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What was the need to introduced MEL if MMEL is there and is less restrictive than MEL ?

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The Master Minimum Equipment List is issued by the manufacturer with the approval of the regulator and can be considered a "global" document you might say. The manufacturer will work hard to build the mathematical case to make the requirements as broad and flexible as it can. It's an ongoing process, and as the type is introduced and matures, the OEM will continue to increase the scope of the MMEL, because each new item added improves Dispatch Reliability (because more more and more components and subsystems can be inop without needing to be fixed right away).

The MMEL is the foundation for the Minimum Equipment List that the airline uses. The MEL is an in-house document created by the airline and approved by the local authority. It'll be based on the MMEL but may have additional limitations imposed by the local authority as it deems necessary for that operation.

It's similar to the Maintenance Program. The OEM will issue a Maintenance Program for a given aircraft type, but each airline will have its own variations by negotiation with its local authority.

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  • $\begingroup$ The airline can also opt to not include some items in MEL if it considers ability to dispatch with them inoperative isn't worth the additional procedures it would have to define and train for that case, no? $\endgroup$ – Jan Hudec Mar 24 at 21:58
  • $\begingroup$ Possible theoretically, but I can't think of a case where an airline would do that. Since items are added to the MMEL to improve dispatch reliability by allowing dispatch with inoperative items, by definition the inclusion of an item means there is a net benefit to deferring an inoperative component. The alternative is a delay or cancellation, and I've never seen a case where that is preferable to an "M" procedure (deactivation by techs) or an "O" procedure (flight crew mitigating action). Each plane will normally have a box full of deactivation parts like blanking plates in the cargo compt. $\endgroup$ – John K Mar 24 at 22:26
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The MMEL should list every piece of equipment and systems with which the aircraft was originally certified. It may place each piece of equipment into the category of being necessary for specific operations. This designates whether an aircraft continues to be airworthy and meets the standards of its Type Certification.

The MEL is a list of equipment that is allowed to be inoperative and the aircraft still be considered airworthy by the operators. While the MMEL is created by the manufacturer in conjunction with the civil aviation authority responsible for type certification and airworthiness standards and determination. The MEL is created by the owner/operators of the aircraft using the MMEL and heir own SOPs as a starting point.

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