Continuing Airworthiness

Continuing airworthiness is defined by ICAO as 'The set of processes by which an aircraft, engine, propeller or part complies with the applicable airworthiness requirements and remains in a condition for safe operation throughout its operating life.'

From: Airworthiness (Third Edition) , 2016

Continued Airworthiness and Operation

Filippo De Florio , in Airworthiness (Second Edition), 2011

Continuing airworthiness requirements

(1)

The continuing airworthiness of aircraft and components shall be ensured in accordance with the provisions of Annex I (Part M).

(2)

Organizations and personnel involved in the continuing airworthiness of aircraft and components, including maintenance, shall comply with the provisions of Annex I and where appropriate with those specified in Articles 4 and 5.

(3)

By derogation from Paragraph 1, the continuing airworthiness of aircraft holding a permit to fly shall be granted on the basis of the specific continuing airworthiness arrangements as defined in the permit to fly issued in accordance with the Part 21.

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The ICAO and the Civil Aviation Authorities

Filippo De Florio , in Airworthiness (Third Edition), 2016

3.1.1.2 Annex 8. Airworthiness of aircraft

This Annex contains the standards defining the minimum level of airworthiness for the development of the type certification requirements as a basis for the international recognition of the certificates of airworthiness for aircraft (according to Article 33 of the Convention) to fly to and land in the Contracting States.

Each state is free to develop its own comprehensive and detailed code of airworthiness or to select, adopt, or accept a code established by another Contracting State. The level of airworthiness that must be maintained by a national code is indicated by the broad standards of Annex 8.

Airworthiness Manual (Doc 9760) containing guidance material has been published by ICAO. States are invited to consider that the material in the Airworthiness Manual is intended to guide them in the development of their detailed and comprehensive national codes with a view to introducing uniformity in those national codes.

The content of Annex 8:

PART I. Definitions

PART II. Procedures for certification and continuing airworthiness 6

Chapter 1. Type Certification

Chapter 2. Production

Chapter 3. Certificate of Airworthiness

Chapter 4. Continuing Airworthiness of Aircraft

Chapter 5. Safety Management

PART III. Large Aeroplanes

PART IIIA. Aeroplanes over 5700   Kg for which application for certification was submitted on or after 13 June 1960 but before 2 March 2004

PART IIIB. Aeroplanes over 5700   kg for which application for certification was submitted on or after 2 March 2004

PART IV. Helicopters

PART IVA. Helicopters for which application for certification was submitted on or after 22 March 1991 but before 13 December 2007

The Standards of this part shall apply to helicopters intended for the carriage of passengers or cargo or mail in international air navigation.

PART IVB. Helicopters for which application for certification was submitted on or after 13 December 2007. Except for those Standards and Recommended Practices which specify a different applicability, the Standards and Recommended Practices of this part shall apply to helicopters greater than 750   kg, maximum certificated take-off mass intended for the carriage of passengers or cargo or mail in international air navigation.

PART V. Small aeroplanes – Aeroplanes over 750   kg but not exceeding 5700   kg for which application for certification was submitted on or after 13 December 2007

Except for those Standards and Recommended Practices which specify a different applicability, the Standards and Recommended Practices of this part shall apply to all aeroplanes having a maximum certificated take-off mass greater than 750   kg but not exceeding 5700   kg intended for the carriage of passengers or cargo or mail in international air navigation.

PART VI. Engines – Except as noted below, the Standards of this part are applicable to engines of all types, used as primary propulsion units, as required in Parts IIIB, IVB, and V. The Standards of this part are applicable to an engine type at the time of submission of an application to the appropriate national authority for a type approval. (…)

PART VII. Propellers – The Standards of this part are applicable to all propellers, as required in Parts IIIB and V. The Standards of this part are applicable to a propeller at the time of submission of an application to the appropriate national authority for a type approval.

An annex that is not directly linked to airworthiness but also capable of influencing the airworthiness requirements is Annex 13.

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Continuing1 Airworthiness and Air Operator's Certification

Filippo De Florio , in Airworthiness (Third Edition), 2016

10.2.5.7 Instructions for continued airworthiness

A holder of the repair approval shall furnish at least one complete set of those changes to the Instructions for Continued Airworthiness that result from the design of repair, comprising descriptive data instructions prepared in accordance with the applicable requirements, to each operator of aircraft incorporating the repair.

We may question why a major repair needs an Instruction for Continued Airworthiness to be added to the instruction of the relevant product. The answer is that major repairs can change the existing maintenance practices or inspection intervals. As an example, major structural repairs may need more frequent inspection.

The person holding the inspection authorization or authority to approve the return to service is responsible for determining whether any changes are necessary to the existing product Instructions for Continued Airworthiness resulting from the major repair.

NOTE: Fig. 10.1A and B, extracted from EASA AMC&GM for Part 21, although appearing complicated at first sight, give a clear idea of the repair process approval for products for which the state of design is an EU Member State and when the state of design is not a Member State.

Figure 10.1. Repair process approval where the state of design is (A) not an EU Member State and (B) an EU Member State.

APPENDIX 10.2 is a summary of EASA Maintenance/Continued Airworthiness requirements:

EASA Maintenance/Continued Airworthiness
Part Applicability Notes
M Continuing airworthiness requirements
1.

The continuing airworthiness of aircraft and components shall be ensured in accordance with the provisions of Annex I (Part M).

2.

Organisations and personnel involved in the continuing airworthiness of aircraft and components, including maintenance, shall comply with the provisions of Annex I (Part M) and where appropriate those specified in Articles 4 and 5.

3.

By derogation from paragraph 1, the continuing airworthiness of aircraft holding a permit to fly shall be ensured on the basis of the specific continuing airworthiness arrangements as defined in the permit to fly issued in accordance with Annex I (Part 21) to Commission Regulation (EU) No 748/2012.

 Article 3 of Commission Regulation (EU) No 1321/2014 of 26 November 2014
Amended by Commission Regulation (EU) 2015/1088 of 3 July 2015
145 Maintenance organisation approvals
1.

Organisations involved in the maintenance of large aircraft or of aircraft used for commercial air transport, and components intended for fitment thereto, shall be approved in accordance with the provisions of Annex II (Part 145).

2.

Maintenance approvals issued or recognised by a Member State in accordance with the JAA requirements and procedures and valid before the entry into force of Regulation (EC) No 2042/2003 shall be deemed to have been issued in accordance with this Regulation.

3.

Personnel qualified to carry out and/or control a continued airworthiness nondestructive test of aircraft structures and/or components, on the basis of any standard recognised by a Member State prior to the entry into force of Regulation (EC) No 2042/2003 as providing an equivalent level of qualification, may continue to carry out and/or control such tests.

4.

Certificates of release to service and authorised release certificates issued before the date of entry into force of Regulation (EC) No 1056/2008 by a maintenance organisation approved under the Member State requirements shall be deemed equivalent to those required under points M.A.801 and M.A.802 of Annex I (Part M), respectively.

 Article 4 (same as above)
66 Certifying staff
1.

Certifying staff shall be qualified in accordance with the provisions of Annex III (Part 66), except as provided for in points M.A.606(h), M.A.607(b), M.A.801(d), and M.A.803 of Annex I (Part M) and in point 145.A.30(j) and Appendix IV to Annex II (Part 145).

2.

Any aircraft maintenance licence and, if any, the technical limitations associated with that licence, issued or recognised by a Member State in accordance with the JAA requirements and procedures and valid at the time of entry into force of Regulation (EC) No 2042/2003, shall be deemed to have been issued in accordance with this Regulation.

3.

Certifying staff holding a licence issued in accordance with Annex III (Part 66) in a given category/subcategory are deemed to have the privileges described in point 66.A.20(a) of the same Annex corresponding to such a category/subcategory. The basic knowledge requirements corresponding to these new privileges shall be deemed as met for the purpose of extending such licence to a new category/subcategory.

4.

Certifying staff holding a licence including aircraft which do not require an individual type rating may continue to exercise his/her privileges until the first renewal or change, where the licence shall be converted following the procedure described in point 66.B.125 of Annex III (Part 66) to the ratings defined in point 66.A.45 of the same Annex.

5.

Conversion reports and Examination credit reports complying with the requirements applicable before Regulation (EU) No 1149/2011 applied shall be deemed to be in compliance with this Regulation.

6.

Until such time as this Regulation specifies requirements for certifying staff:

(i)

for aircraft other than aeroplanes and helicopters;

(ii)

for components;

the requirements in force in the relevant Member State shall continue to apply, except for maintenance organisations located outside the European Union where the requirements shall be approved by the Agency.

 Article 5 (same as above)
147 Training organisation requirements (Article 6)
1.

Organisations involved in the training of personnel referred to in Article 5 shall be approved in accordance with Annex IV (Part 147) to be entitled:

(a) to conduct recognised basic training courses; and/or (b) to conduct recognised type training courses; and (c) to conduct examinations; and (d) to issue training certificates.

2.

Any maintenance training organisation approval issued or recognised by a Member State in accordance with the JAA requirements and procedures and valid at the time of entry into force of Regulation (EC) No 2042/2003 shall be deemed to have been issued in accordance with this Regulation.

3.

Type training courses approved before the approval of the minimum syllabus of certifying staff type rating training in the operational suitability data for the relevant type in accordance with Regulation (EU) No 748/2012 shall include the relevant elements defined in the mandatory part of that operational suitability data not later than 18 December 2017 or within two years after the operational suitability data was approved, whichever is the latest.

 Article 6 (same as above)

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Continuing safety

Duane Kritzinger , in Aircraft System Safety, 2017

11.1.3 Objectives

The objectives of this chapter are to explore the ability for the SSA to remain 'live' so that:

•

the Continuing Airworthiness Maintenance Organisation (CAMO) can challenge some of the operating and maintenance assumptions made in the SSA and translated to the Instructions for Continued Airworthiness (ICA);

•

the Design Organisation can efficiently correct any unforeseen failure conditions as well as crew error and maintenance error vulnerabilities;

•

the CAMO (and their contracted Maintenance Organisations) can benefit from the failure diagnostic data [(e.g. Fault Tree Analysis (FTAs)] generated in the SSA;

•

future additions/upgrades/modification of the system or any of its constituent parts can be efficiently accomplished within the framework of previous SSA deliverables.

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Basic Concepts

He Ren , ... Yong Chen , in Reliability Based Aircraft Maintenance Optimization and Applications, 2017

2.23 Software maintenance

The term software maintenance is something of a misnomer because software maintenance is essentially a continuation of the developmental design process. Failures in airborne software are normally found in unusual operational modes—"bugs" that need to be redesigned out of the system, having been undetected in initial development testing. There remains a significant continuing airworthiness and thus safety concerns, however, that airborne software management processes sustain the same level of integrity as all other critical aircraft systems. It is unusual for civil operators to have authority to modify their own software while some military systems have been established on the basis of experience dating back to the 1960s.

One software characteristic is that a small change introduced in one part of the program, perhaps late in development or aimed at resolving a problem, may have serious implications in other, apparently unrelated areas of code. Operational improvements may be sought by way of similar, apparently minor changes.

Detailed consideration of this topic is beyond the scope of this course. It is important, however, to appreciate the need for careful risk management in tackling airborne software problems and also the need for observing the same critical standards for making and recording changes to the software as applied in the original design, development, and testing.

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Type Certification

Filippo De Florio , in Airworthiness (Third Edition), 2016

5.1 The International Civil Aviation Organization Type Certification

The basis for type certification is included in Chapter 1 of International Civil Aviation Organization (ICAO) Annex 8.

Paragraph 1.2 Design aspects of the appropriate airworthiness requirements describes the following prerequisites:

1.2.1

The design aspects of the appropriate airworthiness requirements, used by a Contracting State for type certification in respect of a class of aircraft or for any change to such type certification, shall be such that compliance with them will ensure compliance with the Standards of Part II 1 of this Annex and, where applicable, with the Standards of Parts III, IV, V, VI, or VII of this Annex. 2

NOTE: The above-mentioned standards represent the minimum standard for the development of regulations like Federal Aviation Regulations (FAR)/CS-23, -25, -27, and others.

For clarity, we will report some subparagraphs of Chapter 1:

1.3.1

There shall be an approved design consisting of such drawings, specifications, reports, and documentary evidence as are necessary to define the design of the aircraft and to show compliance with the design aspects of the appropriate airworthiness requirements.

1.3.2

The aircraft shall be subjected to such inspections and ground and flight tests as are deemed necessary by the State to show compliance with the design aspects of the appropriate airworthiness requirements.

1.4.1

The State of Design, upon receipt of satisfactory evidence that the aircraft type is in compliance with the design aspects of the appropriate airworthiness requirements, shall issue a type certificate to define the design and to signify approval of the design of the aircraft type.

1.4.2

When a Contracting State, other than the State of Design, issues a type certificate for an aircraft type, it shall do so on the basis of satisfactory evidence that the aircraft type is in compliance with the design aspects of the appropriate airworthiness requirements.

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The ICAO and the Civil Aviation Authorities

Filippo De Florio , in Airworthiness (Second Edition), 2011

3.7.1.2 Aircraft Certification Directorates

The Small Airplane Directorate (Central Region) is responsible for FAR 23 and 31; technical guidance for restricted category small airplanes; airworthiness criteria for gliders and airships; technical guidance for primary category airplanes; FAR 23, glider and airship import TC projects; issuance of ADs for the above products; and participation in consensus standard development for light sport aircraft. 20

The Transport Airplane Directorate (Northwest Mountain Region) is responsible for FAR 25 and technical guidance for restricted category transport airplanes 20 ; FAR 25 import type-certification projects and issuance of ADs for the above products.

The Rotorcraft Directorate (Southwest Region) is responsible for FAR 27 and 29; technical guidance for restricted category rotorcraft, powered lift aircraft, and primary category rotorcraft; FAR 27 and 29 import TC projects; and issuance of ADs for the above products.

The Engine and Propeller Directorate (E&PD, New England Region) is responsible for FAR 33 and 35, and technical guidance on auxiliary power units (APUs); FAR 33 and 35 import TC projects; and issuance of ADs for the above products.

The Service also has other functions.

International Policy Office. This office, including the staff in Brussels and Singapore, is responsible for policy guidance on bilateral agreements, import and export of aeronautical products, and other international airworthiness issues, programs, and procedures.

Aircraft Certification Offices (ACOs). Each directorate incorporates three or more ACOs within their geographical areas issuing the actual certification of aircraft and products. They work directly with the applicant and provide the main interface between the public and the FAA.

Aircraft Evaluation Group. A Flight Standards group is colocated with each directorate and it is responsible for determining operational acceptability and continuing airworthiness requirements of newly certified or modified aircraft, engines, propellers, and parts.

Before describing in more detail the four Directorates mentioned above, we introduce some useful definitions.

Aircraft Certification Office (ACO). The aircraft certification directorate's engineering operational element. This office administers and secures compliance with agency regulations, programs, standards, and procedures governing the type design of aircraft, aircraft engines, or propellers. It offers certification expertise on investigating and reporting aircraft accidents, incidents, and service difficulties. The term "ACO" refers to the Engine Certification Office (ECO), the Rotorcraft Certification Office (RCO), the Special Certification Office (SCO), the Airplane Certification Office (ACO), and all other ACOs.

Manufacturing Inspection Office (MIO). The MIO oversees Manufacturing Inspection District Offices (MIDO) and Manufacturing Inspection Satellite Offices (MISO) in its geographic area and provides organizational leadership and technical guidance to these offices. The MIO manages all geographically located production facilities and designees. They administer the airworthiness certification policies, office staffing, and internal budget allocation.

Manufacturing Inspection District Office (MIDO). This is a subordinate office to the MIO in its geographical area. This office oversees production certification, airworthiness certification, approval holders (manufacturing facilities), and designees in its geographical area. MIDOs support ACOs during type-certification programs; they investigate and submit enforcement reports on noncompliance with applicable FARs. MIDOs investigate and ensure corrective measures for service difficulties, which are implemented as identified in the quality system.

Manufacturing Inspection Satellite Office. This subordinate geographically remotes office reports to an MIDO and is responsible for the same activities as of the MIDO.

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Certificates of Airworthiness and Permits to Fly

Filippo De Florio , in Airworthiness (Third Edition), 2016

8.2.2.3 The approval of the flight conditions

Flight conditions include: (21.A.708)

(a)

the configuration(s) for which the permit to fly is requested;

(b)

any condition or restriction necessary for safe operation of the aircraft, including:

1.

the conditions or restrictions put on itineraries or airspace, or both, required for the flight(s);

2.

the conditions and restrictions put on the flight crew to fly the aircraft;

3.

the restrictions regarding carriage of persons other than flight crew;

4.

the operating limitations, specific procedures, or technical conditions to be met;

5.

the specific flight test programme (if applicable);

6.

the specific continuing airworthiness arrangements including maintenance instructions and regime under which they will be performed;

(c)

the substantiation that the aircraft is capable of safe flight under the conditions or restrictions of point (b);

(d)

the method used for the control of the aircraft configuration, in order to remain within the established conditions.

NOTE: The GMs to Part 21.A.708 (of May 2013) provide useful information about the interpretation of this complex and important point. Particularly interesting is the GM No. 3 to 21.A.708(c) Operation of Overweight Aircraft.

According to point 21. A.710:

(a)

When approval of the flight conditions is related to the safety of the design, the flight conditions shall be approved by:

1.

the Agency; or

2.

an appropriately approved design organisation, under the privilege of point 21.A.263(c)(6).

(b)

When approval of the flight conditions is not related to the safety of the design, the flight conditions shall be approved by the competent authority, or the appropriately approved organisation that will also issue the permit to fly.

(c)

Before approving the flight conditions, the Agency, the competent authority, or the approved organisation must be satisfied that the aircraft is capable of safe flight under the specified conditions and restrictions. The Agency or the competent authority may make or require the applicant to make any necessary inspections or tests for that purpose.

The GM 21.A.710 explains:

1.

The approval of flight conditions is related to the safety of the design, when:

a)

the aircraft does not conform to an approved design; or

b)

an Airworthiness Limitation, a Certification Maintenance Requirement, or an Airworthiness Directive has not been complied with; or

c)

the intended flight(s) are outside the approved envelope

d)

the permit to fly is issued for the purpose of 21.A.701(a)(15).

2.

Examples when the approval of flight conditions is not related to the safety of the design are:

a)

production flight testing for the purpose of conformity establishment;

b)

delivery/export flight of a new aircraft the design of which is approved;

c)

demonstrating continuing conformity with the standard previously accepted by the Agency for the aircraft or type of aircraft to qualify or requalify for a (restricted) certificate of airworthiness.

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Airworthiness Requirements

Filippo De Florio , in Airworthiness (Third Edition), 2016

4.4 European Aviation Safety Agency Regulations

In Chapter 3, we have mentioned the various steps of the transfer of competencies to the EASA through a series of Regulations and Commission Regulations, up to a Basic Regulation Structure covering all the present areas of responsibility.

In the EASA system, there are three main levels of Regulatory material:

1.

The Basic Regulation itself, adopted by the European Parliament and the Council, binding in all its elements.

2.

Implementing Rules to the Basic Regulation, adopted by the European Commission.

3.

Soft law, adopted by EASA.

The Basic regulation empowers EASA to develop three different types of soft law: Certification Specifications (CS), Acceptable Means of Compliance (AMC), and Guidance Material (GM).

For the scope of this book, we recall the following:

4.4.1 Basic Regulation

Regulation (EC) No 216/2008 of 20 February 2008

1.

This Regulation is applied to the following:

(a)

the design, production, maintenance, and operation of aeronautical products, parts, and appliances, as well as personnel and organisations involved in the design, production, and maintenance of such products, parts, and appliances;

(b)

personnel and organisations involved in the operation of aircraft.

2.

'This Regulation shall not apply when products, parts, appliances, personnel and organisations referred to in point 1 are engaged in military, customs, police, or similar services. The Member States shall undertake to ensure that such services have due regard as far as practicable to the objectives of this Regulation'.

4.4.1.1 Initial airworthiness

Commission Regulation (EU) 8 No 748/2012 of 3 August 2012 describing implementing rules for the airworthiness and environmental certification of aircraft, and Annex1 to this document is Part 21, 'Certification of aircraft and related products, parts and appliances, and design and production organizations'.

4.4.1.2 Continuing airworthiness

Commission Regulation (EU) No 1321/2014 of 26 November 2014 on the continuing airworthiness of aircraft and aeronautical products, parts, and appliances, and on the approval of organisations and personnel involved in these tasks.

This Commission Regulation, dealing with 'Continuing Airworthiness' contains an update of the:

1.

Part M: Continuing Airworthiness Requirement (Annex I)

2.

Part 145: Maintenance Organizations Approval (Annex II)

3.

Part 66: Certifying Staff (Annex III)

4.

Part 147: Training Organizations Requirements (Annex IV)

NOTE: These regulations have been amended by the Commission Regulation (EU) 2015/1088 of 3 July 2015 containing alleviations for maintenance procedures for general aviation aircraft.

4.4.1.3 Air operations 9

Through the:

Commission Regulation (EU) No 965/2012 of 5 October 2012

Commission Regulation (EU) No 800/2013 of 14 August 2013

Commission Regulation (EU) No 379/2014 of 7 April 2014

the following Annexes were issued:

1.

Definitions

for terms used in Annexes II–VIII (Annex I)

2.

Part-ARO

Authority Requirements for Air Operations (Annex II)

3.

Part-ORO

Organisation Requirements for Air Operations (Annex III)

4.

Part-CAT

Commercial Air Transport Operations (Annex IV)

5.

Part-SPA

Specific Approvals (Annex V)

6.

Part-NCC

Non-Commercial Air Operations With Complex Motor-Powered Aircraft (Annex VI)

7.

Part-NCO

Non-Commercial Air Operations With Other-Than-Complex Motor-Powered Aircraft (Annex VII)

8.

Part-SPO

Specialised Operations (Annex VIII)

NOTE: See in Chapter 9, paragraph 9.3 'EASA operational standards'.

Fig. 3.2 in Chapter 3 gives an overall table of the Basic Regulation.

4.4.1.4 Advisory material

The AMCs are nonbinding standards adopted by EASA to illustrate means to establish compliance with the Basic Regulation and its Implementing Rules.

The AMCs issued by EASA are not of a legislative nature. They cannot create additional obligations on the regulated persons, who may decide to show compliance with the applicable requirements using other means.

However, as the legislator aimed for such material to provide for legal certainty and to contribute to uniform implementation, it provided the AMC adopted by EASA with a presumption of compliance with the rules, so that it commits competent authorities to recognise regulated persons complying with an EASA AMC as complying with the law.

4.4.2 List of EASA Requirements Directly or Indirectly Related to Airworthiness Certification

4.4.2.1 Initial airworthiness

PART 21. Implementing rules for the airworthiness and environmental certification of aircraft and related products, parts, and appliances, as well as for the certification of design and production organisations.

See relevant paragraph in this chapter (4.5.4.2).

CERTIFICATION SPECIFICATIONS. 10

CS-Definitions

Definitions and abbreviations used in CS for products, parts, and appliances.

CS-22: Sailplanes and Powered Sailplanes

See relevant paragraph in this chapter (4.5.6.1).

CS-23: Normal, Utility, Acrobatic, and Commuter Aeroplanes

See relevant paragraph in this chapter (4.5.6.3).

CS-25: Large Aeroplanes

See relevant paragraph in this chapter (4.5.6.4).

CS-27: Small Rotorcraft

See relevant paragraph in this chapter (4.5.6.5).

CS-29: Large Rotorcraft

See relevant paragraph in this chapter (4.5.6.6).

CS-VLR: Very Light Rotorcraft

See relevant paragraph in this chapter (4.5.6.7).

CS-VLA: Very Light Aeroplanes

See relevant paragraph in this chapter (4.5.6.2).

CS-E: Engines

This part prescribes airworthiness standards for the issue of type certificates for aircraft engines and changes to those certificates. Subparts B and C deal specifically with piston aircraft engines, and Subparts D and E deal specifically with turbine aircraft engines. Subpart F deals with Environmental and Operational Design.

CS-P: Propellers

The CS-P contains airworthiness specifications for the issue of type certificates and changes to those certificates, for propellers, in accordance with Part 21.

The applicant is eligible for a propeller type certificate when compliance with subparts A, B, and C has been demonstrated. If the additional compliance with subpart D 11 has not also been shown, this must be stated in the propeller type certificate data sheet.

CS-34: Aircraft Engine Emission and Fuel Venting

See relevant paragraph in this chapter (4.5.6.9).

CS-36: Aircraft Noise

See relevant paragraph in this chapter (4.5.6.10).

CS-APU: Auxiliary Power Units

See relevant paragraph in this chapter (4.5.6.11).

CS-ETSO: European Technical Standard Orders

See relevant paragraph in Chapter 5 (5.3.2.2).

CS-31 HB: Hot Air Balloons

See relevant paragraph in this chapter (4.5.6.8).

4.4.2.2 Continuing airworthiness

See list of pertinent regulations in 4.4.1.2.

Single regulations are discussed in Chapter 10.

4.4.2.3 Air operations

See list of pertinent regulations in 4.4.1.3.

Single regulations are discussed in Chapter 9.

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Introduction and overview

Chun H. Wang , Cong N. Duong , in Bonded Joints and Repairs to Composite Airframe Structures, 2016

1.2 Criticality of Structure and Damage

Aircraft structures are generally classified as follows in terms of criticality of the structure:

•

critical structure, whose integrity is essential in maintaining the overall flight safety of the aircraft (e.g., principal structural elements in transport category aircraft);

•

primary structure carries flight, ground, or pressurization loads, and whose failure would reduce the aircraft's structural integrity;

•

secondary structure that, if it was to fail, would affect the operation of the aircraft but not lead to its loss; and

•

tertiary structure, in which failure would not significantly affect operation of the aircraft.

Inspection, damage assessment, and repair requirements differ significantly between these classifications. However, even within a single component, the allowable damage type and size (and consequently acceptable repair actions) will vary according to the criticality of the damaged region. The original equipment manufacturer (OEM) generally zones an aircraft component in terms of these regions, and specifies repair limits and the pertinent repair procedures in the structural repair manual (SRM). Damages outside the scope of the SRM, particularly to critical regions of primary structure, require engineering design disposition and approval by the OEM (or its delegate); this book describes some new design options demonstrated by recent research results.

Foreign object impact is usually the main type of damage concerning composite aircraft structures. To ensure continuing airworthiness, it is necessary to identify damage severity and detectability as part of the ongoing maintenance process. Current airworthiness regulations ( FAA, 2010) classify various damage types into five categories, as indicated in Figure 1.2 that illustrates the relationship between design strength and damage size:

Figure 1.2. Allowable strength versus damage size.
•

Category 1: Allowable damage or allowable manufacturing defects that do not degrade structural integrity, and hence may go undetected by scheduled inspections. Structures containing this type of damage are capable of sustaining the ultimate load for the life of the aircraft structure. Some examples include barely visible impact damage (BVID), small delamination, porosity, small scratches, and so forth. No repairs are needed.

•

Category 2: Damage that can be reliably detected at scheduled inspection intervals. This type of damage should not grow or, if slow or arrested growth occurs, the residual strength of the damaged structure during the inspection internal is sufficiently above the limit load capability. Some examples include visible impact damage, deep gouges or debonding, and major local overheating damage. Repairs are needed to restore the design ultimate load capability.

•

Category 3: Damage that can be readily detected, within a few flights, by operations or maintenance personnel without special skills in composite inspection. The structure can still maintain limit or near limit load capability. Repairs are required immediately to restore design ultimate load capability.

•

Category 4: Discrete source damage that will reduce the structural strength to below the design limit load such that flight maneuvers become limited (i.e., structure can maintain safe flight at reduced levels). Examples include rotor burst, bird strikes, tire burst, and severe in-flight hail. Repairs are needed immediately after flight.

•

Category 5: Severe damage outside design but is self-evident and known to operations, such as anomalous ground collision with service vehicles, flight overload conditions, abnormally hard landings, and so forth. Immediate repair is required.

Analytical methods for assessing the residual strength of damaged composite components are needed to ensure that only necessarily required repairs are undertaken. Essentially, one of the following decisions must be made:

•

No repair action—damage is negligible.

•

Only needed correction is cosmetic or sealing repair because damage is minor.

•

Structural repair is required (if feasible) because strength is reduced below ultimate design allowable, or has the potential to be reduced in subsequent service.

•

Replacement is required as repair is not economically or technically feasible and component must be replaced.

For BVID, quite large areas of damage (typically 25   mm diameter) can be tolerated for older generation carbon/epoxy systems (and brittle high-temperature systems) without failures occurring below the ultimate design strain allowable, generally around 5000 microstrain for quasi-isotropic laminates made of unidirectional (tape) lamina. Recently, advanced computational modeling techniques have been shown to be able to accurately predict the residual strength of composite laminates containing holes of various sizes and shapes (Wang et al., 2011a; Ridha et al., 2014). Thus, the residual strength assessment of a structure following impact damage can be performed similarly by using these advanced computational methods.

Fatigue studies have also shown that BVID will not grow under realistic cyclic strain levels for typical carbon/epoxy laminates. This is an important point because BVID will often not be detected until a 100% nondestructive inspection is undertaken. Even though there is a possibility of damage growth and residual strength degradation under hygrothermal cycling conditions, this appears to be a serious concern only under severe cycling conditions. This possibly catastrophic flaw growth under severe hygrothermal cycling may result from expansion of entrapped moisture due to freezing or steam formation on heating during supersonic flight.

For safety-critical structures, coupons, structural details, elements, and subcomponents are required to be tested under fatigue loading to determine the sensitivity of structure to damage growth and to demonstrate their compliance with either no-growth or slow-growth requirements. This is to ensure that a damaged structure should not be exposed to an excessive period of time when its residual strength is less than the ultimate. Once the damage (greater than the allowable damage size under category 1) is detected, the component is either repaired to restore ultimate load capability or replaced.

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https://www.sciencedirect.com/science/article/pii/B9780124171534000013