When does the calendar interval for the next aircraft or component maintenance task start?
In a normal scenario :
- The date of signing the certificate of release to service (CRS) should be considered to be the date of the accomplishment.
- The next due date should be calculated using this date.
However, there may be different considerations that render the normal scenario no longer applicable. For example:
Case 1: The interval of the maintenance task has been previously subject to a one-time extension using an approved procedure included in the aircraft maintenance programme (refer to Appendix I to AMC M.A.302 point 4) normally called ‘permitted variation’ or ’tolerance’. In this case the next due date should be calculated using the previous due date (as opposed to accomplishment date) or as agreed by the competent authority.
For aircraft regulated by Part-ML the situation is different when applying the tolerance of 1 month foreseen in ML.A.302(d), the next interval shall be calculated from the accomplishment date (refer to ML.A.302(d)(1) and AMC1 ML.A.302(d)).
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 on aircraft.
Case 3: The task is released as part of a maintenance check/visit, where the duration of the check/visit is significant compared to the interval of the task. In this case, there may be significant difference between date of accomplishment and date of release. For example, a check/visit that lasts for 2 months and an inspection that has an interval of 3 months. In this case, either the task is carried out on the last days of the maintenance check/visit and the next due date is calculated from the CRS, or the task is carried out at the beginning of the visit and the next due date should be calculated from the date of accomplishment.
There may be other examples, but the key principle is to use sound engineering judgment and the guidance provided in the Instructions for Continuing Airworthiness to calculate the next due date.