this case, it is reasonable to think that the performance of this task would need to be planned for the last days of the maintenance check, when possible. Otherwise, the inspection also can be done on the first day, but in that case, it is reasonable to expect that it will be released the same day (then the next due date would be 3 months after the CRS is signed). It also applies to the specific cases of mandatory tasks (ADs, CMRs, ALIs, etc.) defining repetitive action with a calendar limit.
There are many other examples, the key is to use sound engineering judgment and the guidance provided in the Instructions for Continuing Airworthiness to calculate the next due date.