FAA Form 337: The document is only the beginning

opinion

by CJI Expert

August 19, 2026

When reviewing aircraft maintenance records, few documents are more important – or more frequently misunderstood – than FAA Form 337, Major Repair and Alteration. Words by Ryan Johnson, president of ESI Aviation.

A Form 337 documents a major repair or major alteration performed on an aircraft, power plant, propeller, or appliance. It provides a record of the work accomplished and the data and approvals associated with that work. The FAA identifies Form 337 as a means of documenting major repairs and major alterations and requires the form to be completed in accordance with applicable FAA requirements and guidance.

But during a comprehensive aircraft records audit, finding a Form 337 is not necessarily the end of the review – it is often the beginning.

What Is an FAA Form 337?

FAA Form 337, officially titled Major Repair and Alteration (airframe, power plant, propeller, or appliance), is used to document major repairs and major alterations. The form identifies the aircraft or component, describes the work accomplished, and identifies the data or methods used to accomplish the repair or alteration.

Depending on the alteration, the Form 337 may reference an STC, engineering data, manufacturer data, FAA-approved data, DER-approved data, an FAA field approval, or other supporting documentation.

The important question during a records audit is therefore not simply: “Is there a 337?”

The more important question is: “Does the complete documentation package support what the 337 says was accomplished?”

The supporting documentation matters

A Form 337 may reference technical documents that are essential to understanding and supporting the alteration.

Depending on the installation or repair, supporting records may include:

Supplemental Type Certificate (STC)
Approved engineering data
FAA Form 8110-3
FAA field approval documentation
Instructions for Continued Airworthiness (ICA)
Aircraft Flight Manual Supplement (AFMS)
Airworthiness limitations
Installation drawings
Engineering drawings
Structural substantiation
Equipment installation manuals
Weight and balance documentation
Equipment lists
Instructions, limitations, or inspection requirements
Additional compliance or testing documentation

For example, an FAA Form 8110-3 can document a determination of compliance with applicable airworthiness standards and may support a major repair or major alteration. The form itself identifies the data being approved or recommended for approval and the applicable requirements.

Similarly, FAA guidance recognises that certain alterations may require additional data beyond what a DER is authorised to approve, including matters involving flight manual supplements, airworthiness limitations, instructions for continued airworthiness, critical structure and life-limited parts.

That is why an ESI Aviation records audit does not stop when a 337 is located.

The 337 and its references must be connected

One of the most common records deficiencies ESI Aviation encounters is a Form 337 that references supporting documentation that cannot be located in the aircraft’s records.

For example, FAA Form 337: Installation of a modification in accordance with STC XYZ.

A complete records review should not simply check the 337 off a list. ESI will look for the associated STC documentation and determine what additional documentation the STC requires to support the installation.

That may include an:

AFMS
ICA
Equipment list revision
Weight and balance revision
Placards
Installation drawings
Operating limitations
Maintenance or inspection requirements

If the 337 references approved engineering data, ESI will look for the associated data and approval documentation.

If an FAA Form 8110-3 is referenced, ESI will verify that the referenced approved data is actually available and corresponds to the alteration.

If an FAA field approval was used, ESI will look for the appropriate approval documentation and supporting data.

The objective is to establish a complete chain:

ALTERATION → FORM 337 → APPROVED DATA → SUPPORTING DOCUMENTATION → CONTINUING AIRWORTHINESS REQUIREMENTS

If one of those links is missing, the records may not tell the complete story.

Why AFMS and ICA documentation matters

Some alterations create continuing obligations that extend well beyond the date the modification was installed.

An Aircraft Flight Manual Supplement (AFMS) may contain operating information or limitations associated with an installed modification.

Instructions for Continued Airworthiness (ICA) may establish inspection, maintenance, or other continuing-airworthiness requirements associated with the alteration.

If those documents are referenced by the applicable installation or approval data, they need to be available and appropriately incorporated into the aircraft’s records so future maintenance personnel can determine what requirements apply.

This is especially important during an aircraft sale. A buyer is not simply purchasing the physical modification. The buyer is purchasing an aircraft with a configuration that must be understood, documented, maintained, and supported.

A 337 audit should go beyond “present or missing”

At ESI Aviation, we don’t believe a comprehensive records audit should simply produce a checklist showing whether a Form 337 exists.

Each significant alteration or repair should be evaluated for:

Identify what was installed, repaired, or altered?

Applicability Does the documentation clearly apply to this specific aircraft, component, serial number, or configuration?

Approval Basis What approved data supports the work?

Supporting Documentation Are the referenced STCs, engineering documents, 8110-3s, field approvals, drawings, and other documents available?

Continuing Airworthiness Are the associated ICA, AFMS, inspection requirements, limitations, and other continuing requirements available?

Configuration Does the current aircraft configuration match what the documentation describes?

Records Continuity Can the alteration be followed through subsequent maintenance and inspection records?

Traceability Can future maintenance personnel determine exactly what was installed and what documentation governs it?

Finding a missing 337 is only one type of finding

An aircraft records audit may uncover several different scenarios:

337 present — supporting documentation complete

The alteration is adequately documented and the associated records can be verified.

337 present — supporting documentation missing

The Form 337 exists, but referenced STC, engineering data, AFMS, ICA, 8110-3, or other documentation cannot be located.

Supporting documentation present — 337 Missing

The aircraft contains evidence of an alteration, but the corresponding Form 337 cannot be located in the records.

Documentation conflicts

The 337, STC, equipment records, aircraft configuration, or maintenance tracking information do not agree.

Alteration has continuing requirements

The installation documentation identifies an AFMS, ICA, inspection, limitation, or other requirement that is not readily identifiable in the current records.

Each situation requires additional research rather than simply marking the item “complete” or “incomplete”.

Why this matters during an aircraft transaction

These discrepancies can become particularly important during a pre-purchase evaluation.

A buyer’s technical team may ask:

Where is the supporting STC?
Where is the 8110-3?
Where is the AFMS?
Where are the ICA?
Where is the engineering data?
Where are the installation drawings?
Where is the revised weight and balance?
What inspection requirements apply?
Are those requirements being tracked?
Does the aircraft configuration match the approved data?

If the answers cannot be readily supported by the records, the buyer may require additional research, corrective action, or documentation before closing. The best time to identify these issues is before the buyer does.

ESI Aviation’s approach to FAA Form 337 reviews

ESI Aviation specialises in detailed aircraft records audits that go beyond simply locating documents. Our team reviews Form 337s in the context of the entire aircraft records package.

We identify major repairs and alterations, review the referenced data, locate the associated supporting documentation, and evaluate whether the records provide a complete and logical history of the alteration.

When documentation is missing, ESI can conduct additional research using available aircraft records, maintenance tracking systems, previous operators, maintenance facilities, OEM resources, engineering organisations and other available sources.

Where appropriate, ESI can also assist with records reconstruction and organisation to bring fragmented documentation together into a more complete and usable aircraft records package.

The goal is simple: Don’t just find the 337. Understand the alteration. Don’t just identify the STC. Verify the supporting documentation. Don’t just find the AFMS or ICA. Determine whether the applicable continuing-airworthiness information is accounted for. Don’t just check the box. Follow the documentation chain.

The ESI Aviation difference

An aircraft’s maintenance records should tell a complete story. Every major repair and alteration should be supported by documentation that allows a future owner, maintenance organisation, auditor, or regulatory authority to understand what was done, how it was approved, what configuration resulted, and what continuing requirements apply.

That is the level of detail ESI Aviation brings to aircraft records audits. Whether you are purchasing an aircraft, preparing an aircraft for sale, reconstructing missing records, or simply bringing years of documentation under control, ESI Aviation can help identify discrepancies before they become expensive transaction problems.

Your 337s are telling the story of your aircraft. ESI Aviation makes sure the supporting documentation tells the rest of the story. Protect the aircraft. Protect the transaction. Protect the value.