Supplemental Type Certificates (STCs) allow operators to install additional equipment or modify existing aircraft systems to meet operational needs. While STC installations are common, they frequently introduce uncertainty when an item becomes inoperative and operators look to Minimum Equipment List (MEL) authority for relief.
A common misconception is that STC-installed equipment may be deferred simply because it does not appear in the Master Minimum Equipment List (MMEL). In practice, FAA policy places specific limitations on when and how STC-installed items may be deferred. Understanding those limitations is essential for maintaining regulatory compliance and avoiding inspection findings.
Many operators assume that if equipment is not listed in the MMEL, it falls outside of MEL control. This assumption often stems from an incomplete understanding of how the MMEL, the operator’s MEL, and STC relief approvals interact.
Confusion typically arises from several overlapping factors, including differences between type-design equipment and STC-installed equipment, as well as misunderstandings about who may authorize relief. FAA guidance is clear that STC-installed items may not be deferred unless specific relief has been approved and incorporated into the operator’s MEL.
Improper deferral of STC-installed equipment continues to be a frequent source of inspection findings. FAA inspectors regularly reference MMEL Policy Letter 109 when reviewing MEL entries associated with STC installations, particularly when deferral authority is not clearly supported.
The MMEL defines which systems and equipment may be deferred by default for a specific aircraft make and model. Equipment not listed in the MMEL does not automatically qualify for MEL relief.
STC-installed equipment that is not addressed in the MMEL requires separate FAA evaluation. That evaluation is performed by the FAA Aircraft Evaluation Group (AEG) as part of the Flight Operations Evaluation Board (FOEB) process.
Only the STC holder may request relief approval from the AEG. Operators are not permitted to request STC relief on behalf of the STC holder. When relief is approved, the AEG issues an STC relief approval letter authorizing operators to incorporate specific deferral provisions into their MEL.
This process establishes a clear pathway for relief, but it must be followed precisely.
STC relief approval does not modify or amend the MMEL. The MMEL remains unchanged unless an item is later addressed through a subsequent MMEL revision.
Instead, approved STC relief is incorporated into the operator’s MEL. Operators may only defer STC-installed items after the approved relief has been formally incorporated into their FAA-approved MEL.
In practical terms, the process follows a defined sequence:
Operators may not self-approve relief or defer STC-installed items prior to MEL approval. Deferring STC-installed equipment without approved MEL relief is non-compliant with 14 CFR 91.213 and applicable operating rules.
STC-installed equipment that often leads to deferral confusion includes systems that may appear operationally non-essential, but still require approved relief before deferral. Common examples include:
While these items may not be required for basic aircraft operation, deferral authority must always trace back to approved MEL language. Without approved STC relief incorporated into the MEL, these items may not be deferred.
When an STC-installed item becomes inoperative, operators should take a structured approach to determine whether deferral is permitted.
Operators should first verify whether an approved STC relief approval letter exists and confirm that the approved relief has been incorporated into the operator’s MEL. If no MEL relief exists, the item must be repaired prior to flight.
For Part 91 operations, applicability of 14 CFR 91.213(d) may be evaluated for equipment not required by rule or type design, provided all regulatory conditions are satisfied. Operators should also coordinate with the STC holder to determine whether relief exists or may be requested.
Clear documentation of discrepancies, corrective actions, and MEL references is critical. MELs, manuals, and procedures should be kept current to prevent improper deferrals and avoid return-to-service delays.
Responsibility for obtaining STC relief rests solely with the STC holder. Only the STC holder may submit relief requests to the AEG, and operators may not initiate this process themselves.
Once approved, STC relief remains valid through possession of the AEG approval letter unless it is revised or revoked. Operators rely on approved STC relief by incorporating it into their MEL through the standard approval process.
A clear understanding of this workflow helps prevent improper MEL entries, reduces inspection risk, and supports timely return to service.
Accurate MEL documentation plays a critical role in managing STC-installed equipment. Developing and maintaining FAA-compliant MELs, incorporating approved STC relief, and ensuring ongoing alignment with aircraft configuration changes or MMEL revisions all require careful technical oversight.
Sky Safety Solutions supports operators with MEL development, STC relief incorporation, and documentation accuracy to help maintain compliant, inspection-ready operations.
If you need assistance incorporating approved STC relief into your MEL or evaluating deferral eligibility, Sky Safety Solutions can help ensure your documentation accurately reflects FAA requirements and operational authority.