After an AD has been published, there may be the need to update its content. Depending on the impact of the changes that will be included, an AD can be corrected, revised or superseded.
An AD is superseded if accomplishment of that AD can no longer assure an adequate level of safety, of a single aircraft and/or of the fleet: this may occur, for example, if a more stringent requirement is added (compliance time and / or required action), or if the applicability is expanded to aircraft models and/or serial numbers not included in the original applicability.
This may also occur if an error is identified in the original AD, while it is physically possible to comply with that AD as published.
Consequently, even in compliance with the original (superseded) AD, the owner and/or the operator of an affected aircraft must always comply with the superseding AD.
A superseding AD has a new AD number.