3.2 Conducting Annual & 100-Hour Inspections (14 CFR 43.15 & Appendix D)

3.2 Conducting Annual & 100-Hour Inspections (14 CFR 43.15 & Appendix D)

[!NOTE] Scope vs. Privilege: Technically, an annual inspection and a 100-hour inspection are identical in physical scope and detail; both are performed in accordance with 14 CFR Part 43 Appendix D. However, they differ fundamentally in applicability, sign-off authority, and overflight allowances under 14 CFR 91.409.

Inspecting an aircraft to ensure continued airworthiness is one of the most critical responsibilities in aviation maintenance. For the IA, mastering the distinctions between annual and 100-hour inspections, strictly complying with the performance standards of 14 CFR 43.15, and executing diagnostic procedures such as cylinder compression checks are foundational exam and operational competencies.

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Regulatory Framework: Annual vs. 100-Hour Inspections

Federal aviation regulations establish distinct operating rules for annual and 100-hour inspections based on the nature of aircraft operations.

+-----------------------------------------------------------------------------------+
|                       ANNUAL VS. 100-HOUR INSPECTION COMPARISON                   |
+------------------------------------+----------------------------------------------+
| ANNUAL INSPECTION (91.409(a))      | 100-HOUR INSPECTION (91.409(b))              |
+------------------------------------+----------------------------------------------+
| • Required for all civil aircraft  | • Required only for:                         |
| • Interval: Every 12 calendar mos  |   1. Aircraft carrying persons for hire      |
| • Expires: Last day of 12th month  |   2. Flight instruction for hire (if provided|
| • Authority: IA, Repair Station,   |      by the instructor/flight school)        |
|   or Aircraft Manufacturer         | • Interval: Every 100 hours of time in service|
| • Overflight: ZERO hours allowed   | • Authority: A&P Mechanic, IA, Repair Station|
|   (Requires Special Flight Permit) | • Overflight: Up to 10 hours to reach facility|
| • Satisfies 100-hour requirement?  | • Satisfies annual requirement?              |
|   YES                              |   NO                                         |
+------------------------------------+----------------------------------------------+

1. Applicability Rules

• Annual Inspection (14 CFR 91.409(a)): Mandatory for all U.S.-registered civil aircraft operating under Part 91. It must be performed within the preceding 12 calendar months, expiring at 23:59 on the last day of the 12th month following the month in which it was completed (e.g., an annual completed on September 3, 2025, expires on September 30, 2026).
• 100-Hour Inspection (14 CFR 91.409(b)): Mandatory only if an aircraft is operated: (1) carrying any person (other than a crewmember) for hire, or (2) provided by any person giving flight instruction for hire. If an aircraft is flown solely for personal pleasure, non-commercial business transportation, or flight training where the student provides their own aircraft, a 100-hour inspection is not legally required.

2. Sign-Off and Return-to-Service Authority

• Who Can Sign an Annual? Only an FAA-certificated mechanic holding an Inspection Authorization (IA) under 14 CFR 65.95, an appropriately rated certificated repair station under 14 CFR 145, or the aircraft manufacturer under 14 CFR 43.7(d) exercising the inspection privilege of 14 CFR 43.3(j)(3). An A&P mechanic without an IA cannot approve an annual inspection.
• Who Can Sign a 100-Hour? An A&P mechanic (under 14 CFR 65.85 and 65.87), an IA, a certificated repair station, or the manufacturer.
• Asymmetric Equivalency: An annual inspection satisfies any outstanding 100-hour inspection requirement because the annual is certified by a higher level of inspection authority. Conversely, a 100-hour inspection never satisfies an annual inspection requirement.

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The 10-Hour Overflight Allowance (14 CFR 91.409(b))

Under 14 CFR 91.409(b), the 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. However, this rule contains strict legal parameters:

1. Purpose Limitation

The 10-hour buffer is strictly for en-route transit to an inspection facility. An operator cannot use the 10 hours for additional commercial revenue flights or flight training sessions.

2. Mandatory Calculation Deduction

The excess time used to reach the maintenance base must be deducted from the next 100-hour interval. The regulatory clock does not reset from the overflight hour mark; it resets from the original due point.

[!WARNING] Overflight Reset Math (Classic Exam Calculation):

• Previous 100-hour inspection completed at: 1,400.0 hours.
• Next 100-hour inspection legally due at: 1,500.0 hours.
• Aircraft flies 6.0 hours over limit to reach repair station: Arrives at 1,506.0 hours.
• When is the next 100-hour inspection due?
  • Correct Answer: 1,600.0 hours (1,500.0 + 100.0 = 1,600.0 hours).
  • Incorrect Trap: 1,606.0 hours (the 6 overflown hours are deducted: 100 - 6 = 94 hours of operation remaining).

3. Absolute Prohibition on Annual Overflight

Does the 10-hour overflight rule apply to an annual inspection? NO. 14 CFR 91.409(a) contains zero overflight allowance. The moment midnight strikes on the last day of the 12th calendar month, the aircraft is grounded. To fly an aircraft with an expired annual to a maintenance facility, the owner must obtain a Special Flight Permit (ferry permit) under 14 CFR 21.197.

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14 CFR 43.15 General Inspection Performance Rules

Under 14 CFR 43.15(c), each person performing an annual or 100-hour inspection must comply with two non-negotiable performance mandates:

1. Mandatory Checklist Usage (43.15(c)(1))

Each person performing an inspection must use a checklist while performing the inspection. The checklist may be:

• Formulated by the manufacturer of the aircraft or equipment.
• Designed by the inspector's own organization.
• Obtained from another commercial or industry source. Regardless of origin, the checklist must include the scope and detail of the items listed in 14 CFR Part 43 Appendix D.

2. Mandatory Engine Operational Run-Up (43.15(c)(2) and (c)(3))

Note the exact trigger: the run-up requirement attaches to the person approving the aircraft for return to service, and it must be done before that approval. For a reciprocating-engine-powered aircraft, 43.15(c)(2) lists exactly four things the run-up must determine, in accordance with the manufacturer's recommendations:

1. Power output — static and idle r.p.m.
2. Magnetos
3. Fuel and oil pressure
4. Cylinder and oil temperature

Under 43.15(c)(3), a turbine-engine-powered aircraft also requires a run-up before approval for return to service after an annual, 100-hour, or progressive inspection, but the rule states only that the run-up determines satisfactory performance in accordance with the manufacturer's recommendations — it does not enumerate parameters. Two exam-relevant asymmetries follow: the four-item list is reciprocating-only, and progressive inspections trigger a run-up requirement for turbine aircraft but are not named in 43.15(c)(2).

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Part 43 Appendix D: The 10 System Groups

14 CFR Part 43 Appendix D specifies the mandatory minimum scope and detail for annual and 100-hour inspections. Learn it by its paragraph letters (a) through (j), because the exam asks where a specific item lives. Paragraph (a) is a preliminary work requirement, not an inspection group, and there are nine inspection groups, (b) through (j). Every group is qualified "where applicable."

Three points the exam builds distractors on: cylinder compression lives in the engine and nacelle group, ¶ (d)(3) — not in a "fuselage" or "engine compartment" paragraph; batteries are inspected under the cabin and cockpit group, ¶ (c)(6), not under a separate electrical group; and Appendix D has a radio group, not an "avionics and electrical" group. The engine run-up is not in Appendix D at all — it is § 43.15(c)(2) and (c)(3).

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Differential Cylinder Compression Testing (AC 43.13-1B Chapter 8)

Under Appendix D, paragraph (d)(3), the internal engine must be inspected "for cylinder compression and for metal particles or foreign matter on screens and sump drain plugs," and, if there is weak cylinder compression, for improper internal condition and improper internal tolerances. The industry standard method is the differential pressure compression test performed in accordance with AC 43.13-1B, Chapter 8 and engine manufacturer technical service instructions (e.g., Continental SB03-3 or Lycoming SI 1191).

                       DIFFERENTIAL COMPRESSION TESTER SCHEMATIC
                                    0.040" Orifice
Regulated Shop Air (80 psi) ──[Regulator]──[Gauge 1]───[===|===]───[Gauge 2]──> To Cylinder Adapter
                                                       (Holding Pressure)

1. Equipment Setup and Safety Precautions

• Air Supply: Regulated clean compressed air source set to exactly 80 psi.
• Tester Specifications: Standard dual-gauge tester equipped with a restrictor orifice of 0.040-inch diameter, 0.250 inch long, with a 60-degree approach angle. AC 43.13-1B specifies this orifice for engines of less than 1,000 cubic inches displacement. Engine manufacturers layer their own requirements on top: Continental (SB03-3, as revised) requires the acceptable minimum to be established with a master orifice tool on the day of the test rather than by a fixed number.
• SAFETY CAUTION: When 80 psi is applied to a cylinder with a 5-inch piston diameter, approximately 1,570 pounds of linear force is exerted against the piston crown. If the piston is slightly off Top Dead Center, the propeller will rotate violently. Personnel must stand completely clear of the propeller arc and restrain the prop firmly.

2. Test Execution Procedure

1. Run the engine until oil and cylinder head temperatures reach normal operating ranges (warm engine ensures rings, oil film, and piston barrel expansions reflect real operating tolerances).
2. Remove all spark plugs (top or bottom plugs; removing all plugs prevents accidental engine starting).
3. Install the spark plug threaded adapter into the cylinder being tested.
4. Rotate the propeller by hand in the normal direction of rotation until the cylinder is on its compression stroke (air pushes out of adapter hole).
5. Bring the piston to exact Top Dead Center (TDC), where both the intake and exhaust valves are completely closed.
6. Restrain the propeller securely and slowly open the regulator valve to pressurize the cylinder to 80 psi on the input gauge (Gauge 1).
7. Record the holding pressure displayed on the cylinder gauge (Gauge 2) (e.g., 74/80).

3. Diagnostic Acoustic Isolation of Leakage

If a cylinder displays pressure below the acceptable threshold (commonly 60/80 psi for Lycoming, or below the master orifice reading for Continental), the IA must listen for escaping air to isolate the defect:

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Common Exam Traps & IA Watchpoints

[!WARNING] Critical Exam Traps for Section 3.2:

• Trap: 10-Hour Overflight Resetting the Interval: The 10 hours allowed under 91.409(b) does NOT push back the next inspection. If a 100-hour inspection is due at 800 hours and completed at 808 hours, the next inspection is due at 900 hours, NOT 908 hours.
• Trap: 100-Hour Overflight Applied to Annual: Never choose an answer that allows an annual inspection to exceed 12 calendar months by 10 hours. The 10-hour overflight rule applies exclusively to 100-hour inspections.
• Trap: Staking Valves vs. Condemning Cylinders: When valve leakage is detected, AC 43.13-1B and manufacturer service bulletins recommend staking the valve with a fiber mallet and brass drift to dislodge carbon before condemning the cylinder. If compression improves above minimums, the cylinder is serviceable.