Aircraft Structural Repair Manual (SRM) Inspection Criteria

When an aircraft suffers a dent, crack, gouge, or corrosion attack, the Aviation Maintenance Technician's first question should not be "how do I fix this?" but rather "does the manufacturer's Structural Repair Manual tell me I can fix this at all — and if so, exactly how?" The Aircraft Structural Repair Manual (SRM) is the definitive, type-specific document that establishes damage classification, allowable damage limits (ADLs), and approved repair procedures for a certificated aircraft's airframe. Understanding how to navigate and apply SRM inspection criteria is both a practical maintenance skill and a knowledge-test staple for the AMT General written exam.

The SRM is produced by the aircraft manufacturer and accepted by the FAA as data approved for use in the maintenance and alteration of that specific aircraft type. For transport-category aircraft, SRMs are developed in accordance with FAA-accepted substantiation methods and form part of the Instructions for Continued Airworthiness (ICA) required under 14 CFR Part 25 and referenced in Part 43. For general aviation aircraft, equivalent structural maintenance data may appear in the Aircraft Maintenance Manual (AMM) or a separate SRM, but the fundamental inspection logic is the same. Without an SRM — or other FAA-approved or acceptable data — an AMT cannot legally certify a structural repair.

What the SRM Contains

A well-organized SRM divides the airframe into zones and structural elements, then walks the technician through a systematic inspection and decision process. Major sections typically include:

• Introduction and general information: Defines key terms such as "allowable damage," "repairable damage," and "damage beyond limits." It also specifies which revision of the SRM applies, what units of measurement are used, and what special tools or materials are required.
• Damage classification: Categorizes damage by type (dent, scratch, gouge, crack, delamination for composites, corrosion, etc.) and assigns each type specific measurement parameters — depth, length, area, or a combination.
• Allowable Damage Limits (ADLs): Charts, tables, and diagrams that define the maximum damage a structure can sustain and still be considered airworthy without repair. Damage at or within these limits may be left in service (often with a notation in the maintenance records) or requires only cosmetic treatment such as blending or polishing.
• Repair schemes: Step-by-step instructions, material callouts (alloy type, temper, thickness), fastener specifications, and torque values for returning damaged structure to an airworthy condition. Each repair scheme includes its own applicability statement — it applies only to damage within the stated limits and only on the specified structural element.
• Zone and access information: Structural diagrams keyed to zone numbers so the technician can quickly locate which chapter governs the damaged area.

How Allowable Damage Limits Work

The ADL concept is central to SRM inspection. Engineers calculate the load-carrying capability of each structural member under limit and ultimate load conditions. They then define a damage envelope below which the remaining material still meets the airworthiness requirements applicable to that aircraft's certification basis — generally 14 CFR Part 25 for transport-category aircraft, or Part 23 for general aviation aircraft (noting that many newer GA aircraft are certificated under the amended, performance-based Part 23 rewrite while older aircraft may be certificated under earlier Part 23 versions). This envelope becomes the ADL published in the SRM.

In practice, ADLs are expressed in specific, measurable terms. A typical entry for a skin panel might read: "Dent depth not to exceed 0.030 inch (0.76 mm), with a minimum distance of 2.0 inches (50.8 mm) from any fastener, stringer, or frame." Every element of that sentence is testable and safety-critical. If the dent is 0.031 inch deep, it is outside limits — full stop. The technician cannot average the measurement, cannot round down, and cannot use engineering judgment to override the ADL without additional FAA-approved data (such as an FAA Form 337 with DER-approved engineering data, or a Supplemental Type Certificate).

For composite structures, ADLs are often expressed as delamination area (in square inches) detected by tap testing or ultrasonic inspection, or as fiber damage depth relative to the number of plies. Composite SRM sections also specify environmental conditioning requirements for repairs and reference concepts such as "Barely Visible Impact Damage" (BVID) and "Clearly Visible Impact Damage" (CVID). BVID is not defined by a single standardized depth threshold; rather, it describes impact damage that is difficult to detect visually during a routine inspection, with actual detectability criteria varying by material, structure, and the specific manufacturer's SRM.

The Inspection Decision Flow

A logical, SRM-driven inspection follows a clear decision tree:

1. Identify and measure the damage precisely. Use the tools and methods the SRM specifies — depth gauges, calipers, dye-penetrant, eddy current, tap hammer — not guesswork. Document every measurement.
2. Locate the applicable SRM chapter and zone. Confirm the correct aircraft serial number and modification status fall within the SRM's applicability. An SRM revision may not apply to all tail numbers.
3. Compare measurements to the ADL table. If damage is within allowable limits, document findings and, if required by the SRM, blend or polish the area to remove stress risers. Enter the findings in the maintenance record per 14 CFR §43.9.
4. If damage exceeds ADLs, check for an applicable repair scheme. A listed repair scheme is pre-approved data. Follow it exactly — substituting a different fastener size or material alloy is not permitted unless the SRM explicitly allows alternatives.
5. If no repair scheme covers the damage, stop. The damage is "beyond limits" and beyond SRM coverage. The aircraft is unairworthy until new FAA-approved data is obtained — typically through a Designated Engineering Representative (DER) repair approval, a Supplemental Type Certificate, or manufacturer-issued Structural Repair Instructions or an Engineering Order.

Why SRM Compliance Is Legally Required

14 CFR §43.13 requires that maintenance be performed using methods, techniques, and practices acceptable to the Administrator, and using data that is current and applicable. The SRM, accepted by the FAA as part of the aircraft's type design data or ICA, meets this standard. Deviating from SRM repair schemes without additional FAA-approved data constitutes an unapproved alteration — a serious legal and safety violation. The FAA's Aviation Maintenance Handbook (FAA-H-8083-30) reinforces this principle: the technician is not authorized to improvise structural repairs simply because they look reasonable; the data must be traceable and approved.

Additionally, the manufacturer may issue Service Bulletins (SBs) or Alert Service Bulletins (ASBs) that revise or supplement SRM data. AMTs must verify they are working from the current revision of the SRM and check for any applicable SBs before beginning an inspection or repair. An outdated SRM may list limits that the manufacturer has since tightened based on fleet experience.

Key Numbers and Rules

• ADL compliance is binary: damage is either within limits or it is not. There is no "close enough" in structural inspection.
• Fastener edge distance matters: SRMs frequently require minimum distances from fasteners, doublers, frames, and stringers because stress concentrations are highest at these locations.
• Composite BVID: Barely Visible Impact Damage is a qualitative concept referring to impact damage that is difficult to detect during a standard visual inspection; the specific detectability criteria and any associated dimensions are defined by the applicable manufacturer SRM rather than a single universal value.
• Record-keeping is mandatory: 14 CFR §43.9 requires that every inspection finding and maintenance action be documented with a description of the work performed, the date, and the certificated person's signature and certificate number.
• Repair schemes include material callouts: substituting a different alloy, temper, or thickness invalidates the pre-approved scheme and makes the repair unapproved.
• Serial number applicability: Always verify the SRM applies to the specific aircraft being inspected; post-modification aircraft may have different structural limits.

Common Test Traps

• Confusing "allowable" with "repairable." Allowable damage requires no structural repair (though it may need blending). Repairable damage exceeds the ADL but falls within an SRM repair scheme. These are distinct categories with different actions required.
• Assuming any SRM repair scheme applies to any damage of that type. Each scheme has an explicit applicability section specifying the structural element, damage location, and maximum dimensions it covers. A wing skin repair scheme does not automatically apply to a fuselage skin, even if they look similar.
• Overlooking SRM revision currency. An AMT using a superseded SRM revision may apply limits or procedures that are no longer valid. Always confirm you have the current approved revision.
• Substituting materials without authorization. The test often presents a scenario where a listed fastener is unavailable and asks whether the technician may substitute a "similar" fastener. The correct answer is no — only substitutions explicitly permitted by the SRM or other FAA-approved data are legal.
• Failing to document within-limits findings. Students sometimes assume that if damage is within allowable limits and requires no repair, no paperwork is needed. In fact, 14 CFR §43.9 still requires a maintenance record entry describing the inspection and outcome.