100-Hour Inspection Rules (FAR 91.409): Guide for Planning

If you operate aircraft for hire or provide flight instruction under FAR 91.409, 100-hour inspections repeat—and they depend on accurate hour tracking. Many operators reconcile Hobbs, tach, and flight logs that don't match. When posting slips, you often discover the compliance gap when the inspection is already tight or overdue.

This guide turns FAR 91.409 into repeatable steps for hour capture, due forecasting, slot planning, inspection execution, closeout, and correct use of the 10-hour en route allowance.

Main Takeaways

• 100-hour inspections apply under FAR 91.409(b) when you carry persons or property for hire, or when you provide the aircraft for paid instruction. The clock is 100 hours of time in service, not Hobbs or tach.
• Time in service is airborne time (wheels-off to wheels-on) under 14 CFR 1.1. If you track Hobbs/tach for operations, reconcile them to your compliance method on a fixed cadence.
• A 100-hour inspection follows Part 43 Appendix D, and an A&P can approve return to service. The annual runs separately and requires an IA unless an IA performs and documents the inspection as an annual.
• Use a repeatable workflow to avoid overflown intervals. Capture and post hours quickly, forecast due time, reserve a slot with buffer, execute to scope, and close out an audit-ready record package.
• The 10-hour en route allowance is limited and requires payback. It only applies while repositioning to the inspection location, and you must subtract the excess time from the next 100-hour interval.

What FAR 91.409 Requires for 100 Hour Inspections

14 CFR 91.409(b) requires a 100-hour inspection for certain operations, due every 100 hours time in service. The scope aligns to 14 CFR Part 43 Appendix D. "Time in service" is airborne time per 14 CFR 1.1.

A rated A&P mechanic can perform and approve return to service for a 100-hour inspection. An annual requires an Inspection Authorization (IA).

Which Operations Trigger the 100-Hour Requirement

A 100-hour inspection is required when:

• The aircraft carries persons or property for compensation or hire, or
• Flight instruction is provided and the operator/instructor provides the aircraft.

Decision rule (fast check):

• Carry persons for hire? Yes → 100-hour required
• Instruction where you provide the aircraft? Yes → 100-hour required
• Owner-provided aircraft for instruction? No → annual only

For-Hire Carriage and Instruction-for-Hire Rules Under 91.409(b)

Any aircraft carrying passengers or property for compensation or hire is covered. This triggers the 100 hour requirement. It applies to charter operations, air taxi, and cargo flights where compensation changes hands.

Use this table to map common scenarios to the requirement and confirm edge cases where "who provides the aircraft" must be clear in documentation and practice.

100-hour inspection applicability matrix

How the 100-Hour Inspection Differs from the Annual

The 100 hour and annual inspections share the same scope. They differ in the trigger and sign-off authority. They also differ in planning impact.

This table compares the requirements side by side so you can plan around the correct trigger and sign-off authority. Use it to confirm which inspection is due, who can approve it, and whether any allowance applies.

100-Hour vs. Annual Inspection Requirements at a Glance

Key planning point: 100-hour does not replace the annual. If an IA performs and documents the inspection as an annual, it can satisfy the annual requirement.

How to Track Time in Service for 100-Hour Compliance

The FAA defines time in service as airborne time. Hobbs and tach are common operational proxies, but they don't consistently match airborne time.

Why Hobbs and Tach Don't Match Airborne Time

• Hobbs often runs from engine start to shutdown and can overstate airborne time.
• Tach varies with RPM and can understate time depending on profile.
• Either can drive early inspections (cost) or late inspections (noncompliance).

How to Reconcile Hobbs, Tach, and Flight Logs

Keep it simple and consistent:

• Define your primary time source and why it aligns to 14 CFR 1.1.
• Reconcile on a fixed cadence (weekly for high utilization is common).
• Define a variance threshold that triggers review.
• Resolve discrepancies before updating due times in the maintenance system.

Some operators feed flight data directly into planning to reduce late posting and manual errors. Flight Operations tools support this workflow.

The Seven-Step 100-Hour Inspection Workflow

Use the same steps every cycle to prevent due-time drift and closeout gaps.

1. Capture Flight Hours From the Source of Truth

Capture hours from flight logs or digital capture that best reflects airborne time. Use Hobbs/tach as cross-checks—not the primary compliance clock unless your policy supports it.

2. Post Hours Within 24 Hours

Post flight hours within 24 hours. Late posting is a common reason inspections go overdue unexpectedly.

3. Forecast the Next 100-Hour Due Time

Forecast when the aircraft will reach the next 100-hour threshold based on current utilization and update forecasts when utilization shifts.

Planning note: FAA surveys show instructional hours are up almost 30% year over year, which can tighten inspection windows for training fleets (AOPA).

4. Set a Scheduling Buffer and Reserve a Maintenance Slot

Use a buffer (10–15 hours is common) and reserve a slot based on expected TAT, staffing, and parts availability.

Capacity note: MRO capacity limits can increase lead times, and a 10% mechanic shortage in 2025 further compresses scheduling flexibility (ATEC Pipeline Report).

5. Build the Work Package Before the Aircraft Arrives

Use Part 43 Appendix D as baseline scope, then add known discrepancies and deferred items that must be closed during the event. Pre-stage parts and confirm technician coverage.

6. Execute the Inspection and Record Findings on the Work Order

Perform the inspection and record findings/corrective actions on the work order as they occur. Real-time documentation reduces closeout errors.

7. Close Out Records and Complete the Audit-Ready Package

Before return to service, confirm the package includes:

• Completed logbook entry (14 CFR 43.11 fields)
• Work order closure with findings and corrective actions
• Parts trace documentation for installed items (e.g., 8130-3 or equivalent)
• Discrepancy closure proof for each finding

How the 10-Hour En Route Allowance Works

FAR 91.409(b) allows exceeding the 100-hour limit by up to 10 hours only while en route to the inspection location. Excess time must be included in computing the next interval (payback).

Worked examples:

• If you fly 6 hours past 100 to reach the inspection location, the next due time is 194 (100 + 100 − 6).
• If you use the full 10 hours, the next due time is 190.

This is not a rolling extension. Before dispatching past 100 hours, confirm the flight is genuinely en route to the inspection location (not continued revenue operations). If there's risk of exceeding 110 hours, don't fly—swap aircraft or ground it.

Logbook Entries and Records That Survive an Audit

A compliant 100-hour record depends on 14 CFR 43.11, with current guidance from FAA AC 43-9D (issued September 2025).

What Your 100-Hour Logbook Entry Must Include (14 CFR 43.11)

At minimum:

• Work performed description referencing 100-hour inspection and Part 43 Appendix D
• Completion date
• Aircraft total time in service at completion
• Signature of approving person
• Certificate number and certificate type (A&P or IA as applicable)
• RTS/airworthy statement if required by your procedures

Note: AC 43-9D replaced AC 43-9C in September 2025 and aligns recordkeeping expectations to current FAA policy.

Use this checklist to confirm each required field is present and to avoid the most common audit findings.

100-hour inspection logbook entry checklist

Example entry structure (informational only; confirm with your IA or local FSDO):

"I certify that this aircraft has been inspected in accordance with a 100 hour inspection and was determined to be in airworthy condition."
[Date] | [Aircraft Total Time: XXXX.X hrs] | [Signature] | [Certificate #] | [A&P]

Build the Supporting Record Package  (Work Orders + Traceability)

Auditors often verify the logbook line against supporting records:

• Work order reference/attachment
• Discrepancy closures (what was found + how it was corrected)
• Parts trace documentation for installed items (8130-3 or equivalent)

Electronic Records and Signatures (LOA/OpSpec A025)

FAA Notice 8900.726 (December 2024) simplified OpSpec/LOA A025 for electronic records/signatures. If you use digital sign-offs, confirm access controls, audit trails, and data integrity controls meet your authorization requirements.

Common Documentation Errors That Trigger Audit Findings

Common mistakes that create findings include missing aircraft total time at inspection. Auditors can't verify interval compliance without it. Other issues include unsigned or undated entries. Another issue is no Appendix D reference. Discrepancies can also be listed without closure proof. These issues can occur before return to service.

Progressive Inspections as an Alternative to 100-Hour and Annual

14 CFR 91.409(d) allows a FAA-approved progressive inspection program that can replace both 100-hour and annual requirements. The trade-off is less downtime per event, but tighter governance and record discipline.

How Progressive Inspections Are Structured

A progressive inspection breaks Part 43 Appendix D scope into phases completed on a defined cycle (commonly 4–6 phases). Completing all phases equals one full inspection. The program must be approved before you operate under it (by the FAA Administrator or an authorized designee).

To stay compliant, you must maintain:

• A phase schedule showing what's due and when
• Records that prove continuity across phases (no gaps, no missed items)
• A complete cycle closeout within the allowed interval

If you stop the program, notify the FAA and return to standard compliance under 91.409.

When Progressive Inspections Make Sense

Progressive programs fit high-utilization fleets that want to avoid long groundings by spreading downtime across shorter, more frequent events. The downside is that documentation becomes the control—your team must prove each phase is completed and on schedule.

Central tracking helps when you manage multiple aircraft and phase schedules. Fleet Maintenance Software supports progressive program control and standard hour-based intervals.

Centralize Hour Tracking and Inspection Planning with SOMA Software

A 100-hour inspection under FAR 91.409 depends on clean hour capture, accurate forecasting, and disciplined closeout. When hours live in Hobbs, tach, and flight logs, due times drift—and teams find the problem when the inspection is already tight or overdue.

SOMA Software centralizes hour posting, due forecasting, and work order execution in one system. Connect flight logs to schedules to reduce late postings and prevent overflown intervals. Keep digital work orders, structured logbook entries, and supporting documents tied together so the inspection package is audit-ready without chasing paper.

Get a Quote to see how SOMA Software's Aircraft Maintenance Management supports hour-based compliance tracking across your operation.

FAQs About 100 Hour Inspections

What aircraft require a 100 hour inspection?

Any aircraft used to carry persons for hire must comply. Instruction also triggers it when you provide the aircraft. This is under 91.409(b).

Can I use the 10-hour en route allowance more than once between inspections?

No. The allowance is a one-time contingency, and excess hours reduce the next interval. If you use 6 hours, the next inspection is due at 194 hours.

Does completing a 100 hour inspection reset the annual due date?

No. Both requirements run independently. An IA can combine them with correct documentation.

How do I prove to an auditor that my time-in-service tracking is compliant?

Keep a written policy for your time source. Show how it matches the 14 CFR 1.1 airborne-time definition. Keep records of periodic checks.

What fields must be included in a 100 hour inspection logbook entry?

Required fields per 14 CFR 43.11 include the work description. It must reference Part 43 Appendix D. Include the completion date and aircraft total time. Add signature with certificate number and type.