5.6 Airframe Inspection, Records, ADs, and Corrosion Decisions
Key Takeaways
- Annual and 100-hour inspections have the SAME scope and detail (14 CFR Part 43 Appendix D); the difference is who signs (annual needs an IA; a 100-hour can be done by any A&P) and the trigger (12 calendar months vs 100 hours in service).
- A 100-hour limit may be exceeded by up to 10 hours only to reach a place where the inspection can be done, and the overflown time is counted against the next 100 hours.
- An aircraft is returned to service for a required inspection only with a proper logbook entry per 14 CFR 43.11; if found unairworthy, the IA issues a list of discrepancies instead of a sign-off.
- Airworthiness Directives (ADs, 14 CFR Part 39) are mandatory; compliance and method must be recorded with AD number, date, hours, and recurring next-due in the maintenance records.
- Corrosion must be classified by type (surface, pitting, intergranular, exfoliation, galvanic, filiform, stress, fretting) and severity, then removed/treated within approved limits and re-protected, with reinspection if it exceeds limits.
Annual vs 100-Hour: Same Scope, Different Signature
The required inspections live in 14 CFR 91.409 and their content in 14 CFR Part 43, Appendix D ('Scope and Detail of Items to be Included in Annual and 100-Hour Inspections'). A frequent exam trap: the scope and detail are identical for both — the differences are who can sign and what triggers them:
| Annual | 100-hour | |
|---|---|---|
| Trigger | Every 12 calendar months | Every 100 hours time in service (when carrying persons for hire or giving flight instruction for hire) |
| Who signs | A&P with Inspection Authorization (IA) | Any A&P mechanic (no IA needed) |
| Scope | Part 43 App. D | Same Part 43 App. D |
An annual satisfies a 100-hour, but a 100-hour does not satisfy an annual. The 100-hour limit may be exceeded by up to 10 hours, but only while en route to a place where the inspection can be performed, and the excess time is subtracted from the next 100-hour interval. Before the inspection, App. D requires the aircraft be cleaned and opened (panels, cowling, fairings) so the structure and systems can actually be seen.
Records and Return to Service
Inspection is not finished until it is recorded. 14 CFR 43.11 specifies the entry for a required inspection: aircraft total time in service, the inspection type, the date, the signature/certificate number, and a statement that the aircraft was found airworthy (or a reference to the discrepancy list if not). 14 CFR 43.9 covers ordinary maintenance, preventive maintenance, and alteration entries (description of work, date, signature/certificate). Major repairs/alterations also require FAA Form 337.
Key return-to-service logic for the test:
- If a required inspection finds the aircraft unairworthy, the inspector does not sign it off; the IA gives the owner a signed list of discrepancies, and a certificated person must correct them before the aircraft flies.
- Only the holder of the appropriate certificate (A&P, IA, or repair station) may approve for return to service after the work for which they are rated.
- Maintenance records are part of airworthiness — they prove what was inspected, what was found, what data was used, and that the aircraft may fly. Missing or false records can make an otherwise-good aircraft unairworthy.
Airworthiness Directives and Required Inspections
Airworthiness Directives (ADs) are issued under 14 CFR Part 39 to correct an unsafe condition; compliance is mandatory and is a condition of airworthiness. Before signing any inspection the technician must research applicable ADs (airframe, engine, propeller, and appliance) and any Service Bulletins the operator is required to follow. The record entry for an AD must capture the AD number, revision, method of compliance, date, and aircraft time, and for a recurring AD the next-due time or date so it is tracked.
The airframe also has required special/structural inspections beyond the basic walkaround: pressurized-cabin inspections, control-system and cable inspections, emergency-equipment and pitot-static (24-month) checks where applicable, and life-limited part tracking. A checklist (manufacturer's or one meeting App. D) protects against missed areas, but it does not replace judgment — when a finding points to hidden damage (e.g., a smoking rivet, oil-canning skin, or a soft spot), the inspection must expand to find the cause, not just note the symptom.
Corrosion: Classify, Limit, Re-protect
Corrosion is one of the most common airframe findings and must be classified before any decision:
| Type | Recognition |
|---|---|
| Surface (uniform) | General dulling/etching of the finish |
| Pitting | Localized pinholes/cavities (common on aluminum) |
| Intergranular | Attack along grain boundaries (often from improper heat treat) |
| Exfoliation | Intergranular corrosion that lifts/flakes layers — serious in extrusions |
| Galvanic | Two dissimilar metals + moisture (e.g., steel against aluminum) |
| Filiform | Worm-like lines under paint/film |
| Stress / fretting | Corrosion combined with sustained stress or vibration/rubbing |
The decision sequence: clean and identify, determine the type, location, and severity, then remove corrosion within the approved limit (depth/area limits in the SRM/AMM — material removed reduces thickness, so a depth limit protects strength). After removal, treat and re-protect the surface (chemical conversion coating such as Alodine, primer, paint; sealant in joints).
If corrosion exceeds the allowable limit, it becomes structural damage requiring repair or replacement under approved data, plus possible reinspection intervals if a corrosion-prevention/control program (CPCP) applies. As always, the airworthy answer classifies, measures against approved limits, repairs only within authority, re-protects, and documents — never paints over corrosion to hide it.
Inspection Workflow and a Records Worked Example
A practical airframe inspection follows a repeatable workflow that the ACS expects you to reproduce. Before the inspection, review the aircraft and engine/propeller maintenance records, status of ADs, service bulletins, life-limited parts, and prior discrepancies, and confirm the registration and airworthiness certificate are aboard.
During the inspection, work the checklist that meets Appendix D — fuselage and hull, cabin and cockpit, engine and nacelle, landing gear, wings and empennage, propeller, systems, and a final operational/run-up check — opening the panels and fairings needed to actually see the structure. After, make the return-to-service or discrepancy entry, update AD compliance and next-due tracking, and record any parts and data used.
A records worked example ties it together. Suppose an annual reveals an out-of-limits exfoliation area on a wing-root extrusion and a recurring AD that came due. The IA cannot sign the aircraft airworthy until the corrosion is repaired under approved data and the AD is complied with. 9**, recording the corrosion removal/repair, the data used, the AD number, method of compliance, date, and aircraft time, and the recurring AD's next-due. 11** with the airworthiness statement.
The lesson the test drives home: the inspection, the corrosion decision, the AD, and the records are one connected airworthiness chain — a perfect physical inspection with incomplete records, or a cleared AD that was never recorded, still leaves the aircraft unairworthy.
What is the primary regulatory difference between an annual and a 100-hour inspection?
An aircraft due for a 100-hour inspection is 6 hours past the limit while flying to a maintenance base. How is this handled under the rules?
An IA performing an annual finds the aircraft unairworthy. What is the correct action?
Which corrosion type is recognized as intergranular attack that lifts or flakes the metal into layers and is especially serious in aluminum extrusions?