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 (for an individual mechanic, an annual needs an IA; a 100-hour can be done by an appropriately rated A&P) and the trigger (12 calendar months vs 100 hours in service when 91.409(b) applies, subject to 91.409(c) exceptions).
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 approved or disapproved after a required inspection with a logbook entry per 14 CFR 43.11; if it is unairworthy, the person who performed the inspection (an A&P for a 100-hour, an IA for an annual) still makes the entry and gives the owner a signed and dated list of discrepancies.
Airworthiness Directives (ADs, 14 CFR Part 39) are mandatory; the owner's AD status record (14 CFR 91.417) shows, for each AD, the method of compliance, the AD number and revision date, and, if recurring, the time and date when the next action is required.
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 for carrying non-crewmembers for hire, or paid flight instruction in an aircraft supplied by the person giving the instruction, subject to 91.409(c) exceptions |
| Who signs | For an individual mechanic, an A&P with Inspection Authorization (IA) | An appropriately rated 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 unless a person authorized to perform annuals did it and entered it as 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 and extent, the date, the signature, certificate number, and kind of certificate, 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 number, and kind of certificate). Major repairs/alterations also require FAA Form 337, in addition to that entry.
Key return-to-service logic for the test:
- If a required inspection finds the aircraft unairworthy, the inspector does not approve it for return to service. The person who performed the inspection (an A&P for a 100-hour, an IA for an annual) still makes the 43.11 entry and gives the owner a signed and dated list of discrepancies; any appropriately rated mechanic or repair station can then correct them (items allowed to stay inoperative under 91.213(d)(2) are placarded "Inoperative" and added to the list).
- 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 owner's AD status record (14 CFR 91.417) must show, for each applicable AD, the method of compliance and the AD number and revision date, and for a recurring AD the time and date when the next action is required; noting the date and aircraft time of compliance in the entry is good practice.
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. The maintenance record is completed under 14 CFR 43.9, recording the corrosion removal/repair, the data used, and the AD compliance (the AD number and revision date and the method of compliance); the AD status record then shows the recurring AD's next-due time and date. Once the inspection requirements are met and the aircraft is found airworthy, the IA completes the 43.11 inspection entry 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?
The 100-hour has a much larger scope than the annual
Only a repair station may perform an annual
The annual does not require a logbook entry
The scope is the same; for an individual mechanic, the annual takes an A&P with Inspection Authorization while a 100-hour can be done by an appropriately rated A&P
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?
It is an automatic violation regardless of circumstances
Up to 10 hours overflight is allowed only to reach an inspection point, and the excess is deducted from the next 100-hour interval
The overflown time is simply ignored
The aircraft may overfly by up to 50 hours with no consequence
An IA performing an annual finds the aircraft unairworthy. What is the correct action?
Sign the aircraft off anyway since the inspection was completed
Ground the aircraft permanently with no recourse
Make the disapproval entry and give the owner a signed and dated list of discrepancies
Issue a new airworthiness certificate
Which corrosion type is recognized as intergranular attack that lifts or flakes the metal into layers and is especially serious in aluminum extrusions?
Exfoliation corrosion
Filiform corrosion
Uniform surface corrosion
Fretting corrosion
Sections you finish are checked off in the contents.