Subject: INSTRUCTIONS FOR CONTINUED

AIRWORTHINESS: IN-SERVICE INSPECTION

OF SAFETY CRITICAL TURBINE ENGINE

PARTS AT PIECE-PART OPPORTUNITY

Date: 3/8/01

Initiated By: ANE-110

AC No: 33.4-2

Change:

1. PURPOSE. This advisory circular (AC) provides guidance and acceptable methods, but

not the only methods, that may be used to demonstrate compliance with the requirements of

§33.4 of Title 14 of the Code of Federal Regulations, Instructions for Continued Airworthiness.

This AC deals with instructions for in-service inspections of safety critical turbine engine parts.

The need for in-service inspections is determined by a qualitative assessment of the safety

implications of a cracked safety critical part. The in-service inspections should be conducted

each time one of these safety critical parts is completely disassembled, unless the part has been

inspected within the last 100 cycles in service.

2. RELATED REGULATIONS.

 a. Section 33.4, Instructions for Continued Airworthiness.

 b. Part 33 Appendix A, Sections A33.3(a)(6) and A33.3(b)(2), Instructions for Continued

Airworthiness.

3. RELATED REFERENCE MATERIAL.

 a. AC 33-2B, Aircraft Engine Type Certification Handbook, dated June 30, 1993.

 b. AC 33.4-1, Instructions for Continued Airworthiness, dated August 27, 1999.

 c. Joint FAA – Aerospace Industries Association (AIA) report, Propulsion System and

Auxiliary Power Unit (APU) Related Aircraft Safety Hazards, dated October 25, 1999.
AC 33.4-2 3/8/01

Page 2 Par 4

4. DEFINITIONS. The following terms are defined for the purpose of this AC:

 a. Continued Airworthiness Assessment Methodology (CAAM) Level 3 Hazard or Event.

A CAAM level 3 hazard or event is a propulsion system or auxiliary power unit (APU)

malfunction that involves substantial damage to the aircraft or second unrelated system, small

penetrations of aircraft fuel lines or aircraft fuel tanks, significant damage to a second engine

system, uncontrolled fires, rapid cabin depressurization, permanent loss of thrust or power

greater than one propulsion system, inability to climb and maintain flight at least 1000 feet above

terrain, or impairment of aircraft controllability.

 b. CAAM Level 4 Hazard or Event. A level 4 hazard or event is a propulsion system or

APU malfunction that involves forced landing, loss of aircraft (hull loss), fatalities, or serious

injuries.

 c. Critical Feature. A critical feature is an attribute, characteristic, or property of a part that

results in the part having relatively high stress, susceptibility to handling damage or foreign object

damage (FOD), or low tolerance to material or other anomalies.

 d. Damage Tolerance. Damage tolerance is an element of the turbine engine component

design and life management process that accounts for potential component imperfections.

Component imperfections could result from inherent material structure, material processing,

component design, manufacturing or usage. Damage tolerance addresses component

imperfections by incorporating appropriate combinations of fracture-resistant design, fracture

mechanics, process control, and nondestructive inspection.

 e. Eddy Current Inspection (ECI). ECI is a nondestructive testing method in which eddy

current flow is induced in a test object. Variations in the test object cause changes in the flow

that are detected by a nearby coil or a Hall effect device for subsequent analysis.

 f. In-service Inspection. An in-service inspection is a detailed inspection of a specific critical

feature or area of a part.

 g. Fluorescent Penetrant Inspection (FPI). FPI is a surface crack detection process that

uses a penetrating fluid with a fluorescent suspension to enter crack separations by capillary

action. A black light is used to visually detect cracks containing this fluid.

 h. Hazard Ratio. The hazard ratio is the percent of total events of a particular turbine

propulsion system or APU malfunction or failure that has serious or severe consequences (i.e.,

hazard level 3 or 4).
3/8/01 AC 33.4-2

Par 4 Page 3

 i. Line-of-Sight. Line-of-sight refers to the qualitative assessment of the ability to visually

inspect piece-part features. Deep holes or features that are obstructed by adjacent part

features, narrow access passages, etc., are considered to have poor “line-of-sight” inspection

capability.

 j. Non-Destructive Inspection (NDI). An NDI is any inspection that identifies stress, strain,

dimensional, crack or flaw characteristics without compromising the integrity or airworthiness of

the part.

 k. Piece-Part Opportunity. Piece-part opportunity is the opportunity to perform in-service

inspection of safety critical parts when such parts are completely disassembled in accordance

with the manufacturer’s or other FAA-approved maintenance or overhaul manual instructions.

Inspections are triggered by opportunity, not by a time or cyclic interval requirement.

 l. Probability of Detection (POD). POD is a quantitative statistical measure of the capability

to detect a particular type of anomaly (flaw) over a range of sizes using a specific NDI

technique under specific conditions.

 m. Safety Critical Parts. Safety critical parts are those parts of an engine whose failure is

likely to directly present a CAAM level 3 or 4 hazard to the aircraft.

 n. Uncontained Failure. Uncontained failure is the uncontained release of debris caused by

the malfunction of an engine component (blade, disk, spacer, impeller, drum/spool, pressure

vessel). In order to be categorized as uncontained for the purposes of this AC, the debris must

pass completely through the nacelle. Parts or fragments that puncture the nacelle skin but do

not escape or pass completely through are not considered “uncontained.” Parts or fragments

that pass out of the inlet or exhaust opening without passing through any structure are also not

considered “uncontained.”

5. BACKGROUND. Analysis of fifteen years of transport aircraft accident and incident data

shows that the leading cause of engine related CAAM level 3 and 4 accidents for turbofan

engines is the uncontained failure of safety critical parts. The failure of safety critical parts can

present a significant hazard to an aircraft by releasing fragments that can penetrate the cabin or

fuel tanks, damage control surfaces, or sever flammable fluid or hydraulic lines. To significantly

reduce the occurrence of these incidents, part features most critical to safety should be

subjected to in-service inspections at each piece-part opportunity during their service lives,

using methods that detect flaws that could lead to failure.
AC 33.4-2 3/8/01

Page 4 Par 5

 a. In daily operation, many engine parts are exposed to high thermal and mechanical loads.

As a result of these loads, cracks can form. If these cracks are not detected, they can grow

and lead to part failure. Cracks can also form for many other reasons, including the following:

 (1) Material impurities.

 (2) Machining during manufacture or repair.

 (3) Unexpected stress levels due to part design or operation.

 (4) Unanticipated operating conditions.

 (5) Foreign object damage.

 (6) Handling damage during overhaul or repair.

 (7) Corrosion.

 b. As part of the certification plan, the applicant should identify the safety critical parts likely

to result in a CAAM level 3 or 4 event if they fail. When these parts and their associated

inspection instructions are included in the airworthiness limitations section (ALS) of the ICA

required by §33.4, the inspections become mandatory operational restrictions.

 c. The incorporation of damage tolerance design methods acceptable to the Administrator

enables a TC holder or applicant to evaluate the vulnerability of a safety critical part to anomaly

threats. Therefore, TC holders who have designed or assessed safety critical parts using a

damage tolerance design methodology may establish in-service inspections based on the part’s

damage tolerance characteristics and analyses.

6. CRITICAL PARTS AND FEATURES IDENTIFICATION. Effective in-service

inspections include identification of the safety critical parts, the most critical features, and the

inspection processes that reliably detect flaws in these features. Each applicant should conduct

an assessment to establish which parts and part features are candidates for in-service inspection.

 a. Selection of Parts. The primary consideration for the selection of safety critical parts

requiring in-service inspection is the consequence of failure of the part. Parts whose failure is

likely to result in a CAAM level 3 or 4 event, regardless of the probability of occurrence, should

be subject to in-service inspection.
3/8/01 AC 33.4-2

Par 6 Page 5

 (1) Methods for identifying safety critical parts should incorporate the following:

 (a) Service experience of similar parts.

 (b) Kinetic energy analyses of the part at operational levels.

 (c) Characteristics of the surrounding containment structure.

 (2) Safety critical parts should include: fan disks and hubs, high pressure turbine (HPT)

disks, low pressure turbine (LPT) disks, high pressure compressor disks and drum rotors. The

list may also include parts on large engines, such as cooling plates, shafts and spacers, due to

their mass.

 (3) If there is insufficient field experience to accurately determine the likelihood and

consequence of failure, design configurations should be qualitatively evaluated using the most

relevant field experience and the safety assessment conducted for certification. The qualitative

assessment should include, but is not limited to, the following factors:

 (a) Stress and temperature level relative to material capability.

 (b) Crack growth rate.

 (c) Crack path and the most critical fracture pattern.

 (d) Kinetic energy of fragments of similar components that had previous CAAM level

3 and 4 events.

 (e) Potential crack and damage types.

 b. Feature Identification. Once the list of safety critical parts has been established, a

historical review of the features that have caused the failure of similar parts should be

conducted.

 (1) This feature review should incorporate the following factors:

 (a) Failure history.

 (b) Failure root causes.

 (c) Feature history.

 (d) Susceptibility to handling and foreign object damage.
AC 33.4-2 3/8/01

Page 6 Par 6

 (e) Susceptibility to material impurities.

 (f) Susceptibility to corrosion, fatigue, and creep.

 (2) CAAM and mature TC holder databases have been considered acceptable for this

review. The design characteristics of the part should be considered when evaluating features.

 (3) When evaluating a feature for in-service inspection, the following characteristics

should be included as appropriate:

 (a) High stress.

 (b) Residual stress (compressive or tensile).

 (c) Peening effects on crack detection and growth rate.

 (d) Residual life.

 (e) Material characteristics.

 (f) New or novel materials.

 (g) Manufacturing and repair processes.

 (h) Severity of operating environment (temperature, speed, corrosion, etc.).

 (i) Cleaning requirements.

 (4) An applicant may conduct fracture mechanics analyses to support the identification of

the most critical part features.

7. INSPECTION METHODS. FPI and ECI are acceptable methods, but not the only

methods, for crack detection. Whether FPI, ECI or another method is used, the prescribed

inspection method should have a demonstrated reliability of detecting cracks on the targeted

part feature and a well-developed process that minimizes variation and maximizes detection

sensitivity and reliability.

 a. FPI is the most widely used inspection method for detecting surface flaws in turbine

engine parts. Many safety critical parts currently receive global or full field FPI inspections

periodically. However, parts with poor line-of-sight features, surfaces with high residual

compressive stress, rough surface finishes, complex feature details, or parts that are hard to

clean are not usually appropriate candidates for FPI.
3/8/01 AC 33.4-2

Par 7 Page 7

 b. To raise crack detection capabilities to a higher level, other inspection techniques, such as

ECI, should be considered for those features for which FPI is not considered appropriate.

Typically, ECI is appropriate for disk bores, high length to diameter (L/D) ratio holes, dovetail

slots, and other highly stressed features. Considerations in selecting an inspection method

include, but are not limited to, desired process sensitivity and reliability, accessibility, part

condition, feature geometry and POD.

8. FREQUENCY OF INSPECTIONS. To maximize the likelihood of crack detection, inservice inspections should be conducted at each piece-part opportunity, regardless of which

inspection method is chosen. The TC holder should determine at which disassembly level the

part would be sufficiently disassembled to allow for meaningful inspection. Parts that are

configured with riveted, pressed-on, or otherwise attached hardware whose removal is likely to

expose the part to additional damage should not necessarily be further disassembled to

accommodate focused inspection.

9. ENGINE INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (ICA).

 a. The engine manual ICA should contain language to include the in-service inspections.

The ALS of the ICA should incorporate the following factors:

 (1) Definition of “piece-part” or other appropriate disassembly level.

 (2) Identification of the parts requiring in-service inspection.

 (3) Location of the inspection instruction details.

 (4) Specification of when the inspection is required.

 b. TC holder ICAs are not standardized in the description of the ALS; titles such as

“Chapter 5,” “Time Limits Section,” or “Lifing Service Bulletins” are used. Engines with a

certification basis prior to Amendment 9 of part 33 are subject to the establishment of limitations

in accordance with §33.5, Instruction manual for installing and operating the engine, which does

not require a separate ALS. Engines with a certification basis of part 33 Amendment 9 and

later are subject to the requirements of §33.4, Instructions for Continued Airworthiness, which

does require a separate ALS.

 c. The step-by-step instructions for the actual inspections should be placed in the

appropriate engine overhaul manual section or other readily accessible shop document.
AC 33.4-2 3/8/01

Page 8 Par 9

 d. The following, or similar language, should be included in the ALS or Chapter 5 of the

engine manual to assure that the in-service inspections are incorporated into the operator’s

continuous airworthiness maintenance plans:

AIRWORTHINESS LIMITATIONS

MANDATORY INSPECTIONS

1. Perform inspections of the following parts at each piece-part opportunity in

accordance with the instructions provided in the applicable manual chapter(s):

Part Nomenclature Part Number

(P/N)

Inspect per Applicable Manual Chapter

Fan Disk All 72-31-xx-xxx, Fluorescent Penetrant

Inspection

and

72-31-xx-xxx, Eddy Current Inspection

HPT Rotor

Interstage Seal

All 72-53-xx-xxx, Fluorescent Penetrant

Inspection

2. For the purposes of these mandatory inspections, piece-part opportunity means:

 a. The part is considered completely disassembled in accordance with the

disassembly instructions in the engine manufacturer’s maintenance manual; and

 b. The part has accumulated more than 100 cycles in service since the last inservice inspection, provided that the part was not damaged or related to the cause for

its removal from the engine.

 e. Applicants should submit the in-service inspection ICA to the Aircraft Certification Office

(ACO) responsible for overseeing that type certification project for acceptance. The ACO and

an Aircraft Evaluation Group (AEG) will jointly determine the acceptability of the ICA. The

AEG will review the in-service inspection ICA and make recommendations on the maintenance

and operational aspects. The ACO will determine final acceptance of the completed ICA. The

in-service inspection instructions may be incomplete at type certification if a program exists to

ensure its completion either:

 (1) Prior to delivery of the first aircraft with the engine installed; or

 (2) Upon issuance of a standard certificate of airworthiness for the aircraft with the engine

installed, whichever occurs later.
3/8/01 AC 33.4-2

Par 9 Page 9

 f. Supplemental type certificates (STCs) or parts manufacturer approvals (PMAs) that

incorporate safety critical parts should contain in-service inspection ICA as part of the design

approval. The STC or PMA applicant is responsible for producing, distributing and maintaining

the required in-service inspection ICA.

10. RECORD KEEPING. Any person who performs the in-service inspection on a

component or part should make an entry in the maintenance record of that component or part

after inspection. Operators may incorporate the records of these inspections into existing

maintenance record keeping systems.

 /s/

Jay J. Pardee

Manager, Engine and Propeller Directorate

Aircraft Certification Service