Chapter: Engine Inspection

Overview

This chapter covers the systematic inspection of aircraft powerplants, a fundamental responsibility of the Aircraft Maintenance Engineer (AME). Engine inspection is a critical safety function that ensures the airworthiness of the aircraft. This chapter synthesizes the knowledge required to perform thorough inspections, interpret findings correctly, and take appropriate corrective actions. It integrates regulatory requirements, manufacturer's data, and standard maintenance practices as outlined in FAA Advisory Circular AC 43.13-1B and 14 CFR Part 43.

The scope of engine inspection encompasses everything from routine visual checks to detailed internal examinations. The AME must be able to identify discrepancies, determine their severity, and decide whether the engine can remain in service or must be removed for repair. This requires a deep understanding of engine systems, failure modes, and the regulatory framework governing maintenance actions.

Key Concepts Explained in Detail

The Regulatory Framework for Engine Inspection

All maintenance actions are governed by regulations that define what must be done, how it must be done, and what records must be kept.

14 CFR Part 43 is the primary regulation for maintenance, preventive maintenance, rebuilding, and alteration. Key subparts include:

14 CFR 43.2: States that maintenance must be performed in accordance with applicable data. This means the AME must use the current manufacturer's maintenance manual, Instructions for Continued Airworthiness, or other approved data as the basis for all work.
14 CFR 43.9: Requires that after maintenance, a record must be made. This record must include a description of the work performed, the date, the signature and certificate number of the person approving the aircraft for return to service, and, for major repairs or alterations, a reference to the FAA Form 337.
14 CFR 43.13(a): Mandates that each person performing maintenance shall use methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness. If these are not available, the person must use methods, techniques, and practices acceptable to the FAA.
14 CFR 43.15: Specifies the performance rules for inspections, including the requirement to perform the inspection in accordance with the applicable manuals and to identify any defects.
14 CFR 43.11: Requires that the person performing an inspection must report any defects to the owner or operator.

14 CFR 91.7 states that no person may operate a civil aircraft unless it is in an airworthy condition. This is the overarching requirement that drives all inspection and maintenance activities.

FAA Advisory Circular AC 43.13-1B is an accepted means of compliance with 14 CFR 43.13(a). It provides widely accepted methods, techniques, and practices for the inspection, repair, and alteration of aircraft. While not a substitute for manufacturer's data, it is a critical reference for standard practices.

FAA Form 337 is required for major repairs and major alterations as defined in 14 CFR Part 43 Appendix A. A cracked engine mount, for example, is a structural repair and is considered a major repair, thus requiring this form.

The Hierarchy of Maintenance Data

The AME must understand the hierarchy of technical data to make correct decisions. The order of precedence is:

Airworthiness Directives (ADs): These are legally enforceable rules that mandate specific inspections, repairs, or modifications. They take precedence over all other data.
Manufacturer's Maintenance Manuals and Instructions for Continued Airworthiness (ICA): These are the primary source for inspection intervals, allowable damage limits, and repair procedures. They are the "approved data" referenced in 14 CFR 43.13(a).
FAA Advisory Circular AC 43.13-1B: This provides acceptable methods for standard practices when manufacturer's data is not available or does not cover a specific situation. It is a general guide, not a substitute for specific manufacturer instructions.
Service Bulletins (SBs) and Service Letters (SLs): These are manufacturer recommendations that are not mandatory unless incorporated into an AD. They may provide repair procedures, but these procedures may not be FAA-approved unless explicitly stated.

A critical distinction is that a manufacturer's service bulletin alone may not be an FAA-approved data source for a major repair. In such cases, the repair must be accomplished using data approved by the FAA, which may require an STC or a field approval.

Inspection Types and Intervals

Engine inspections are performed at various intervals, each with a specific purpose.

Preflight Inspection: A visual check performed by the pilot or mechanic before each flight to ensure the aircraft is safe for the intended flight. It includes checking for obvious leaks, loose parts, and general condition.
100-Hour Inspection: Required by 14 CFR 91.409 for aircraft operated for hire. It is a more detailed inspection than a preflight, covering specific items listed in the regulations and the manufacturer's inspection checklist.
Annual Inspection: Required by 14 CFR 91.409 for all aircraft. It is a comprehensive inspection of the entire aircraft, including the engine, and is valid for 12 months.
Progressive Inspection: An alternative to the annual inspection, where the aircraft is inspected in phases over a 12-month period.
Manufacturer's Scheduled Inspections: These are inspections required by the engine manufacturer at specific intervals (e.g., every 2,000 hours) to maintain the engine's type design and airworthiness. These may include hot section inspections (HSI) for turbine engines or top overhauls for reciprocating engines.
Life-Limited Parts: Certain components, such as turbine disks, have a mandatory replacement time. These parts must be retired from service at their specified life limit, regardless of their apparent condition. The AME must track these parts meticulously per 14 CFR 91.417(a)(2).

Common Engine Discrepancies and Their Significance

The AME must be able to recognize and interpret common findings during an inspection.

Fluid Leaks:

Oil Leaks: A small leak at a gasket (e.g., accessory case, pushrod housing) is a common finding. The correct action is to replace the gasket, not to over-tighten the fasteners, which can distort the housing. A leaking propeller governor gasket is a minor defect that can be corrected by replacing the gasket.
Fuel Leaks: A fuel leak is a serious safety hazard and must be corrected immediately. A small leak at a B-nut connection may be corrected by tightening, but if the leak persists, the line or fitting must be replaced. Applying sealant is not an approved repair for fuel systems.
Chafing: A hose that is chafing against a bracket is a serious finding. If the chafing has worn through the outer braid, exposing the inner liner, the hose is unairworthy and must be replaced. The cause of the chafing must also be corrected.

Hose and Line Condition:

Dry Rot and Cracking: Flexible hoses showing signs of dry rot and cracking on the outer cover must be replaced, even if they are not leaking. The outer cover deterioration indicates that the inner layers may also be compromised, making the hose susceptible to failure under pressure.
Blocked Breather Tube: A blocked crankcase breather will cause pressure to build up in the crankcase, forcing oil past seals and gaskets, leading to oil leaks and potential loss of oil.

Engine Components:

Compressor and Turbine Blades: Leading edge erosion, pitting, and nicks are common findings. The severity must be evaluated against the manufacturer's allowable damage limits. If the damage is within limits, it is acceptable. If it exceeds limits, the engine must be removed from service for repair or replacement.
Cracks: Cracks in critical components like turbine blades are generally not acceptable. If the manufacturer's manual states that cracks are not allowed, the engine is not airworthy and must be removed from service. Blending out cracks is not an approved repair unless explicitly allowed by the manufacturer.
Exhaust Manifold Clamps: Cracked clamps can lead to exhaust leaks and fire hazards. They must be replaced.
Engine Baffles: Baffles are essential for engine cooling. A broken baffle must be repaired or replaced.
Engine Mount Isolators: Excessive play in an isolator indicates wear and requires replacement.
Crankshaft Flange Runout: Runout beyond the manufacturer's service limit indicates a bent crankshaft or worn main bearing journals. This condition typically requires a major overhaul.

Fasteners and Hardware:

Safety Wire: Safety wire must be installed so that it tightens the fastener when the fastener attempts to loosen. Incorrect safety wire direction is a defect that must be corrected. Missing safety wire on critical fasteners, such as engine mount bolts, is an airworthiness discrepancy.
Cotter Pins: Missing cotter pins in castellated nuts and clevis pins are an airworthiness discrepancy and must be installed correctly.
Torque: Fasteners must be torqued to the manufacturer's specifications. Over-tightening can distort parts. Torque alone is not sufficient for critical fasteners; they must also be secured with safety wire or cotter pins if required by the manual.

Internal Engine Condition:

Metallic Particles in Oil Filter: This is a strong indicator of internal engine wear or damage. The source must be identified before the engine is returned to service. The AME should perform an oil analysis, inspect the filter for material type (ferrous vs non-ferrous), and may need to borescope cylinders or inspect the oil pump.
Low Compression: A compression reading below the manufacturer's minimum indicates a potential issue. The cause must be identified before any corrective action. A differential pressure check helps isolate the leakage path (piston rings, valves, or gaskets). A hissing sound from the oil filler cap during the test indicates air leaking past the piston rings into the crankcase.

Important Procedures and Diagnostic Logic

The Differential Pressure (Compression) Test

This test measures the ability of a cylinder to hold air pressure. A regulated air supply is connected to the cylinder, and the leakage rate is measured. A reading below the manufacturer's minimum (e.g., 70/80) warrants investigation.

The location of the leak can be determined by listening for escaping air:

Hissing from the oil filler cap: Air leaking past the piston rings into the crankcase.
Hissing from the exhaust pipe: A leaking exhaust valve.
Hissing from the carburetor or intake manifold: A leaking intake valve.
Bubbles in the radiator (if liquid-cooled): A cracked cylinder head or blown head gasket.

Borescope Inspection

A borescope is an optical device used to inspect the internal condition of an engine without disassembly. It is used to inspect cylinders, combustion chambers, turbine blades, and other components. The AME must be able to interpret the findings and compare them to the manufacturer's allowable limits.

Troubleshooting Logic

Effective troubleshooting is a systematic process of elimination.

Gather Information: Talk to the pilot or operator about the reported problem. What are the symptoms? When did they occur?
Perform a Visual Inspection: Look for obvious signs of trouble, such as leaks, loose parts, or damage.
Formulate a Hypothesis: Based on the symptoms and inspection, what is the most likely cause?
Test the Hypothesis: Use diagnostic tools and procedures to confirm or eliminate the suspected cause.
Take Corrective Action: Repair or replace the faulty component.
Verify the Repair: Run the engine or perform a functional test to ensure the problem is resolved.

For example, a pilot reports a high oil temperature reading. The first step is to check the oil level and look for a blocked oil cooler. This is a logical first step before replacing components.

A pilot reports a fuel smell in the cabin. If no leaks are found in the engine compartment, the next logical step is to check the cabin heater system, which may use a heat exchanger that can develop internal leaks.

Common Relationships and Critical Thinking

Inspection findings are not always defects. A nick in a fan blade that is within the manufacturer's allowable limits is acceptable. The finding should be documented, but no repair is required.
The manufacturer's manual is the final authority. When a condition is not addressed by the available maintenance manual, the AME must seek approved data from the manufacturer before making any repair decision.
A service bulletin is not always an approved data source. For major repairs, the data must be FAA-approved.
Life-limited parts must be retired at their life limit, regardless of condition. A minor nick within allowable limits does not change the life limit.
A simple defect can be corrected with a simple repair. A leaking gasket is corrected by replacing the gasket, not by overhauling the entire component.
A serious defect requires a serious response. Metallic particles in the oil filter require a thorough investigation to identify the source before the engine is returned to service.

Documentation is mandatory. All maintenance actions, including inspections and repairs, must be recorded per 14 CFR 43.9.

The AME's role is not just to find discrepancies but to make sound judgments based on technical data, regulatory requirements, and a commitment to safety. The decision to return an aircraft to service is a significant responsibility that requires a thorough understanding of all the concepts presented in this chapter.

Chapter 3: Engine Inspection