ED Decision 2003/1/RM Final 17/10/2003 European Aviation Safety Agency The Executive Director DECISION NO. 2003/1/RM OF THE EXECUTIVE DIRECTOR OF THE AGENCY of 17 October 2003 on acceptable means of compliance and guidance material 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 (“AMC and GM to Part 21”) THE EXECUTIVE DIRECTOR OF THE EUROPEAN AVIATION SAFETY AGENCY, Having regard to Regulation (EC) No 1592/2002 of 15 July 2002 on common rules in the field of civil aviation and establishing a European Aviation Safety Agency1 (hereinafter referred to as the “Basic Regulation”), and in particular Article 13 thereof. Having regard to the Commission Regulation (EC) No 1702/2003 of 24 September 2003 laying down implementing rules for the airworthiness and environmental certification of aircraft and related products, parts and appliances,2 as well as for the certification of design and production organisations. Whereas: (1) The Agency should issue certification specifications, including airworthiness codes and acceptable means of compliance, as well as any guidance material for the application of the Basic Regulation and its implementing rules. (2) The Agency has, pursuant to Article 43 of the Basic Regulation, consulted widely interested parties on the matters which are subject to this Decision and following that consultation provided a written response to the comments received, 1 OJ L 240, 7.09.2002, p. 1. 2 OJ L 243, 27.09.2003, p. 6. ED Decision 2003/1/RM Final 17/10/2003 HAS DECIDED AS FOLLOWS: Article 1 The acceptable means of compliance and guidance material to be used in the airworthiness certification of products, parts and appliances and the approval of organisations involved in their design or manufacture are those laid down in the Annex to this Decision, unless otherwise provided in certification specifications. Article 2 This Decision shall enter into force on 17 October 2003. It shall be published in the Official Publication of the Agency. Done at Brussels, 17 October 2003. For the European Aviation Safety Agency, Patrick GOUDOU Executive Director European Aviation Safety Agency AMC and GM to Part 21 Acceptable Means of Compliance and Guidance Material 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 PART 21 Acceptable Means of Compliance and Guidance Material TABLE OF CONTENTS TERMINOLOGY...........................................................................................................................................6 SECTION A..................................................................................................................................................7 Subpart A - General....................................................................................................................................7 GM 21A.3(a) The system for collection, investigation and analysis of data.............................................7 GM 21A.3(b) Occurrence reporting...........................................................................................................7 AMC 21A.3(b)(2) Reporting to the Agency...............................................................................................7 GM 21A.3B(d)(4) Defect correction – Sufficiency of proposed corrective action.....................................7 AMC 21A.3B(b) Unsafe condition.........................................................................................................18 GM 21A.3B(b) Determination of an unsafe condition.............................................................................18 AMC 21A.4 Transferring of information on eligibility and approval status from the design holder to production organisations.........................................................................................................................23 Subpart B – Type-certificates..................................................................................................................25 GM 21A.14(b) Eligibility for alternative procedures................................................................................25 AMC 21A.14(b) Alternative Procedures..................................................................................................25 GM 21A.16B Special Conditions.............................................................................................................30 GM 21A.33 Investigation and Tests........................................................................................................30 GM 21A.35 Flight Tests..........................................................................................................................30 GM 21A.35(b)(2) Objective and Content of Function and Reliability Testing.........................................30 GM 21A.35(f)(1) Flying Time for Function and Reliability Testing..........................................................31 GM 21A.35(f)(2) Flying Time for Function and Reliability Testing..........................................................31 (Subpart C – Not applicable)...................................................................................................................32 Subpart D – Changes to type-certificates..............................................................................................33 GM 21A.91 Classification of changes to a type design..........................................................................33 GM 21A.93(b) Major Changes: Application............................................................................................40 GM 21A.101 Establishment of the type-certification basis of Changed Aeronautical Products.............40 Appendix 1 to GM 21A.101 CLASSIFICATION OF CHANGES.............................................................51 Appendix 2 to GM 21A.101 PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING LATEST REQUIREMENTS TO A CHANGED PRODUCT.....................................................................81 Appendix 3 to GM 21A.101 THE USE OF SERVICE EXPERIENCE IN THE CERTIFICATION PROCESS...............................................................................................................................................89 Subpart E – Supplemental type-certificates..........................................................................................92 GM 21A.112B Demonstration of capability for supplemental type-certificate cases..............................92 Subpart F – Production without production organisation approval...................................................96 GM No. 1 to 21A.121 Applicability - Individual product, part or appliance..............................................96 GM No. 2 to 21A.121 Applicability – Applicable design data..................................................................96 AMC No. 1 to 21A.122 Eligibility – Link between design and production...............................................96 AMC No. 2 to 21A.122 Eligibility – Link between design and production..........................................97 GM 21A.124(a) Application – Application form.......................................................................................99 GM 21A.124(b)(1)(i) Applicability - Inappropriate approval under Subpart G.........................................99 GM 21A.124(b)(1)(ii) Certification or approval needed in advance of the issue of a POA...................100 GM 21A.124(b)(2) Application - Minimum information to include with the application.........................100 GM No. 1 to 21A.125 Letter of agreement - Meaning of individual......................................................101 GM No. 1 to 21A.125(b) Letter of agreement - Contents of the Manual..............................................101 GM No. 2 to 21A.125(b) Letter of agreement - Production Inspection System: Functional Tests........102 GM 21A.125(c) Letter of agreement - Assistance................................................................................102 GM No. 1 to 21A.125B(a) Uncontrolled non-compliance with applicable design data.........................102 GM No. 2 to 21A.125B(a) Examples for level one findings..................................................................102 GM 21A.126 Production Inspection System.........................................................................................102 GM 21A.126(a)(1) Production Inspection System – Conformity of supplied parts, appliances and material.................................................................................................................................................103 GM 21A.126(a)(2) Production Inspection System - Identification of incoming materials and parts.....103 GM No. 1 to 21A.126(a)(3) Production Inspection System - List of specifications...............................104 GM No. 2 to 21A.126(a)(3) Production Inspection System - Means of checking of the production processes..............................................................................................................................................104 GM 21A.126(a)(4) Production Inspection System – Applicable design/production data procedures..104 2 Acceptable Means of Compliance and Guidance Material for PART 21 TABLE OF CONTENTS GM 21A.126(b)(1) Production Inspection System - Inspection of parts in process..............................105 GM 21A.126(b)(2) Production Inspection System – Suitable storage and protection..........................105 GM 21A.126(b)(3) Production Inspection System – Use of derived data instead of original design data ..............................................................................................................................................................105 GM 21A.126(b)(4) Production Inspection System – Segregation of rejected material.........................105 GM 21A.126(b)(5) Production Inspection System – Engineering and manufacturing review procedure ..............................................................................................................................................................106 GM 21A.126(b)(6) Production Inspection System – Recording and record keeping............................106 GM 21A.127 Approved production ground and flight tests...................................................................107 GM No. 1 to 21A.128 Acceptable functional test - Engines..................................................................107 GM No. 2 to 21A.128 Acceptable functional test –Variable pitch propellers........................................107 GM No. 3 to 21A.128 Acceptable functional test - Engines and Propellers.........................................107 GM 21A.129(a) Availability for inspection by the Competent Authority................................................107 AMC No. 1 to 21A.129(c) Obligations of the manufacturer – Conformity of prototype models and test specimens.............................................................................................................................................108 AMC No. 2 to 21A.129(c) Obligations of the manufacturer – Conformity with Applicable Design Data ..............................................................................................................................................................108 AMC No. 3 to 21A.129(c) Obligations of the manufacturer – Condition for safe operation..................108 AMC No. 1 to 21A.130(b) Statement of Conformity for Complete Aircraft...........................................109 AMC No. 2 to 21A.130(b) Statement of Conformity for Products (other than complete aircraft), parts, appliances and materials - The Authorised Release Certificate (EASA Form 1).................................112 Subpart G – Production organisation approval for products, parts and appliances......................120 GM 21A.131 Scope – Applicable design data...................................................................................120 GM 21A.133(a) Eligibility – Approval appropriate for showing conformity............................................120 AMC No. 1 to 21A.133(b) and (c) Eligibility – Link between design and production organisations......121 AMC No. 2 to 21A.133(b) and (c) Eligibility – Link between design and production organisations122 GM 21A.134 Application – Application form and manner.....................................................................124 GM No. 1 to 21A.139(a) Quality System...............................................................................................124 GM No. 2 to 21A.139(a) Quality System – Conformity of supplied parts or appliances.......................125 GM 21A.139(b)(1) Quality System – Elements of the quality system...................................................126 GM No. 1 to 21A.139(b)(2) Quality System – Independent quality assurance function.......................126 GM No. 2 to 21A.139(b)(2) Quality System – Adequacy of procedures and monitoring function........126 GM 21A.143 Exposition – Production organisation exposition.............................................................127 GM 21A.145(a) Approval Requirements...............................................................................................127 GM 21A.145(b)(2) Approval Requirements – Airworthiness, noise, fuel venting and exhaust emissions /production data procedures.................................................................................................................128 GM 21A.145(c)(1) Approval Requirements – Accountable manager...................................................128 GM 21A.145(c)(2) Approval Requirements – Responsible managers.................................................128 AMC 21A.145(d)(1) Approval Requirements – Certifying staff.............................................................129 AMC 21A.145(d)(2) Approval Requirements – Record of certifying staff.............................................129 AMC 21A.145(d)(3) Approval requirements – Evidence of authorisation.............................................130 GM 21A.147(a) Changes to the approved production organisation – Significant changes..................130 AMC 21A.148 Changes of location – Management during change of location....................................131 GM 21A.149 Transferability..................................................................................................................132 GM 21A.151 Terms of approval – Scope and categories.....................................................................132 AMC 21A.153 Changes to the terms of approval – Application for a change to the terms of approval134 GM 21A.157 Investigations – Arrangements........................................................................................134 GM No. 1 to 21A.158(a) Uncontrolled non-compliance with applicable design data...........................134 GM No. 2 to 21A.158(a) Examples of level one findings......................................................................135 GM 21A.159(a)(3) Evidence of a lack of satisfactory control..........................................................135 AMC 21A.163(c) Computer generated signature.................................................................................135 AMC 21A.163(d) Privileges – Maintenance..........................................................................................136 GM 21A.165(a) Obligations of the holder – Basic working document..................................................137 GM No. 1 to 21A.165(c) Obligations of the holder – Conformity of prototype models and test specimens............................................................................................................................................137 GM No. 2 to 21A.165(c) Obligations of holder – Conformity with type design................................137 GM No. 3 to 21A.165(c) Obligations of the holder – Condition for safe operation...............................137 GM No. 4 to 21.165(c) Airworthiness Release or Conformity Certificate.............................................139 GM 21A.165(d) and(h) Obligations of the holder – Recording and archiving system.....................140 Subpart H – Airworthiness certificates................................................................................................141 Subpart I – Noise certificates................................................................................................................142 Subpart J – Design organisation approval..........................................................................................143 3 Acceptable Means of Compliance and Guidance Material for PART 21 TABLE OF CONTENTS GM No. 1 to 21A.239(a) Design assurance system.............................................................................143 GM No. 2 to 21A.239(a) Design assurance system for minor changes to type design or minor repairs to products................................................................................................................................................147 AMC 21A.239(a)(3) Design assurance system - Independent system monitoring..............................148 AMC 21A.239(b) Design assurance system - Independent checking function of the showing of compliance............................................................................................................................................148 GM 21A.239(c) Design assurance system...........................................................................................148 AMC No. 1 to 21A.243(a) Data requirements.......................................................................................148 AMC No. 2 to 21A.243(a) Data requirements - Model content of handbook for organisations designing minor changes to type design or minor repairs to products..................................................................150 GM No. 1 to 21A.243(d) Statement of qualifications and experience...................................................151 GM No. 2 to 21A.243(d) Data requirements - Statement of the qualification and experience- Organisations designing minor changes to type design or minor repairs to products..........................153 GM No. 1 to 21A.245 Requirements for approval.................................................................................153 GM No. 2 to 21A.245 Requirements for approval - Organisations designing minor changes to type design or minor repairs to products......................................................................................................154 GM 21A.247 Significant changes in the design assurance system......................................................154 GM 21A.249 Transferability..................................................................................................................156 GM No. 1 to 21A.251 Terms of approval..............................................................................................156 GM No. 2 to 21A.251 Terms of approval - Organisations designing minor changes to type design or minor repairs to products......................................................................................................................156 GM 21A.257(a) Investigations..............................................................................................................157 GM 21A.263(b) DOA privilege related to compliance documents........................................................157 AMC No. 1 to 21A.263(c)(1) Procedure for the classification of changes to type design and repairs as minor and major....................................................................................................................................157 AMC No. 2 to 21A.263(c)(1) Privileges - Organisations designing minor changes to type design or minor repairs to products : classification procedure.............................................................................159 AMC No. 1 to 21A.263(c)(2) Procedure for the approval of minor changes to type design or minor repairs...................................................................................................................................................160 AMC No. 2 to 21A.263(c)(2) Privileges - Organisations designing minor changes to type design or minor repairs to products : procedure for the approval of minor changes to type design or minor repairs ..............................................................................................................................................................161 GM 21A.263(c)(3) Issue of information or instructions.........................................................................162 GM 21A.263(c)(4) Procedure for the approval of documentary changes to the Aircraft Flight Manual163 AMC 21A. 265(a) Administration of the Handbook...............................................................................165 GM 21A.265(b) Use of the Handbook...................................................................................................166 Subpart K – Parts and appliances........................................................................................................167 GM No. 1 to 21A.303(c) Standard Parts..............................................................................................167 GM No. 2 to 21A.303(c) Officially recognised Standards.....................................................................167 GM 21A.307 Release of Parts and Appliances for Installation............................................................167 (Subpart L – Not applicable)..................................................................................................................168 Subpart M - Repairs................................................................................................................................169 GM 21A.431(a) Scope..........................................................................................................................169 GM 21A.431(d) Repairs to articles.......................................................................................................169 AMC 21A.433 (a) and 21A.447 Repair design and Record Keeping...................................................169 GM 21A.435(a) Classification of repairs...............................................................................................170 GM 21A.437 Issue of repair design approval.......................................................................................172 GM 21A.437(a) Issue of repair design approval...................................................................................172 AMC 21A.437(b) Issue of repair design approval................................................................................173 GM 21A.439 Production of repair parts...............................................................................................173 GM 21A.441 Repair Embodiment.........................................................................................................174 GM 21A.443 Limitations........................................................................................................................174 GM 21A.445 Unrepaired damage....................................................................................................174 (Subpart N – Not applicable).................................................................................................................176 Subpart O – European Technical Standard Order Authorisations....................................................177 AMC 21A.602B(b)(2) Procedures for ETSO authorisations.................................................................177 AMC 21A.608 Declaration of Design and Performance....................................................................178 (Subpart P – Not applicable)..................................................................................................................180 Subpart Q – Identification of products, parts and appliances...........................................................181 SECTION B..............................................................................................................................................182 Subpart A – General...............................................................................................................................182 GM 21B.20 Responsibility for implementation......................................................................................182 4 Acceptable Means of Compliance and Guidance Material for PART 21 TABLE OF CONTENTS GM 21B.25(a) Organisation..................................................................................................................182 GM 21B.25(b) Resources.....................................................................................................................183 GM 21B.25(c) Qualification and training...............................................................................................184 AMC 21B.30(a) Documented procedures.............................................................................................184 AMC 21B.35(a) Changes......................................................................................................................185 GM 21B.40 Principles for the resolution of disputes.............................................................................185 GM No. 1 to 21B.45 Co-ordination with other related activities............................................................185 GM No. 2 to 21B.45 Co-ordination........................................................................................................186 GM No. 3 to 21B.45 Reporting - Information relevant to registers established by the Agency............186 GM No. 1 to 21B.50 Standardisation findings by the Agency...............................................................186 GM No. 2 to 21B.50 Standardisation - Means established by the Agency...........................................186 GM 21B.55 Record keeping for design approvals transferred to the Agency......................................186 Subpart B – Type-certificates and restricted type-certificates..........................................................189 (Subpart C – Not applicable).................................................................................................................190 Subpart D – Changes to type-certificates and restricted type-certificates......................................191 Subpart E – Supplemental type-certificates........................................................................................192 Subpart F - Production without production organisation approval..................................................193 AMC 21B.120(a) Investigation team - Qualification criteria for the investigation team members........193 AMC 21B.120(c)(1) Evaluation of applications.....................................................................................193 GM 21B.120(c)(3) Investigation preparation and planning...................................................................195 GM 21B.120(c)(5) and (6) Auditing and investigation findings.............................................................195 AMC 21B.130 Issue of the letter of agreement.....................................................................................196 GM 21B.130(b) Issue of the letter of agreement..................................................................................196 AMC 21B.140 Amendment of a letter of agreement.............................................................................196 GM 21B.143(a) Objective evidence......................................................................................................196 GM 21B.150(d) Record keeping - Traceability of release certificates..................................................196 Subpart G- Production organisation approval....................................................................................197 GM 21B.220(a) Investigation team.......................................................................................................197 AMC 21B. 220(c) Procedures for investigation - Evaluation of applications........................................197 GM No. 1 to 21B.220(c) Procedures for investigation - Investigation preparation and planning.........199 GM No. 2 to 21B.220(c) Procedures for investigation – General.........................................................200 GM No. 3 to 21B.220(c) Procedures for investigation - POA applications received from organisations with facilities/partners/suppliers/subcontractors located in a third country...........................................216 GM No. 4 to 21B.220(c) Procedures for investigation – Competent Authority surveillance of suppliers of a POA holder located in other Member States.....................................................................................217 GM 21B.225(a) Objective evidence......................................................................................................223 AMC 21B.225(a) Notification of findings..............................................................................................223 AMC No. 1 to 21B.230 Issue of the certificate......................................................................................223 AMC No. 2 to 21B.230 Approval reference number............................................................................223 GM 21B.235(a)(4) Guide to the conduct of monitoring production standards......................................224 GM 21B.235(b) Maintenance of the POA - Work allocation within the Competent Authority...............225 GM 21B.235(b) and (c) Continued surveillance....................................................................................225 AMC 21B.235(c) Continuation of POA..................................................................................................225 AMC No. 1 to 21B.240 Application for significant changes or variation of scope and terms of the POA ..............................................................................................................................................................225 GM 21B.245 Continued validity............................................................................................................227 AMC 21B.245 Corrective action plan....................................................................................................227 Subpart H - Airworthiness Certificates.................................................................................................228 GM 21B.320(b)(6) Investigation............................................................................................................228 GM 21B.325(a) Airworthiness Certificates............................................................................................228 GM 21B.325(b) Completion of the Airworthiness Review Certificate by a Member State....................228 Subpart I - Noise certificates.................................................................................................................229 GM 21B.425(a) Noise Certificates........................................................................................................229 Subpart J – Design organisation approval..........................................................................................230 Subpart K – Parts and appliances........................................................................................................231 (Subpart L – Not applicable)..................................................................................................................232 Subpart M – Repairs...............................................................................................................................233 (Subpart N – Not applicable).................................................................................................................234 Subpart O – European Technical Standard Order authorisations....................................................235 (Subpart P – Not applicable)..................................................................................................................236 Subpart Q – Identification of products, parts and appliances...........................................................237 5 AMC & GM for PART 21 TERMINOLOGY For information purposes: The “Basic Regulation” means Regulation (EC) No 1592/2002 of 15 July 2002. “Certification Specifications” (CS) refers when used in the text to the airworthiness codes and associated acceptable means of compliance developed by the Agency in accordance with Articles 13(b) and 14.2(a) of the Basic Regulation. “Acceptable Means of Compliance” (AMC) illustrate a means, but not the only means, by which a specification contained in an airworthiness code or a requirement in an implementing rule can be met. “Guidance Material” (GM) helps to illustrate the meaning of a specification or requirement. “Competent Authority” should be understood in accordance with 21.1 (see Part 21). 6 SECTION A/Subpart A AMC & GM for PART 21 SECTION A Subpart A - General GM 21A.3(a) The system for collection, investigation and analysis of data In the context of that requirement the word “Collection” means, the setting up, of systems and procedures which will enable relevant malfunctions, failures and defects to be properly reported when they occur. GM 21A.3(b) Occurrence reporting For occurrence reporting, refer to AMC 20-8, in AMC 20. AMC 21A.3(b)(2) Reporting to the Agency Within the overall limit of 72 hours the degree of urgency for submission of a report should be determined by the level of hazard judged to have resulted from the occurrence. Where an occurrence is judged by the person identifying the possible unsafe condition to have resulted in an immediate and particularly significant hazard the Agency (or the competent authority of the Member State as required) expects to be advised immediately and by the fastest possible means (telephone, fax, email, telex, etc.) of whatever details are available at that time. This initial report must be followed up by a full written report within 72 hours. A typical example would be an uncontained engine failure resulting in damage to aircraft primary structure. Where the occurrence is judged to have resulted in a less immediate and less significant hazard, report submission may be delayed up to the maximum of three days in order to provide more details. GM 21A.3B(d)(4) Defect correction – Sufficiency of proposed corrective action This GM provides guidelines to assist in establishing rectification campaigns to remedy discovered defects. 1. STATUS This document contains GM of a general nature for use in conjunction with engineering judgement, to aid airworthiness engineers in reaching decisions in the state of technology at the material time. While the main principles of this GM could be applied to small private aeroplanes, helicopters, etc. the numerical values chosen for illustration are appropriate to large aeroplanes for public transport. 2. INTRODUCTION 2.1 Over the years, target airworthiness risk levels underlying airworthiness requirements have developed on the basis of traditional qualitative airworthiness approaches; they have been given more precision in recent years by being compared with achieved airworthiness levels (judged 7 SECTION A/Subpart A AMC & GM for PART 21 from accident statistics) and by the general deliberations and discussions which accompanied the introduction of rational performance requirements, and more recently, the Safety Assessment approach in requirements. Although the target airworthiness risk level tends to be discussed as a single figure (a fatal accident rate for airworthiness reasons of not more than 1 in 10,000,000 flights/flying hours for large aeroplanes) it has to be recognised that the requirements when applied to particular aircraft types will result in achieved airworthiness levels at certification lying within a band around the target level and that thereafter, for particular aircraft types and for particular aircraft, the achieved level will vary within that band from time to time. 2.2 The achieved airworthiness risk levels can vary so as to be below the target levels, because it is difficult if not impossible to design to the minimum requirements without being in excess of requirements in many areas; also because aircraft are not always operated at the critical conditions (e.g., aircraft weight, cg position and operational speeds; environmental conditions - temperature, humidity, degree of turbulence). The achieved level may vary so as to be above the target level because of undetected variations in material standards or build standards, because of design deficiencies, because of encountering unforeseen combinations of failures and/or combinations of events, and because of unanticipated operating conditions or environmental conditions. 2.3 There is now a recognition of the need to attempt to monitor the conditions which tend to increase the level and to take appropriate corrective action when the monitoring indicates the need to do so in order to prevent the level rising above a predetermined "ceiling". 2.4 The Agency also has a duty in terms of providing the public with aviation services and therefore should consider the penalties associated with curtailment or even removal (by "grounding") of aviation services when establishing the acceptability of any potential variation in airworthiness level. 2.5 Thus, the purpose of this GM is: (a) To postulate basic principles which should be used to guide the course of actions to be followed so as to maintain an adequate level of airworthiness risk after a defect has occurred which, if uncorrected, would involve a potential significant increase of the level of risk for an aircraft type. (b) For those cases where it is not possible fully and immediately to restore an adequate level of airworthiness risk by any possible alleviating action such as an inspection or limitation, to state the criteria which should be used in order to asses the residual increase in risk and to limit it to an appropriate small fraction of the mean airworthiness through life risk. 3. DISCUSSION 3.1 Several parameters are involved in decisions on safety matters. In the past the cost of proposed action has often been compared with the notional 'risk cost', i.e. the cost of a catastrophe multiplied by its probability of occurrence. 3.2 This can be a useful exercise, but it should be held within the constraint of acceptable airworthiness risk levels, i.e., within airworthiness risk targets which represent the maximum levels of risk with which an aircraft design must comply, i.e., in the upper part of the 'band'. Currently for large aeroplanes the mean airworthiness risk level is set at a catastrophe rate for airworthiness reasons of not more than one in every ten- million flights/flying hours. The constraint is overriding in that any option, which could be permitted on risk cost considerations, or other grounds, is unacceptable if it leads to significant long-term violation of this safety requirement. 3.3 While it should clearly be the objective of all to react to and eliminate emergency situations, i.e., those involving a potentially significant increase of airworthiness risk levels, without unreasonable delay, the Agency should be able finally to rule on what is a minimum acceptable 8