The ALS is part of the certificated product (aircraft, engine, propeller) type design (Part 21.A.31.a), that contains the mandatory scheduled maintenance items and the limitations for part replacement, necessary to maintain compliance with that type design. For each individual aircraft, an approved aircraft maintenance programme (AMP) must be created, initially containing the ALS at the revision level applicable at the time of the aircraft’s first certificate of airworthiness.
The ALS content is regulated by the airworthiness codes, e.g. CS 25.1529, Appendix H25.4. Each subsequent change to the ALS is approved by EASA. In view of the nature of the tasks contained in the ALS, failure to comply with an ALS revision (i.e. new or more restrictive tasks) would – in general – lead to an unsafe condition. Since EASA has, under ICAO guidelines, the duty to notify new or more restrictive ALS tasks as ‘Mandatory Continued Airworthiness Information’ to ICAO Contracting States, AD action will be taken for these specific ALS revisions.
In case an ALS revision only provides ‘relief’ (less restrictive), no AD will be issued, as there would be no ‘safety’ justification for such action. The existing EASA AD for a previous ALS revision, however, always allows the use of ‘later approved revisions’ to ensure compliance with the AD, which includes the use of any extended compliance time(s).
Under Commission Regulation (EU) 1321/2014 (Part M), operators are required by Part M.A.302 (a) to have an approved AMP, which establishes compliance – Part M.A.302 (d)(ii) – with the instructions for continued airworthiness (ICA) published by the (S)TC holder of the affected product, with the ALS being part of that ICA.
However, Part M does not specify exactly when, after publication of an ALS revision, an AMP must demonstrate compliance with that particular ALS revision and, consequently, from which time the new or more restrictive ALS tasks must be complied with. In practice, any new on-aircraft task may not be accomplished until after the AMP has been updated, i.e. after approval of that AMP update by the competent authority.
These are additional reasons why EASA issues an AD for an ALS revision: to require, from the effective date of the AD (even before the AMP update), the accomplishment of the on-aircraft tasks (before exceeding their due date) and to require the update of the AMP within a clear (and reasonable) compliance time.