Minimum Equipment Lists (MEL): Repair Categories, the LOA, (M)/(O) Procedures, and the CDL
Updated: Sep 2
Aircraft systems occasionally fail or become inoperative, but that doesn't always mean the aircraft is grounded. The FAA provides a structured way to determine whether an aircraft can be legally and safely flown with certain inoperative equipment through a Minimum Equipment List. But the MEL has real operational depth that trips up pilots and owners: the repair categories (A, B, C, and D) each carry a specific time clock that determines how long you can fly with an item deferred, a Part 91 MEL isn't legal without a Letter of Authorization, and the MEL is only half the story — the (M) and (O) procedures documents must be carried and followed. Understanding how MELs actually work — categories, clocks, and required documents — is essential for anyone flying complex or commercially operated aircraft, and it's a common checkride and interview topic.
This post covers MELs in practical depth: what an MEL is and how it relates to the MMEL, the repair category time limits (the key operational detail), the Letter of Authorization that makes a Part 91 MEL valid, the (M) and (O) procedures, the documents that must be carried, the CDL as the MEL's structural cousin, and the 91.213 alternative when there's no MEL.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What a Minimum Equipment List Is
An MEL is an FAA-approved document specifying which equipment may be inoperative while still allowing the aircraft to be operated under specific conditions.
An MEL is:
Aircraft-specific (by make, model, and sometimes serial number/configuration)
Approved by the FAA
Legally binding once adopted by the operator
The operator's authorization to fly with specified inoperative equipment
The core function:
It lists equipment that CAN be inoperative
Under what conditions and limitations
With what procedures (maintenance and operational)
For how long (the repair category)
The "if it's not listed" rule:
If an item is NOT in the MEL, it generally must be operational
(Unless it's not required equipment at all)
The MEL is the authorization — no listing, no deferral (under the MEL)
Items not in the MEL that ARE required must work
The airworthiness effect:
With an approved MEL, the MEL becomes the standard for what's required
Flying per the MEL = airworthy (for the deferred items)
Flying contrary to the MEL = unairworthy
The MEL defines airworthiness for equipment on it
MEL vs. MMEL: The Relationship
Understanding the MMEL-to-MEL derivation, which the basic treatment covers well but can be sharpened.
Master Minimum Equipment List (MMEL):
Created by the manufacturer and an FAA board (the Flight Operations Evaluation Board)
Approved by the FAA
A baseline TEMPLATE for the aircraft type
Lists the equipment that MAY be deferrable (the maximum)
NOT usable by an operator directly
Minimum Equipment List (MEL):
DERIVED from the MMEL
Customized for a specific operator and aircraft (their configuration)
Can be MORE restrictive than the MMEL (never less)
Approved for the operator (via the LOA — below)
Legally usable
The derivation:
The MMEL is the starting point (the type's deferrable items)
The operator builds their MEL from it (for their specific aircraft/operation)
The MEL can remove items (be more restrictive) but not add deferrals beyond the MMEL
The MEL is tailored to the operator
Why you can't use the MMEL directly:
The MMEL is generic (the type template)
It doesn't include the operator's specific procedures ((M) and (O))
It's not approved for a specific operator
You need the derived, approved MEL
The "more restrictive" point:
The MEL can be MORE restrictive than the MMEL
It cannot be LESS restrictive (can't defer more than the MMEL allows)
The MMEL is the ceiling of deferrability
The MEL fits within it
The Repair Category Time Limits: A, B, C, and D
This is the single most important operational detail the basic treatment names but doesn't explain: each MEL item has a repair category that sets a CLOCK for how long you can fly with it deferred.
The four repair categories:
Category A:
No standard time interval — the repair interval is specified in the "Remarks or Exceptions" column of the MEL item itself
Varies by item (could be a number of flights, days, or a specific condition)
Read the specific item's requirement
Item-specific timing
Category B:
Must be repaired within 3 consecutive calendar days (72 hours conceptually, but counted in calendar days)
Excluding the day the malfunction was recorded
The clock starts the day after discovery
3 calendar days
Category C:
Must be repaired within 10 consecutive calendar days
Excluding the day of discovery
The clock starts the day after
10 calendar days
Category D:
Must be repaired within 120 consecutive calendar days
Excluding the day of discovery
The longest standard interval
120 calendar days
How the clock works:
The day the malfunction is discovered/recorded is DAY ZERO (excluded)
The count begins the next calendar day
Example (Category C): item found inoperative on the 1st → the 10 days start on the 2nd → must be repaired by the end of the 11th
Calendar days (not flight days)
Why the categories matter:
The category tells you HOW LONG you can defer the item
Exceeding the repair interval = the deferral is no longer valid = unairworthy
You must repair (or the aircraft is grounded) by the category's deadline
Track the repair deadline for each deferred item
The extension possibility:
Some operators (Part 121/135) can get a one-time extension for certain categories (e.g., a Category C extended by another 10 days) with FAA authorization
Not automatic
Governed by the operator's program
Category B, C, D may be extendable per the operations specifications
The tracking imperative:
Each deferred item has a repair deadline (based on its category)
Miss the deadline and the aircraft is grounded (for that item)
Track deferrals and deadlines carefully
A common compliance failure
The Letter of Authorization (LOA)
A crucial document the basic treatment doesn't mention: a Part 91 MEL isn't legal without an LOA.
What the LOA is:
A Letter of Authorization issued by the FAA (the FSDO)
Authorizes the specific operator to use the MEL for the specific aircraft
Required for a Part 91 operator to use an MEL
The legal authorization to operate under the MEL
Why it's needed:
The MEL itself is the document; the LOA is the authorization to USE it
For Part 91, the FSDO issues an LOA authorizing the MEL
Without the LOA, the MEL isn't authorized
The LOA + MEL together enable MEL operations
What the LOA does:
Names the operator and aircraft
Authorizes use of the MEL (and the associated documents)
Makes the MEL legally usable for that operator
The FAA's approval in letter form
The Part 91 process:
A Part 91 operator wanting an MEL requests it from the FSDO
The FSDO issues the MEL and the LOA (for the specific aircraft)
The operator can then use the MEL
The LOA is part of the authorization
Carrying the LOA:
The LOA (along with the MEL and procedures) must be carried/available
Part of the required documentation
Available for inspection
Proves the authorization
The Documents That Must Be Carried
The MEL is only part of the package — several documents must be aboard.
The MEL document itself:
The list of items, conditions, and repair categories
Aircraft-specific
Must be aboard/available
The core document
The (M) and (O) procedures documents:
The maintenance (M) and operational (O) procedures referenced by MEL items
These are SEPARATE documents (often a manual)
Must be available (the procedures must be accessible)
The MEL references them; they contain the actual procedures
The LOA (for Part 91):
The Letter of Authorization
Authorizes the MEL use
Carried/available
The authorization
The preamble:
The MMEL preamble (definitions, general rules)
Part of the MEL package
Explains the categories, definitions, etc.
Reference material
Why all of these:
The MEL alone isn't enough — you need the procedures and the authorization
An MEL deferral requires following the (M) and (O) procedures (which are in the separate documents)
The full package must be available
Incomplete documentation invalidates the MEL operation
The "MEL is part of the AFM" concept:
Once adopted, the MEL is considered part of the aircraft's operating limitations
It's an FAA-approved document with regulatory force
Carried with the aircraft
Part of the required documentation
Required Procedures: (M) and (O)
The procedures that make a deferral valid, expanded.
(M) Maintenance Procedures:
Procedures that must be completed (typically before flight) for the deferral
Usually performed by qualified maintenance personnel
Example: deactivating and securing an inoperative component
Must be done and documented
The (M) nuance:
(M) procedures are usually performed by maintenance personnel
BUT some simple (M) procedures may be performed by the pilot if they hold the appropriate authorization/rating or if allowed (e.g., pulling a circuit breaker as a simple deactivation)
Generally, though, (M) means maintenance does it
Check who's authorized for the specific procedure
(O) Operational Procedures:
Procedures the FLIGHT CREW must perform during operation
Example: a limitation or a monitoring procedure while flying with the item inoperative
Performed by the pilot/crew
Must be followed
Both must be completed:
If an item has (M) and (O) procedures, BOTH must be done
(M) before flight (maintenance), (O) during operation (crew)
Both documented/complied with
Skipping either invalidates the deferral
The documentation:
The deferral must be recorded (the item, the date, the category/deadline, the placard)
The (M) procedure completion documented
The (O) procedure is the crew's responsibility in flight
Proper records required
The Placard and Deferral Process
How an MEL deferral actually happens.
The steps:
An item is found inoperative
Check the MEL — is the item listed as deferrable?
If yes, verify the conditions/limitations can be met
Complete the (M) maintenance procedure (deactivate/secure as required)
Placard the item "INOPERATIVE"
Record the deferral (item, date, repair category/deadline)
Comply with any (O) operational procedure in flight
Repair by the category's deadline
The placard:
Inoperative items are placarded "INOPERATIVE"
Alerts the crew
Required
Standard for deferrals
The record:
The deferral is recorded (often in a deferral log or the maintenance records)
Includes the repair deadline (per the category)
Tracks the open deferral
Required documentation
The go/no-go:
With a valid MEL deferral (procedures done, placarded, within the deadline), the aircraft is airworthy
The pilot verifies the deferral is valid before flight
The pilot can decline to fly even with a valid deferral (PIC authority)
Legal ≠ mandatory to fly
The CDL: The MEL's Structural Cousin
A related document the basic treatment doesn't mention, often confused with the MEL.
What the CDL is:
Configuration Deviation List
Lists EXTERNAL STRUCTURAL parts that may be MISSING for flight
Example: a missing access panel, fairing, or similar external part
The structural counterpart to the MEL
The MEL vs. CDL distinction:
MEL: inoperative EQUIPMENT/SYSTEMS (things that don't work)
CDL: missing external STRUCTURAL parts (things that are absent)
MEL = equipment; CDL = structure
Different documents for different situations
How the CDL works:
If an external part is missing (a panel, fairing), check the CDL
The CDL says whether the aircraft can fly missing that part
With performance penalties or limitations (often)
Part of the AFM (for aircraft that have one)
Why they're confused:
Both allow flight with something not "complete"
MEL: a system doesn't work; CDL: a part is missing
Both are approved documents
Know the difference (equipment vs. structure)
The practical point:
Inoperative radio → MEL
Missing wheel fairing → CDL
Different documents
Both about flying with a deviation from the complete configuration
The 91.213(d) Alternative: No MEL
For aircraft without an MEL, the fallback process (which most GA uses).
When 91.213(d) applies:
The aircraft does NOT have an MEL
Most general aviation aircraft (no MEL)
The default inoperative-equipment process
The "four-step" (really a multi-part) rule
The 91.213(d) determination — the equipment must NOT be:
Part of the VFR-day type certificate requirements
Required on the aircraft's equipment list or KOEL for the kind of operation
Required by 91.205 (or other rule) for the specific operation
Required by an Airworthiness Directive
If it passes:
Deactivate or remove the item
Placard it "INOPERATIVE"
A pilot or mechanic determines it's not a hazard
Then the aircraft can fly with it inoperative
The MEL vs. 91.213(d) choice:
With an MEL: use the MEL (more items may be deferrable, structured process)
Without an MEL: 91.213(d) (only non-required equipment)
An MEL generally allows MORE flexibility (deferring some required items under conditions)
91.213(d) only allows deferring non-required equipment
Why an MEL offers more:
An MEL can allow deferring some equipment that 91.213(d) couldn't (with conditions/procedures)
91.213(d) can't defer required equipment at all
The MEL is more flexible (but more work to establish and maintain)
The tradeoff
The "can't mix" rule:
Once you adopt an MEL, you must use it (not 91.213(d))
You can't switch between them
The MEL becomes your method
A key rule
MELs and Airworthiness
The compliance stakes, expanded.
Operating contrary to an MEL:
Renders the aircraft unairworthy
Can result in FAA enforcement action
May invalidate insurance coverage
A serious compliance issue
The specific violations:
Flying with an item inoperative that's NOT deferrable per the MEL
Exceeding a repair category deadline
Not completing the (M) or (O) procedures
Missing placards or documentation
The pilot's responsibility:
Understand and comply with the MEL before every flight
Verify open deferrals are valid (within deadlines, procedures done)
The PIC determines airworthiness
The MEL is part of that determination
The ramp-check reality:
Ramp checks review MEL compliance
Open deferrals, deadlines, placards, documentation
Non-compliance is discovered
Keep it clean
Common MEL Mistakes
The errors, expanded.
Assuming the MEL is optional after adoption:
Once adopted, the MEL MUST be used
Not optional
Can't revert to 91.213(d)
A common misunderstanding
Missing required (M) or (O) procedures:
Both procedures must be completed
Skipping either invalidates the deferral
(M) before flight, (O) in flight
Frequently missed
Exceeding repair time limits:
Each category has a deadline (A: per item, B: 3 days, C: 10 days, D: 120 days)
Exceeding it grounds the aircraft (for that item)
Track the deadlines
A common failure
Improper or missing placards:
Inoperative items must be placarded "INOPERATIVE"
Missing placards are a violation
Required
Often overlooked
Incomplete maintenance log entries:
The deferral and procedures must be documented
Incomplete records invalidate the deferral
Document properly
A frequent deficiency
Confusing the MEL and CDL:
MEL for inoperative equipment; CDL for missing structural parts
Using the wrong document
Know the difference
A conceptual error
Final Thoughts
Minimum Equipment Lists provide a structured, safety-driven approach to managing inoperative equipment. Understanding how they actually work — the MMEL-to-MEL derivation, the A/B/C/D repair categories and their calendar-day clocks, the Letter of Authorization that makes a Part 91 MEL legal, the (M) and (O) procedures, and the documents that must be carried — turns the MEL from an abstract concept into a manageable system. And knowing the alternatives (91.213(d) without an MEL) and the related documents (the CDL for missing structural parts) rounds out the picture.
Whether you're flying under Part 91, 135, or 121, understanding MELs is essential to maintaining airworthiness and making sound go/no-go decisions. Pilots who truly understand MELs — categories, clocks, procedures, and documents — are better decision-makers and safer operators.
On the Written Test and Checkride
MELs appear on tests and checkride orals (especially for commercial, CFI, and turbine transitions). The most commonly tested topics:
What an MEL is and the MEL vs. MMEL distinction
The repair categories (A/B/C/D) and their time limits
The (M) and (O) procedures
The 91.213(d) alternative (no MEL)
The "can't switch back and forth" rule
MEL vs. CDL
Quick Reference
MEL:
FAA-approved list of equipment that may be inoperative (with conditions)
Aircraft-specific, legally binding once adopted
Defines airworthiness for listed items
MEL vs. MMEL:
MMEL: manufacturer/FAA template (the type baseline, not usable directly)
MEL: derived, operator-specific, approved (usable)
MEL can be MORE restrictive than MMEL, never less
Repair Categories (the CLOCKS):
Category | Repair Within |
A | Per the item's Remarks/Exceptions (varies) |
B | 3 consecutive calendar days |
C | 10 consecutive calendar days |
D | 120 consecutive calendar days |
Day of discovery excluded (clock starts next day)
Exceeding the deadline = grounded (for that item)
Letter of Authorization (LOA):
FAA (FSDO) authorization to USE the MEL (Part 91)
Required for a Part 91 MEL to be legal
Carried with the MEL
Documents to Carry:
The MEL
The (M) and (O) procedures documents
The LOA (Part 91)
The preamble
(M) and (O) Procedures:
(M) Maintenance: usually maintenance personnel, before flight
(O) Operational: flight crew, during operation
BOTH must be completed and documented
Deferral Process:
Item inoperative → check MEL
Verify conditions
Complete (M) procedure
Placard "INOPERATIVE"
Record (with repair deadline)
Comply with (O) in flight
Repair by the category deadline
CDL (Configuration Deviation List):
Missing external STRUCTURAL parts (not equipment)
MEL = inoperative equipment; CDL = missing structure
Both allow flight with a deviation
91.213(d) (No MEL):Equipment can be deferred only if NOT:
Required for VFR-day type certification
On the equipment list/KOEL
Required by 91.205 for the operation
Required by an AD
Then: deactivate/remove, placard, determine not a hazard
MEL vs. 91.213(d):
MEL: more flexible (can defer some required equipment with conditions)
91.213(d): only non-required equipment
Once you adopt an MEL, you must use it (can't switch)
Key Principle:
An MEL (derived from the MMEL, authorized by an LOA for Part 91) lists equipment that may be inoperative under specified conditions, with (M) maintenance and (O) operational procedures that must both be completed. Each item's repair category sets the clock — B: 3 days, C: 10 days, D: 120 days (day of discovery excluded) — and exceeding it grounds the aircraft. Without an MEL, 91.213(d) allows deferring only non-required equipment. The CDL is the structural counterpart for missing external parts.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.