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The MEL, minimum equipment list, defines whether fault of some equipment on the aircraft prevents it from being dispatched for flight or not.

However, sometimes (e.g. in comments to this question) there is misunderstanding regarding what it means to say something is on the MEL. So, when I say something is on the MEL, does it mean that:

  • the aircraft can't be dispatched without it, i.e. the list contains the required items, or
  • that the aircraft can be dispatched without it, i.e. the list contains the optional items?

This answer suggests the former, but I've heard/read it used in the other way, so I would like some clarification.

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    $\begingroup$ As a little background from the past, in the 1990s I flew for two different 747 carriers, both of which were known for their less than optimal maintenance, except that it was optimal from the standpoint of their bottom lines. At both airlines a little yellow sticker about 0.75 inch on each side was affixed next to cockpit items that were inop. Inop items not in the cockpit had their stickers on the aluminum maintenance lot cover. Rarely was there ever a flight for which there were no stickers. The typical flight would have half a doxen stickers as I remember. A dozen was not unheard of. $\endgroup$ – Terry Jun 18 '16 at 22:17
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    $\begingroup$ Both answers in the referenced question aviation.stackexchange.com/questions/12070/… agree with the later, that it is a list of items that can be inop and the aircraft dispatched. They are light on details, but are in agreement with my answer here and comment on the other post. From the linked answer: "it details which equipment is allowed to be inoperative without grounding the aircraft." $\endgroup$ – OSUZorba Jun 19 '16 at 4:32
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The MMEL and MEL list items that can be inop while still dispatching the aircraft, it also lists any required restrictions of that system being inop.

I think FAA Policy Letter 34, which is the preamble for all MMELs, gives some of the best information:

First they set the basis of an MEL, that every system/function of the aircraft must be operation to dispatch. This restriction can be superseded by the MEL:

The following is applicable for authorized certificate holders operating under Title 14 Code of Federal Regulations (14 CFR) Parts 121, 125, 129, 135: 14 CFR require that all equipment installed on an aircraft in compliance with the Airworthiness Standards and the Operating Rules must be operative. However, the Rules also permit the publication of a Minimum Equipment List (MEL) where compliance with certain equipment requirements is not necessary in the interests of safety under all operating conditions.

Then they tell you that MEL includes items that are allowed to be inoperative along with any limitations:

The FAA approved MMEL includes those items of equipment related to airworthiness and operating regulations and other items of equipment which the Administrator finds may be inoperative and yet maintain an acceptable level of safety by appropriate conditions and limitations

The individual operator's MEL, when approved and authorized, permits operation of the aircraft with inoperative equipment. Equipment not required by the operation being conducted and equipment in excess of 14 CFR requirements are included in the MEL with appropriate conditions and limitations. The MEL must not deviate from the Aircraft Flight Manual Limitations, Emergency Procedures or with Airworthiness Directives. It is important to remember that all equipment related to the airworthiness and the operating regulations of the aircraft not listed on the MMEL must be operative.

The image below is from the 767 MMEL, as you can see it lists items that can be inoperative and the limitation. Some items carry many restrictions, such as anti-skid or a pack, others carry none, such as reading lights. The MMELs are available on the FAA's website, so it can be an interesting read.

enter image description here

I do not know why the terminology is backwards, I assume it comes from a time aircraft were much less complex and an list of required items was realistic. But if it only listed items that were required for flight, you could end up with hundreds of systems inop, with no direct requirement to repair them. PL-34 addresses this a little bit:

The MEL is intended to permit operation with inoperative items of equipment for a period of time until repairs can be accomplished. It is important that repairs be accomplished at the earliest opportunity. In order to maintain an acceptable level of safety and reliability the MMEL establishes limitations on the duration of and conditions for operation with inoperative equipment.

and

Operators are responsible for exercising the necessary operational control to ensure that an acceptable level of safety is maintained. When operating with multiple inoperative items, the interrelationships between those items and the effect on aircraft operation and crew workload will be considered.

Basically the take away from this is that an operator can not issue every MEL in the book and dispatch the aircraft. For example, it may be okay to MEL one AC pack or one AC pack temp control valve, but it would not be okay to MEL one pack and the temp control valve on the other pack.

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    $\begingroup$ I am not sure the list of required items would be shorter. But for required items, there is not much to say: they are simply required. It is the items that can be inoperable under certain conditions that need something said about them: the conditions. $\endgroup$ – Jan Hudec Jun 20 '16 at 6:14
  • $\begingroup$ Technically, an aircraft can be dispatched with a non-MEL item inoperative; you just need a ferry permit from the FAA and can't carry any passengers (even non-revenue) or cargo. $\endgroup$ – Sean Apr 3 '19 at 20:58
  • $\begingroup$ @Sean this true, thanks for adding it. I will clarify that ferry flights are Part 91, not Part 121 or 135. $\endgroup$ – OSUZorba Apr 7 '19 at 3:28
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The minimum equipment list is a list that requires a letter of authorization in order to make it valid, and it contains items that may be inoperative or removed from the aircraft.

If something breaks or is removed, and it isn't on the MEL, then the flight is a no-go.

Source: §91.213 Inoperative instruments and equipment.

Authorization for Use

(a) Except as provided in paragraph (d) of this section, no person may take off an aircraft with inoperative instruments or equipment installed unless the following conditions are met:

(1) An approved Minimum Equipment List exists for that aircraft.

(2) The aircraft has within it a letter of authorization, issued by the FAA Flight Standards district office having jurisdiction over the area in which the operator is located, authorizing operation of the aircraft under the Minimum Equipment List. The letter of authorization may be obtained by written request of the airworthiness certificate holder. The Minimum Equipment List and the letter of authorization constitute a supplemental type certificate for the aircraft.

Regulations stating that the important items are not allowed to be listed on the MEL

**This suggests that it is a list of items that may be removed or inoperative.

(3) The approved Minimum Equipment List must—

(i) Be prepared in accordance with the limitations specified in paragraph (b) of this section; and

(ii) Provide for the operation of the aircraft with the instruments and equipment in an inoperable condition.

(4) The aircraft records available to the pilot must include an entry describing the inoperable instruments and equipment.

(5) The aircraft is operated under all applicable conditions and limitations contained in the Minimum Equipment List and the letter authorizing the use of the list.

(b) The following instruments and equipment may not be included in a Minimum Equipment List:

(1) Instruments and equipment that are either specifically or otherwise required by the airworthiness requirements under which the aircraft is type certificated and which are essential for safe operations under all operating conditions.

(2) Instruments and equipment required by an airworthiness directive to be in operable condition unless the airworthiness directive provides otherwise.

(3) Instruments and equipment required for specific operations by this part.

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  • $\begingroup$ Your source part went incomplete. $\endgroup$ – SMS von der Tann Jun 18 '16 at 20:29
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    $\begingroup$ How did this terminology get reversed? "Minimum equipment" implies that in order to fly the plane must have, at a minimum, all the items on this list. Somehow it means the opposite. $\endgroup$ – TomMcW Jun 18 '16 at 20:49
  • $\begingroup$ To a Brit, the term "minimal equipment list" would seem a better (but still rather cryptic) description of what it is than "minimum". In other words, "you can get by without this stuff if you have to, but you should really get it fixed ASAP". $\endgroup$ – alephzero Jun 19 '16 at 1:18
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    $\begingroup$ @TomMcW Well, it is the document which allows you to determine the minimum equipment required for dispatch. It just goes about it in a manner which may seem odd at first, but consider that there are often restrictions associated with inoperative equipment that need to be listed, and that they don't want to implicitly allow flight with something that was forgotten. It is important that the implications for each item being inoperative are individually considered. If it isn't listed, it's required and therefore is part of the minimum equipment! $\endgroup$ – Lnafziger Jul 24 '16 at 12:39

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