MINIMUM EQUIPMENT LISTS (MEL) - AUSTRALIA GUIDE

A Minimum Equipment List (MEL) is a regulatory document that allows an aircraft to operate with certain equipment inoperative, provided specified conditions and time limits are met. The MEL defines which items may be deferred, the operational or maintenance procedures required, and how long the defect can remain before repair.

In Australia, MELs are approved by CASA or an Authorised Person and must be based on the aircraft type’s Master Minimum Equipment List (MMEL).

An approved MEL specifies:

• Which equipment may be inoperative before flight

• Conditions that must be followed for dispatch

• Maintenance actions required

• The time limit for repair (Category A, B, C or D)

Need a MINIMUM EQUIPMENT LIST fast? Direen Aviation offers structured MEL package pathways ranging from client-prepared inputs through to fully supported drafting, stronger post-issue amendment protection and digital-ready MEL data support.

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WHAT IS A MINIMUM EQUIPMENT LIST (MEL)?

A Minimum Equipment List (MEL) is an approved document that allows an aircraft to be dispatched with specified items inoperative, but only when the stated conditions, limitations and procedures are met. It tells flight crews and maintenance personnel what may be deferred, what actions are required, and when the defect must be rectified to keep the flight safe and compliant.

In Australia, an MEL is approved under the applicable CASA regulatory framework and must be based on the Master Minimum Equipment List (MMEL) for the aircraft type. In practice, it is a risk control document: it manages the effect of an inoperative item by applying operational limitations, placarding requirements, and any required operational or maintenance procedures before dispatch.

A key part of that control is the use of rectification interval categories A, B, C and D. Category A items have the time limit stated in the MEL entry, Category B items must be repaired within 3 consecutive calendar days, Category C within 10 consecutive calendar days, and Category D within 120 consecutive calendar days. These limits help operators keep aircraft in service while repairs are arranged, without allowing temporary defects to become the normal operating state.

WHAT AN MEL DOES, AND DOESN’T

An MEL does:

• Let you dispatch with specific items inoperative, but only under defined conditions;

• Set risk controls: time limits (A/B/C and D), operating restrictions, and required placarding, operations and maintenance procedures;

• Create a consistent decision process for pilots, maintenance control (CAMO), and aircraft maintenance engineers.

An MEL doesn’t:

• Mean “fly unserviceable whenever you want” or replace sound judgement;

• Override the AFM/POH limitations or other mandatory requirements;

• Allow dispatch if the aircraft is not capable of safe flight for that specific mission.

HOW AN MEL MANAGES RISK IN DAY-TO-DAY OPERATIONS

An MEL (Minimum Equipment List) is an approved, practical rulebook for managing defects in service. When something is inoperative, the MEL tells you whether the aircraft can still be dispatched and, critically, what you must do first. That includes operational restrictions (for example, limiting certain types of operations), required flight crew actions (operations procedures) and maintenance actions (maintenance procedures), and any configuration or placarding requirements so the aircraft is operated in a controlled, known state.

A big part of the MEL’s safety value is that it manages risk over time. The rectification interval categories A/B/C and D limit how long you can continue operating before the item must be repaired, which prevents “temporary” defects becoming normal. Those time limits, combined with the MEL’s restrictions and procedures, let you keep the aircraft operating while repairs are organised (parts, personnel, facilities etc.) without drifting outside acceptable safety margins.

Just as important is what an MEL doesn’t do: it is not blanket permission to fly with defects, and it does not replace command decision making. Even if an item appears in the MEL, you still need to confirm the aircraft is safe and suitable for the specific flight, and that all MEL conditions can be met before dispatch. This is particularly important when there are multiple defects, because their combined effect can change the risk profile.

WHEN DO I NEED A CASA APPROVED MEL FOR MY OPERATION?

• Part 121 aeroplanes: CASA approved MEL required (always).

• Part 133 rotorcraft: CASA approved MEL required if a Master MEL exists for the rotorcraft and the flight is IFR and/or begins or ends outside Australian territory.

• Part 135 aeroplanes, Subpart 121.Z aeroplanes, or operations under EX74/24: CASA approved MEL required if the flight is IFR (and a Master MEL exists) and/or begins or ends outside Australian territory (international). For international flights, the MEL trigger applies regardless of whether a Master MEL exists or not.

If you’re not in those categories, a CASA approved MEL may not be mandatory for your operation, but you can still apply for an MEL approval under Part 91 to gain the operational and risk control benefits an MEL provides.

Not every operator needs the same level of MEL support.

Some operators are prepared to complete the preparation inputs internally, while others want Direen Aviation to lead the drafting process, provide stronger amendment protection, or prepare MEL data for digital systems. Compare MEL Core, MEL Protect, MEL Protect Plus and MEL Digital to find the right fit.

CASA MEL REQUIREMENTS EXPLAINED AND HOW DIREEN AVIATION CAN HELP?

CASA’s MEL requirement is mainly triggered by what ruleset you operate under (Part 91/121/133/138/135) and your flight profile, especially IFR and international legs. The intent is straightforward: higher complexity operations and higher consequence flight profiles require a formal, approved framework for operating with inoperative items.

Part 121 operators need an MEL as a baseline requirement. For Part 133 rotorcraft and Part 135/121.Z aeroplanes (and EX74/24 operations), the requirement kicks in for IFR flights where a master MEL exists and for flights beginning or ending outside Australian territory (international).

Part 138 (aerial work) is a little different. Part 138 doesn’t create a blanket “every operator must have an MEL” rule on its own, but it does require you to have procedures to ensure required equipment is operative, unless inoperative operation is permitted under the Part 138 MOS (including via permitted unserviceability and, where used, an MEL approved under Part 91.Y). Importantly, if you’re using an MEL in a Part 138 context, it can’t allow an item to remain unserviceable longer than the Part 138 MOS permits.

If you want the fastest, least painful path, the winning move is getting the MEL right the first time (correct master/MMEL basis, aircraft configuration alignment, clear operations, maintenance and placarding procedures, and clean defect deferral and rectification control process). Direen Aviation can build your MEL and guide the approval pathway, and where eligible within CASA’s delegation framework, a Direen Aviation CASA Delegate can be appointed to approve, vary, or extend MELs under CASR 91.Y.

If you tell us your aircraft type, operating Part (91/121/133/135/138), and whether you fly IFR or international, we’ll tell you in one message whether you need a CASA approved MEL, and the cleanest way to get it approved.

MMEL VS MEL

An MMEL (Master Minimum Equipment List) is the aircraft type’s baseline list of what may be inoperative and still allow dispatch, under specific conditions. An MEL (Minimum Equipment List) is the operator’s approved, aircraft specific document built from the MMEL and tailored to the operator’s configuration, operation and Australian regulations, so it can be used day-to-day for compliant dispatch decisions in Australia (CASA).

Think of the MMEL as the source template and the MEL as the approved “working rulebook”. The MMEL is produced for an aircraft type and sets the starting point for permissible unserviceability: what can be deferred, what limitations apply, and what procedures must be followed. It is intentionally generic to the type, so it won’t fully reflect your exact aircraft configuration because it accommodates a range of configuration options available from the aircraft manufacturer (OEM). Modifications to the original aircraft configuration, avionics upgrades, STCs, mission kit, or operational profile can significantly impact an MEL approval..

The MEL is where that baseline becomes operationally usable. A proper MEL matches your aircraft configuration and translates MMEL relief into clear, dispatch ready controls: operational restrictions, required flight crew procedures, required maintenance procedures, placarding and configuration actions, and rectification interval categories (A/B/C/D). In other words, the MEL is the document your pilots and maintenance personnel actually rely on to decide “can we dispatch today, and under what conditions?”, while still maintaining safety and compliance.

In Australia, the approval pathway matters: an MEL is not just a document you “have”, it’s a controlled, approved document tied to the operator and aircraft details. That’s why the fastest outcomes usually come from getting the fundamentals right early (correct MMEL basis, configuration alignment, clear operations, maintenance and placarding procedures, and clean defect control procedures), and using an approving body (e.g., a CASA Delegate like Direen Aviation) to keep the process efficient and consistent.

RECTIFICATION INTERVALS (CATEGORIES A, B, C AND D)

Category	Rectification interval
A	Interval stated against the item
B	3 consecutive calendar days
C	10 consecutive calendar days
D	120 consecutive calendar days

A good MEL makes these intervals easy to apply in real life (and avoids ambiguous interpretations across bases and crews).

MEL EXTENSIONS: WHEN THEY’RE POSSIBLE, AND WHEN NOT

Extensions aren’t a “standard operating technique”, they’re a continuity tool for genuine constraints. MEL extensions, typically for categories B or C only, may be permitted for genuine spares procurement problems or circumstances beyond the operator’s control. Category A items cannot be extended.

Good MEL extension requests usually include:

• Clear reason rectification could not be completed within the time allowed;

• Risk controls and limitations deemed necessary to maintain acceptable safety standards;

• A realistic rectification plan and date; and

• Evidence (e.g., parts orders and availability status).

HOW TO USE AN MEL DAY-TO-DAY

Confirm the defect and identify the MEL item correctly:

1. Check the correct MEL revision and applicability;

2. Complete required maintenance procedures (if applicable);

3. Complete required operational procedures (if applicable);

4. Affix Placards as directed;

5. Record the defect deferral per your operating procedures;

6. Continue operations in accordance with the conditions of the MEL;

7. Re-check compliance before each subsequent flight; and

8. Track the rectification interval and arrange for repairs (don’t wait until the last day).

MEL VS SFP: WHEN AN MEL WON’T HELP

An MEL helps when the aircraft remains dispatchable within the MEL’s stated conditions. If the defect falls outside MEL relief, or the aircraft can’t meet normal dispatch requirements, you may need a Special Flight Permit (SFP) for a one-off repositioning flight to a maintenance base.

MEL VS SFP

• Minimum equipment list (MEL): A CASA approved dispatch tool that lets you operate with specific items inoperative, under defined limitations, procedures and rectification intervals. The aircraft remains airworthy and compliant for the flight, because the MEL sets the conditions under which it can legally dispatch.

• Special Flight Permit (SFP): Used only when an aircraft doesn’t meet normal airworthiness requirements, but can still be flown safely for a specific, one-off purpose (such as a ferry to maintenance) with strict operating restrictions.

• Rule of thumb: If it’s a recurring dispatch item → MEL. If it’s a one-off “get it to maintenance” flight → SFP.

HOW TO APPLY FOR APPROVAL OF A MINIMUM EQUIPMENT LIST

The operator prepares the MEL (based on the aircraft’s applicable MMEL, the aircraft configuration, and the operator’s procedures), then applies in writing for approval. In practice, you’ll submit the MEL and supporting material to an approving body (CASA, or a CASA Delegate) and it’s assessed against the required MEL content and whether it enables safe operation.

In practice, an “MEL application” isn’t CASA writing your MEL for you, it’s you submitting an MEL that is already operationally usable: it matches the aircraft configuration, includes clear operating limitations, and describes the operations, maintenance and placarding procedures. It must also detail procedures for how crews and maintenance personnel will defer defect rectification by invoking MEL items. The operator must also establish procedures for how flight crews and maintenance personnel will invoke MEL items (normally they’re in a maintenance control manual, operations exposition or referenced by the MEL).

The clean application to approval pathway is:

• Confirm whether a CASA approved MEL is a mandatory requirement for your operation (CASR Parts, IFR operations or international flights);

• Prepare the MEL, including supporting operator procedures (based on the applicable MMEL and your aircraft’s configuration);

• Submit using the CASA MEL application form (initial issue or variation of an existing MEL).

CASA vs a CASA Delegate for MELs: who prepares it, who approves it, and why operators use delegates

CASA does not write your MEL, the operator is responsible for preparing it. CASA’s role is assessment and approval of the MEL. For this reason many operators use a CASA Delegate, because a delegate can provide a full service: help draft and prepare the MEL in consultation with the operator so it’s approval ready, and where the CASA Delegate’s appointment allows, they can also approve, vary, or extend the MEL, often faster and with fewer back-and-forth cycles.

Think of it as two different models. With CASA direct, you (the operator) still need to produce an MEL that stands up on its own, complete and aligned to the MMEL, and supported by real operator procedures. CASA’s guidance is explicit that operators must develop the operations and maintenance instructions and establish the procedures that flight crews and maintenance personnel will use with the MEL. CASA then assesses and approves the MEL.

With a CASA Delegate pathway, you’re typically buying an end-to-end service and outcome: MEL development plus approval management. CASA’s delegation framework under CASR 91.Y sets out the basis for appointing external delegates to approve, vary or extend MELs, but it also defines scope limits (i.e., what aircraft and operations can be covered). Practically, that means a delegate can often reduce friction by building the MEL in the exact structure reviewers expect, ensuring your procedures are referenced correctly, and issuing the approval when the aircraft and operation is within their appointment scope.

Direen Aviation’s MEL service is positioned as full service: developing operator and aircraft specific MELs aligned to the applicable MMEL, including procedures, rectification intervals and configuration considerations, and issuing MEL approvals directly where permitted under the relevant CASA Delegation.

Need a System of Maintenance (SOM) or Certificate of Airworthiness (CoA) as well?

WHY USE A CASA DELEGATE?

Because CASA doesn’t write your MEL, the operator must prepare it, and a CASA Delegate can typically provide an end-to-end service: draft and structure the MEL so it’s approval ready, and where their appointment allows, approve it directly. That usually means fewer rework cycles, clearer procedures, and a faster, more predictable outcome.

COMMON MISTAKES THAT DELAY MEL APPROVAL

• Using the wrong MMEL

  Your MEL must be based on the correct MMEL for the aircraft’s State of Design and certification basis, and you should clearly state the MMEL document number and revision you used. This becomes a common problem for aircraft types that hold multiple type certifications (for example FAA and EASA), where an operator (or template provider) inadvertently builds the MEL from the wrong authority’s MMEL, such as using an FAA MMEL for an EASA type certificated aircraft.

  If the MMEL basis is wrong (or not declared), the reviewer can’t confirm that the relief, procedures, and rectification intervals align with the applicable baseline for that aircraft and approval pathway. The result is usually delays and rework, because it often requires correcting the MMEL reference and re-checking multiple MEL entries, definitions, and procedures that may differ between MMELs.

• The MEL doesn’t match the aircraft's configuration

  An MEL has to reflect what is actually installed and approved on the aircraft, and this is probably the most common error. A frequent issue is that the MEL doesn’t specify the installed quantity of an item, which matters because MMEL relief often depends on how many units are fitted and how redundancy is affected.

  This problem is amplified when a single MEL covers multiple aircraft. In that case, each aircraft’s configuration (avionics fit, optional equipment, STCs, tail-specific differences) needs to be clearly identified. Operators often don’t do this, and the MEL becomes “generic” when it actually needs configuration and applicability notes.

  Another common trap is avionics upgrades. When an aircraft has been upgraded from analogue to digital avionics, the original MMEL may not explicitly cover the new digital systems, and operators sometimes leave the legacy analogue items in the MEL even though they no longer exist on the aircraft. That creates a mismatch between the MEL and the real aircraft configuration, which can lead to incorrect dispatch decisions and compliance findings because the MEL is no longer a reliable “rulebook” for what’s actually installed.

• Missing or vague operations and maintenance procedures

  Operations and maintenance procedures are the “how” that makes MEL relief safe and usable. If the instructions are missing, unclear, or written too generically, crews and maintenance can’t consistently apply the same controls across shifts and bases. MEL applications are often bounced where procedures don’t clearly state who does the action, what the action is, and how it’s verified.

• Rectification intervals applied incorrectly

  Rectification categories A, B, C and D are a key risk control, so they must be applied exactly as intended and tracked against the correct operating context. A common trap is aircraft that move between private and commercial operations(for example Part 91 vs Part 135). Relief can be more restrictive under commercial operations, and problems arise when crews apply the more generous “private” relief and then the aircraft is operated commercially. This can easily lead to rectification intervals being overflown, not because anyone intended to push limits, but because the operation type changed and the defect control process didn’t catch it.

  Another recurring issue is using the wrong category when building the MEL, such as taking an MMEL Category B item and recording it as a Category C in the MEL. That effectively grants a longer deferral than the baseline allows. The general rule is that an MEL should not provide more permissive relief than the applicable MMEL for the same circumstances, so mismatched categories are a red flag and can trigger rework or findings.

  The practical fix is a robust control process in two places: first, when building the MEL, to ensure the categories and supporting details are correct; and second, in defect management, so defects are tracked against the correct rectification deadline for the current operation. There should also be a clear gate to prevent dispatch if the aircraft is moving into a more restrictive operating regime without the defect being rectified (or without the correct approved relief applying).

• No clear control process for how MEL relief is invoked and tracked

  Even a well written MEL fails if the operator doesn’t have a clear process for using it. You need a practical method for recording the defect, selecting the correct MEL item, completing required operations and maintenance procedures, placarding, and configuring the aircraft, and tracking the rectification due date (and any extension, if applicable). Without a defined control process, different people make different calls, and it becomes easy to miss time limits or fly outside the stated conditions.

• Including items not in the MMEL (or adding relief beyond the baseline)

  Adding items that are not in the MMEL, or proposing relief that is more permissive than the MMEL, typically increases scrutiny because the baseline safety assessment may not exist for that item. If you need additional items or different relief, it usually requires clear justification and an approval pathway that supports that change. If these items are inserted without proper substantiation, it commonly results in rejection or a request for significant rework to remove or reframe the entries.

• CASA regulatory requirements not incorporated into the MEL

  A common mistake is copying MMEL wording into the MEL without converting authority specific regulatory references to the applicable CASA rules. Many MMELs include relief that is conditional on local regulations, for example statements like “Any in excess of those required by 14 CFR operating rule may be inoperative.” That wording makes sense in an FAA context, but if it’s left unchanged in an Australian MEL it becomes ambiguous and potentially incorrect, because the requirements may be different under CASA.

  The fix is to translate those regulatory dependency statements into the Australian framework. Instead of referencing “14 CFR”, the MEL needs to reference the correct CASA operating requirements (and, where relevant, the operator’s Part and flight profile). If this isn’t amended, the MEL can inadvertently allow dispatch with equipment that is actually required under CASA rules, which is both a compliance risk and a common reason for rework during review.

Request a sample MEL

Sample demonstration copies of each package level can be provided on request so operators can compare presentation, navigation features and support inclusions before proceeding.

Frequently Asked Questions ABOUT MINIMUM EQUIPMENT LISTS (MEL)

Still have questions? Take a look at the FAQ or reach out anytime. If you’re feeling ready, go ahead and apply.

• What is the difference between a MMEL and MEL?

  The Master Minimum Equipment List (MMEL) is produced by the aircraft manufacturer and approved by authorities (e.g. FAA, EASA etc.), while the MEL is developed by the operator from the MMEL, tailored to their specific aircraft and operations.

• Are MELs required for all aircraft?

  Not always. An MEL is mandatory for many commercial operations, especially Part 121 air transport operations, but it may not be required for every smaller aircraft or every type of commercial operation. Whether a CASA approved MEL is required depends on the operating rules you’re under (Part 121/138/135/133/91) and the type of flying you do (for example IFR and/or flights that begin or end outside Australian territory).

• What are the MEL categories for repair?

  MEL items are assigned rectification interval categories that limit how long they can remain inoperative: Category A (the time limit is specified against the item), Category B (3 consecutive calendar days), Category C (10 consecutive calendar days), and Category D (120 consecutive calendar days), and with B, C and D items, the clock commences the day after the defect is discovered.

• Does an MEL override the Type Certificate or an Airworthiness Directive?

  No. An MEL cannot be used to override the aircraft’s approved type design or to bypass any requirement of an Airworthiness Directive (AD). If an AD requires a system or item to be installed, operative, or operated under specific limitations, the MEL can’t be used to defer or relax that requirement.

• Can an MEL be extended?

  Yes, some items in Categories B and C may be extended, usually for an additional period equal to the original.

• What is an "(M)" or "(O)" procedure in the MEL?

  An (M) indicates a maintenance procedure must be performed to disable the item, while an (O) indicates an operational procedure (e.g., specific checklist or action) the crew must follow.

• What happens if a piece of equipment is not listed in the MEL?

  If it’s not listed, you can’t use the MEL to defer it. Any item that is required by civil aviation legislation (or is airworthiness critical) must be operative before dispatch, even if it isn’t included in your MEL.

  If the item is not required for the intended operation, the aircraft may still be able to operate only if there’s a valid allowance under the applicable regulation (usually stated in the relevant Manual of Standards) or the defect has been approved as a permissible unserviceability.

• Can one MEL cover multiple aircraft?

  Often yes, particularly for aircraft of the same type, but configuration differences must be clearly identified and managed (e.g. avionics fits, STCs, options, and installed quantities).

  If aircraft configurations vary significantly, it may be better to have separate MELs to reduce complexity and avoid confusion for flight crew and maintenance personnel.

• What’s the difference between a variation and a new approval when something changes?

  If an aircraft operator has not previously held an approved MEL for that aircraft, you’re generally applying for initial approval (a new MEL).

  If you already have an approved MEL and you’re updating it, for example changing operating or maintenance procedures, or updating content to align with a new MMEL revision, that is typically treated as a variation.

Or need help with a System of Maintenance (SOM), Certificate of Airworthiness (CoA), reliability program or Special Flight Permit (SFP) as well?

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