Corrosion is one of the most persistent threats to aircraft structural integrity, and detecting it is only half the battle. What happens after a mechanic spots corrosion — how the finding is described, recorded, and tracked over time — is equally important. Thorough, accurate documentation creates a historical record that allows subsequent inspectors and owners to understand the aircraft's condition, compare the progression of any corrosion that was treated and monitored, and make airworthiness determinations grounded in fact rather than guesswork. For Aviation Maintenance Technicians (AMTs), understanding the documentation process is not optional; it is a core professional and regulatory obligation.
This article walks through the full documentation cycle: identifying what to record, how to write a technically sound entry, what regulatory authority governs maintenance records, and how documentation ties into the broader corrosion control program. Every concept here is grounded in FAA guidance found in the Aviation Maintenance Technician Handbooks and the applicable regulations under Title 14 of the Code of Federal Regulations (14 CFR).
Why Documentation Is a Safety Function
Aircraft corrosion rarely appears in isolation. A small area of pitting corrosion on a wing spar today may indicate an environmental exposure pattern — moisture intrusion, a failed sealant, or inadequate drainage — that will produce corrosion elsewhere tomorrow. Without accurate records, the next inspector has no baseline. They cannot tell whether a finding is new or whether it was previously treated. They cannot gauge the rate of progression. They cannot determine whether prior repairs were completed correctly or whether an area that was once within serviceable limits has now grown beyond them.
Beyond the practical safety value, documentation is a legal requirement. Under 14 CFR Part 43.9, any person who performs maintenance, preventive maintenance, rebuilding, or alteration on an aircraft (other than certain owner-performed preventive maintenance) must make an entry in the maintenance record. That entry must include a description of the work performed, the date of completion, the name and signature of the person performing the work, and the certificate number and type held by that person. Failure to create a proper maintenance record entry can render the maintenance legally invalid and the aircraft unairworthy.
What to Document When Corrosion Is Found
A corrosion entry in the maintenance record should be specific enough that a reader who has never seen the aircraft can form an accurate mental picture of the finding. Vague language like "corrosion noted and treated" is inadequate. A thorough entry covers several key elements:
- Location: Identify the exact structural area using standard aircraft station, buttock-line, and waterline references where applicable, or a clear descriptive reference (e.g., "lower surface of left wing, between Rib 3 and Rib 4, approximately 6 inches inboard of the aileron hinge bracket").
- Type of corrosion: Surface corrosion, pitting corrosion, intergranular corrosion, filiform corrosion, galvanic corrosion, or exfoliation — each has different structural implications, and identifying the type helps future inspectors know what to look for nearby.
- Extent and severity: Describe the approximate area affected, the depth of pitting (shallow, moderate, or deep), and whether the metal has been perforated. Use measurable terms where possible.
- Structural part affected: Note whether the corroded material is a primary structural member (spar cap, bulkhead, skin in a stress-critical zone), a secondary structure, or a non-structural component. This directly affects airworthiness determination.
- Action taken: Describe the treatment method — whether corrosion was mechanically removed, what tools were used, whether chemical conversion coating was applied, what protective finish was restored, and whether the area was sealed.
- Disposition: State clearly whether the part was found serviceable after treatment, was repaired per a specific approved data source, or was removed and replaced. If corrosion was left in place and monitored, explain why and reference the approved data authorizing that decision.
- Approved data reference: Cite the manufacturer's Structural Repair Manual (SRM), an applicable Airworthiness Directive (AD), an FAA-approved repair specification, or other approved data that authorized the treatment or disposition.
Levels of Corrosion Severity and Their Documentation Implications
The FAA and aircraft manufacturers typically classify corrosion by level of severity, and this classification should appear in maintenance records. Level 1 corrosion is generally defined as minor surface corrosion that can be removed and treated without exceeding allowable material removal limits, leaving the structure fully airworthy. Level 2 corrosion involves more significant material removal that still falls within allowable limits but may require a second-level review or engineering disposition. Level 3 corrosion exceeds allowable limits and requires a structural repair or part replacement before the aircraft can be returned to service.
Understanding these levels matters for documentation because the required action — and therefore the record entry — differs for each. A Level 1 finding might be documented as cleaned, treated, and returned to service per SRM limits. A Level 3 finding requires documentation of the repair, the approved data used, and the mechanic's or repairman's certification that the repair restores the structure to its original or equivalent strength.
Regulatory Framework Governing Maintenance Records
Several sections of 14 CFR directly govern how maintenance records must be kept. 14 CFR Part 43.9 establishes the minimum content of a maintenance record entry. 14 CFR Part 43.11 covers entries made when an inspection (such as an annual or 100-hour inspection) is performed. Under Part 43.11, the inspector must record the type of inspection, the date, the aircraft total time in service, the signature and certificate number of the inspector, and either a return-to-service statement or a description of any discrepancies found.
14 CFR Part 91.417 specifies how long maintenance records must be retained. Records of routine maintenance must be kept until the work is superseded by subsequent work of equivalent scope and detail, or for a minimum period specified by regulation. Records of major repairs and major alterations (documented on FAA Form 337) must be kept indefinitely with the aircraft. Because significant corrosion removal that weakens a primary structure can constitute a major repair, AMTs must be alert to when a corrosion finding crosses into major-repair territory and requires Form 337 documentation and, in some cases, FAA approval.
Major vs. Minor Repair Determinations for Corrosion Work
One of the more consequential documentation decisions an AMT faces is whether a corrosion repair is a major repair or a minor repair. This determination flows directly from 14 CFR Part 43, Appendix A, which lists repairs to primary structural members — spars, spar caps, compression members, highly stressed bulkheads, and similar components — as major repairs. Corrosion removal that involves blending out material on a primary structural member can therefore qualify as a major repair, even if the total amount of metal removed seems small.
A major repair requires approved data (an SRM, an FAA-approved engineering order, or a Designated Engineering Representative approval), must be performed by or supervised by an appropriately rated certificated mechanic or repair station, and must be documented on FAA Form 337 in addition to the standard maintenance record entry. One copy of the completed Form 337 is given to the aircraft owner, and the other copy is forwarded (typically through the local FSDO) to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours of approval for return to service.
Corrosion Monitoring Entries and Trend Tracking
When corrosion is found but is within limits and is being monitored rather than immediately removed — a practice sometimes authorized by manufacturers for very early-stage surface corrosion — the maintenance record must reflect this clearly. The entry should state that the area was found with corrosion, that it was evaluated against specific approved limits, that it was found within those limits, that no removal was performed, and that the area is to be re-inspected at a specified future interval. Failing to document a "monitor" decision properly creates the false impression to the next inspector that the area is corrosion-free, which could lead to missed progression and eventual structural failure.
Over multiple inspection cycles, these monitoring entries create a trend record. Trend analysis is a fundamental tool of a sound corrosion prevention and control program (CPCP). If the same area shows documented findings at each inspection, the rate of progression can be calculated and used to determine whether the monitoring interval should be shortened or whether removal and repair should be performed proactively.
Key Numbers and Rules
- 14 CFR 43.9: Requires description of work performed, date, name, signature, and certificate type and number for any maintenance entry.
- 14 CFR 43.11: Governs inspection entries; requires aircraft total time in service, inspection type, and a return-to-service statement or discrepancy list.
- 14 CFR 91.417: Routine maintenance records must be retained until superseded; major repair/alteration records (Form 337) must be kept with the aircraft indefinitely.
- FAA Form 337: Required for major repairs and major alterations; one copy is given to the aircraft owner, and the other is forwarded to the FAA Aircraft Registration Branch (Oklahoma City) within 48 hours.
- 14 CFR Part 43, Appendix A: Defines what constitutes a major repair, including corrosion work on primary structural members.
- Corrosion levels 1, 2, and 3 are manufacturer-defined severity classifications that determine the required repair action and scope of documentation.
Common Test Traps
- Vague entries are not acceptable. Saying "corrosion found and treated" without specifying location, type, extent, and action taken does not meet the requirements of 14 CFR 43.9. The exam may present scenarios where you must identify what is missing from an incomplete entry.
- Not all corrosion work is a minor repair. Any material removal on a primary structural member can qualify as a major repair under Appendix A of Part 43, requiring Form 337 and approved data — even if the area looks small.
- Monitoring without documenting creates an airworthiness hazard. If corrosion is within limits and monitored, the record must explicitly say so, including the approved source and the re-inspection interval.
- Form 337 retention is permanent. Unlike routine maintenance records that can be superseded, Form 337 records for major repairs must travel with the aircraft indefinitely — they are never discarded.
- The certificate holder, not just the mechanic, has records responsibilities. Under 14 CFR 91.417, the aircraft owner or operator is responsible for maintaining and producing maintenance records when required by the FAA. AMTs create the records; owners must keep them.