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Deconstructing Minimum Equipment Lists

A minimum equipment list (MEL) is a list of equipment that may be inoperative, yet still allow an aircraft to operate safely, per FAA regulations. Why are MELs important?

“MELs are important because they enable an operator to fly the aircraft legally with inoperative equipment, thereby increasing operational dispatch reliability,” said Jon Haag, president of Haag Aviation Services, a business aviation consultant and member of NBAA’s Maintenance Committee. “Referring to the MEL also helps operators determine the impact of flying with inoperative systems,” he added.

However, it takes an in-depth understanding of the applicable FAA regulations to be able to develop an MEL that provides the clarity and flexibility to facilitate flying with certain inoperable equipment.

Start With the FARs

So how can business aircraft operators go about creating an MEL?

First, they should begin by reviewing the regulatory guidance on operating an aircraft with inoperative instruments and equipment, which can be found in FAR Part 91.213, 91.1115, 125.201 and 135.179.

Next, operators need to understand that an MEL actually is a list of relief items from several sources:

  • The Master Minimum Equipment List (MMEL), which is provided by each aircraft manufacturer and approved by the FAA
  • Supplemental Type Certificate Relief Approval Letters (SRALs)
  • FAA Letters of Authorization (LOAs) – either OpSpec D095 or MSpec D095

“An MMEL is a master minimum equipment list of aircraft instrument and equipment items that may be inoperative under certain operational conditions while maintaining an acceptable level of safety,” explained Haag. “These items have gone through risk and safety assessments with the aircraft OEM or design change holders (DCHs) – essentially the holders of supplemental type certificates (STCs).”

Each MMEL is developed through a partnership that includes the FAA’s Flight Operation Evaluation Board (FOEB), the OEMs or DCHs, and owners/operators. All Part 23 and Part 25 airplanes and Part 27 and Part 29 rotorcraft have MMELs.

It’s important to note the distinction between an MEL and an MMEL.

“The MMEL includes all equipment and accessories available for the aircraft model, while the MEL is created by the operator for your specific type of aircraft,” explained Elaine Karabatsos, aviation maintenance director for Encompass Health.

Building Your List

As operators look to set up their MELs, the following information should accompany all MMEL relief-item proposals, says Haag:

  • The title and number of the proposed MMEL item
  • A brief system description and intended functions
  • A summary of proposed MMEL relief
  • A brief description of intent
  • The effect of dispatching a flight with this item inoperative
  • The operating rule that requires the item to be operative (if any)
  • The transfer of function when the item is inoperative
  • The next most critical failure
  • The effect on increased flight crew workloads (including risk mitigations)
  • The certification requirements for the item
  • Any conflicts with aircraft flight manual limitations, procedures or ADs
  • If the item affects or is required to accomplish an emergency procedure
  • The effect on aircraft capability in an emergency
  • Maintenance and operations procedures required for the proposed dispatch condition
  • Any information that adds support to the proposal (e.g., wiring diagrams, schematic drawings or STC documents)

Each item approved has a respective MMEL relief-item number, and it’s important to note that some MMEL relief items may have time constraints, depending on their criticality to the flight crew. The FAA’s MMEL Policy Letter 32 denotes the appropriate MMEL format specification to be used by the FOEB to produce a MMEL.

As an operator creates its MEL, it’s important to be keenly aware of a couple of other FAA documents that affect the process:

  • MMEL PL-109, Revision 1, dated Nov. 7, 2019
  • Supplemental Type Certificate MMEL/MEL Relief Process

Remember, the MMEL is a list of relief items, the majority of which were added by the type certificate holder (OEM). However, the list also may include several STC number-specific items. PL-109 discusses the process for inclusion of instruments and equipment installed by STC that would not be covered under an operator non-essential equipment and Furnishings (NEF) list.

To put this in perspective, some examples of STC’ed items that may need to go through the FOEB process include:

  • Passenger seats
  • Third comm units
  • Third IRS systems
  • FANS 1/A+ communication management units
  • ADSB-Out components

“The operator must understand how the instruments and equipment are installed in their aircraft – under a type certificate or under an STC – to see if they are eligible to be included in an MEL.”

Stewart D’Leon NBAA’s Director of Technical Operations

Clearing the Hurdles

What happens if an aircraft does not have an MMEL and has inoperative equipment? In the case of a Part 91 operated aircraft, FAR 91.213 Paragraph D discusses the process of operating with inoperative equipment.

The STC applicant or holder is the point of contact for all matters regarding relief for their STC. Operators desiring MEL relief must consult directly with the STC applicant or holder for such relief. STCs held by applicants or holders that do not support relief for their STC will not be considered for MMEL/MEL relief. The bulk of the more than 70,000 STCs currently being tracked by the FAA have not gone through the MMEL relief-item process or the STC SRAL process.

“The operator must understand how the instruments and equipment are installed in their aircraft – under a type certificate or under an STC – to see if they are eligible to be included in an MEL,” said Stewart D’Leon, NBAA’s director of technical operations.

The operator will need to review the aircraft MMEL for STC number-specific relief items and the SRAL list for STC number-specific approval letters for inclusion into their aircraft specific MEL. If the relief item of the STC installed instruments or equipment that the operator is seeking is not listed, the instruments or equipment must be operable to fly.

Operators may not add items installed via an STC into their MELs, nor may they exercise MEL relief for items installed via an STC until that item has been evaluated by the FAA’s Aircraft Evaluation Group (AEG) and relief is granted.

More Considerations

What happens if the STC DCH has gone out of business?

An STC that is surrendered or abandoned by the STC holder may be a candidate for MMEL/MEL relief on a case-by-case basis. In such instances, upon request to the appropriate AEG office, a “qualified applicant” may act in place of the STC holder regarding the process to gain MMEL/MEL relief for the STC. A “qualified applicant” is an individual or organization that can act on behalf of the STC holder for matters when an STC is considered abandoned or surrendered.

It’s important to note that the SRAL is approved for the qualified applicant only. If the aircraft is sold, the new operator will be required to go through the process as a qualified applicant.

Additional FAA guidance is being developed to assist operators in meeting the requirements of PL-109. However, operators, at a minimum, need to determine if their STC-installed instruments or equipment are currently covered under the aircraft MMEL or SRAL. If not, the operator must remove the relief item from its MEL. Additionally, an operator should contact the STC DCH and ensure they move the process forward for relief consideration.

Finally, an operator needs to inform STC installers of the requirements of PL-109 and that the expectation will be for the DCH to produce or obtain the STC relief approval letter so an operator can add a relief item to their respective MEL.

“Besides ensuring safety, a properly updated MEL helps operators avoid non-compliance, so it’s important to keep MELs up-to-date following changes to equipment or documentation made by the operator, the manufacturer or the FAA,” said Greg Hamelink, NBAA Maintenance Committee chair and senior manager for flight operations and maintenance at Stryker Corp.

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