Amendments to TP9155 – The MMEL/MEL Policy and Procedures Manual - Civil Aviation Safety Alert (CASA) No. 2022-02


Operators of aircraft with an approved Minimum Equipment List (MEL) and TC personnel responsible for MEL oversight

File Classification No.: Z 5000-35 U
RDIMS No.: 18394893
Document No.: CASA 2022-02
Issue No.: 01
Effective Date: 2022-04-13


This Civil Aviation Safety Alert (CASA) is intended to clarify MEL Item Repair Interval requirements and limitations and to announce the introduction of TP9155 Amendment Bulletins.

The current text in TP9155, Section 3.12 and subsequent paragraphs includes some conflicting statements with regard to MEL Item Repair Interval Extensions. This CASA will provide clear direction for industry and TC personnel until TP9155 can be amended.

The introduction of an Amendment Bulletin service will be limited specifically to the TC MMEL/MEL program and will allow targeted dissemination of program and policy changes, if and when needed.

Text will be developed for inclusion in TP9155 to explain the Amendment Bulletin process.


MELs are documents that are approved and issued by TC or Delegated Authorities that allow time-limited dispatch of aircraft with inoperative equipment in accordance with specific limitations and instructions. The intent is to permit safe, continued aircraft operations for a limited period as determined by engineering analysis during aircraft type certification. Operators must repair the inoperative item within the specified period to maintain the aircraft’s certificate of airworthiness. In some cases, a single extension to the original specified repair interval is permitted, if properly justified and agreed to by the TC personnel responsible for the operator’s MEL.

Minimum Equipment Lists are governed by Canadian Aviation Regulations (CAR) 605.07 and mandate the production and use of MELs in accordance with TP9155 – The MMEL/MEL Policy and Procedures Manual. TP9155 is incorporated by reference, and therefore constitutes a Standard with which compliance is mandatory.

The rewrite of TP9155 (3rd Edition) required the involvement of numerous primary stakeholders, including Continuing Airworthiness, National Aircraft Certification, Regional Offices (Airworthiness and Operations) and Commercial Flight Standards. The resulting draft document was then subjected to the full CARAC consultation process on two separate occasions to ensure participation and inclusion by all interested parties.

All comments and suggestions received were considered and implemented or discarded with written, documented justification in the CARAC Disposition of Comments document. The resulting Manual (TP9155, 3rd Edition) represents a best effort of all stakeholders to modernize the program guidance and direction and to be consistent with the FAA and EASA where possible, as we share base documents (MMELs) and operate common types.

Current Situation

TP9155, Section 3.12 and subsequent paragraphs deal with extensions to the various Categories of MEL Item Repair Intervals. Unfortunately, previous versions of TP9155 permitted multiple extensions not in accordance with the Master Minimum Equipment List (MMEL) limitations for authorized, extendable item relief periods. Although a clarification for Category B and C items was added to the 3rd Edition, conflicting statements concerning extensions were overlooked and not removed. This has resulted in contradictory instructions and confusion for the reader who is trying to comply with the requirements.

The MMEL is the controlling document provided by the aircraft manufacturer and approved by the National Authority for the aircraft type, that permits conditional dispatch with inoperative equipment while maintaining the aircraft’s certificate of airworthiness. The MEL, which is the operator’s document and is derived primarily from the MMEL, may never be less restrictive than the MMEL. Accordingly, TC and Delegated Authorities for the issuance of MELs must comply with all stated MMEL restrictions or limitations.

There are four Categories for inoperable MEL items employed during the development of the MMEL that specify the acceptable repair interval for each item listed in the operator’s MEL. These Categories are itemized here, with applicable limits and restrictions based on the TC-approved MMEL for the type.

Category A MEL Item repair interval extensions

Category A MEL items are not evaluated for any extensions during the type certification process and MMEL development because there are not any available or acceptable mitigations to safely permit extended operation. No further relief is available, beyond the period specified for each Category A inoperable item.

Neither EASA, the FAA or other global authorities permit extensions for Category A items for this reason.

Transport Canada had permitted CAT A extensions on the basis of ad hoc analysis by TC personnel responsible for the operator. However, this resulted in unintended, additional liability that cannot be acceptably mitigated and was cancelled with the publishing of TP9155, 3rd Edition. Any CAT A extensions contravene the basis of the type certification by permitting continued operations beyond the maximum specified MMEL time limits, while the equipment remained inoperative. This results in an unairworthy aircraft being operated in violation of its type certificate.

Category B and C MEL Item repair interval extensions

In contrast to Category A restrictions, operators may self-extend Category B, C or D MEL items one time. Aircraft type certification specialists conduct risk assessments during the development of a MMEL that support a single repair interval extension for these items.

EASA, the FAA and TC permit qualified operators to extend Category B and Category C MEL items specified repair intervals one time, based on the MMEL risk assessments conducted for that purpose.

Category D MEL item repair interval extensions

Although the MMEL risk assessment process permits it, the FAA do not allow extensions of Category D items because they have a repair interval of 120 days and are, by definition “excess to the requirements”.

Category D items may be removed without incurring any operational penalty and the FAA believe that the defined 120-day repair interval should be enough time to repair any item or remove it.

Transport Canada previously permitted Category D extensions on the basis that operators must justify extensions to their TC inspectors, and that Category D items did not directly impact safety of flight. For harmonization reasons, and because of the rationale espoused by the FAA, TP9155, 3rd Edition was modified to eliminate self-extensions of Category D items.

However, recent discussions with regional TC personnel responsible for their operators’ MELs has revealed that a new problem has arisen. The ongoing pandemic and other global uncertainties have caused supply chains to become unreliable or in some cases, collapse.

TP9155 repair interval philosophy assumed that parts support and shipping options to achieve the stated repair times were continuously available. Lead times for some parts have become exceedingly long and alternate suppliers may not be available at all. As the recent introduction of the Non-Essential Furnishings (NEFs) program stipulate that approved MEL-NEF items are commonly assigned MEL Category D, the supply chain issue for obtaining replacement parts is further compromised.

TP9155 Intended Changes and Clarification

Accordingly, TC management has agreed to reinstate a CAT D self- extension mechanism. This will permit flexibility where need has been demonstrated by the operator. As stated in the MMEL/ MEL preamble, it is intended that operators exhaust all options for repairs before any extensions are considered. Wording is being developed for inclusion in TP9155 to reinstate Cat D extensions.

Category B, C, and D MEL items are evaluated during MMEL promulgation to permit a single extension to their repair interval. For Category B items, that results in one additional three-day repair period, ten days for Category C items and 120 additional days for Category D.

There are no MMEL-compliant mitigation strategies available for any scenario that allows continued operations beyond the stated extension limits. Permitting aircraft operations to continue with unsupported MEL repair interval extensions will invalidate the aircraft’s certificate of airworthiness with resulting liability for all concerned. Insurance certificates are also likely to be negatively affected under these circumstances, where extension non-compliance resulting in operations with an invalid certificate of airworthiness was knowingly tolerated.

In summary, TP9155 is being amended to ensure the following criteria for MEL item repair interval direction is clearly stated:

Category A items are not extendable. An alternate process involving the use of flight permits that prohibit the carriage of passengers or cargo provides a means of ferrying an aircraft to a suitable repair facility or base, when the specified Cat A repair period cannot be met. Reference CAR 605.08(2).

Category B and C MEL items remain self- extendable one time only beyond the initial, specified repair interval, in accordance with the procedure stipulated in TP9155.

The Category D MEL item repair interval will be restored to be self-extendable, one time only beyond the initial, specified repair interval in accordance with the procedure stipulated in TP9155.

It is intended that repairs to inoperable MEL items be effected as quickly as possible, utilizing all means available, regardless of the specified repair interval.

It is not intended that the Aeronautics Act, Section 5.9(2) exemption process be employed for any MEL extensions.

Note: Operators are expected to amend their MELs and operations as necessary to comply with these instructions forthwith. As previously stated, updated text for inclusion in TP9155 is being drafted to be fully consistent with this CASA direction. The following references are included here for the reader’s convenience.

TP9155, Section 3.9.2 MEL Content

a) The operator’s MEL must reflect the current MMEL limitations unless otherwise authorized by a change in the MMEL, or the TC Supplement.

TP9155, Section 3.4 MEL Limitations requires that with the exception of global changes, a MEL cannot be less restrictive than the content of the Approved MMEL and/ or the approved TC Supplement. The operator must be able to demonstrate to Transport Canada that the intent of the MMEL and/ or TC Supplement is met.

TP9155 Section 3.5 Audits of Operator’s MELs requires the Principal Operations Inspector and /or Principal Maintenance Inspector must review an operator’s MEL and conformance on an annual basis, or as determined by the region. Significant non-conformances may result in the MEL approval being withdrawn.

Compliance with this CASA will result in MMEL policy harmonization with other recognized national aviation authorities.

MMEL/MEL Amendment Bulletin

A MMEL/MEL Amendment Bulletin procedure will be developed and incorporated into TP9155, in a format similar to this CASA. This will permit rapid dissemination of approved procedural changes to TP9155 to all stakeholders. Currently, issuing an amendment directly to TP9155 is time consuming and does not address the immediacy of some changes.

Recommended action

It is recommended that this CASA be accepted as written in accordance with the existing MMEL requirements. TP9155, the MMEL/MEL Policy and Procedures Manual will be amended as necessary to incorporate these changes.

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact Commercial Flight Standards in Ottawa by e-mail at

Original signed by
Andrew Larsen

Félix Meunier
Standards Branch


The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.