Lecture 6: Product Development and Design Continued

Chapter 7 - Common (Good) Practices, Chapter 8 - Instructions for Continued Airworthiness

Materials Data for Design

AR-MMPDS

• Metallic Material Properties Development and Standardization (MMPDS) Handbook

• Replacement document for MILHDBK-5

• Recognized internationally as a reliable source of aircraft materials data for aerospace materials selection and analysis

• Statistically based material and fastener allowable properties

• Only publicly available source that the Federal Aviation Administration generally accepts for:

  • Compliance with Federal Aviation Regulations (FARs) for material strength properties;

  • Design values for aircraft certification and continued airworthiness; and

  • Fastener joint allowables

MMPDS Acronym

Metallic Materials Properties Development and Standardization

Drawing Standards

Benefits

• Common look and feel

  • Drawing end-users always know where to find certain information

  • Different drawing creators will produce very similar drawings

    • ie guidance on views, dimensions, call-outs, naming, etc.

Drawing Practices

• Often developed internally based on a standard

• Helps designers identify most common drawing practices related to the company

• Customized look & feel for marketing

• Often identifies internal procedures to follow:

  • File management

  • Naming convention - titles and drawing numbers

  • Checks and approvals

  • Revision history and management

  • Archiving

  • Paper vs. electronic copies

  • Stamps and/or signatures

File Management

• Transport requirement for access to drawing files

  • 5 year rule

• Paper vs electronic pros and cons

• File management system - CAD integrated or Windows folders

  • Organization by project, application, aircraft?

  • Integrated revision control

Definition - Instructions for Continued Airworthiness (ICAs)

Instructions provided by the aeronautical product manufacturer or STC/PDA holder to ensure continued airworthiness.

ICAs are Maintenance Instructions that generally include:

• Product description

• Servicing

• Inspection

• Troubleshooting

• Repair instructions

• Also, the tasks, intervals, and limitations applicable to these actions

Link between ICAs and the Certification Process

• Approval of the ICAs is part of the certification process (establishment of the the maintenance schedule, procedures, and Airworthiness Limitations)

• The ICAs are changed throughout the aircraft’s life as the aircraft is changed and feedback provided from in-service phase

Standards Requiring ICAs

523.1529 (App G) - Normal, Utility, Aerobatic, Commuter

525.1529 (App H) - Transport Category

527.1529 (App A) - Normal (Rotorcraft)

529.1529 (App A) - Transport (Rotorcraft)

531.82 (App A) - Balloons

533.4 (App A) - Engines

535.4 (App A) - Propellers

Composition of ICAs

Engineering - What you have to do.

Maintenance - What you should be aware of.

Composition of ICAs - Engineering

• Airworthiness Limitations section

  • Mandatory replacement time intervals

  • Related structural and systems inspection procedures

• Structural Repair Manual

• Certification Maintenance Requirements

Composition of ICAs - Maintenance

• Description of product

• Maintenance schedule

• Removal/installation instructions

• Diagrams of structural access plates

• Details for special inspection techniques

• Information for protective treatment after inspection

• Data re: structural factors

• List of special tools

• Servicing information

Reference to ICAs

• Approved portions of the ICAs shall be identified in the aircraft/product TCDS.

• As a minimum, the TCDS will specifically identify the location of the Airworthiness Limitations.

Processes for Developing the Maintenance Schedule

• Engineering - System Safety/Functional Hazard Analysis

• Engineering - Structures - Damage Tolerance/Fatigue Evaluation

• Maintenance (Engineering) - Maintenance Review Board (MRB) process

• Maintenance Steering Group process

System Reliability 52_.1309 Elements

• Identification of faults/malfunctions (FHA)

• Determining effect on safety (SSA)

• Mitigation (Design, Operation, Maint.)

Outcome = System Reliability

System Reliability 52_.1309

Failure Conditions that reduce the capability of the aircraft or ability of the crew to cope with adverse conditions:

• Safety margin

• Crew workload

• Misleading indication

• Injuries

• Continued safe operation

• Intended mission

• Landing

Process

1.Functional Hazard Analysis (FHA)

• Preliminary step

• Considers minor, major, hazardous, and catastrophic failure conditions

• Will not identify all possible failure conditions (detailed A/C design not completed)

2.System Safety Analysis (SSA)

• No single failure shall result in a catastrophe, that is, the “fail-safe” design concept

• Identify dormant (hidden) failures and dispositions

• Certification Maintenance Requirements (CMRs)

CMRs Definition

• Mandatory periodic tasks, established during the certification of the aircraft as a limitation of the Type Certificate

• They are systems related tasks, designed to verify that certain failures have not occurred

• Limiting exposure time will have an effect on the overall failure probability of the system

CMR Development AC25-19

• System Safety Analysis identifies need for inspections in order to comply

• Required checks associated with Hazardous or Catastrophic Failure Conditions become Candidate CMRs (CCMRs)

• Each CCMR must be dispositioned through the Certification Maintenance Coordinating Committee (CMCC)

CMR Categorization

One Star

• Tasks and intervals are mandatory and cannot be changed or deleted without regulatory approval

Two Stars

• Intervals may be changed, tasks cannot be changed or deleted without regulatory approval

One star and two star CMRs are mandatory

CMR Examples

• Control Systems Checks: Operational check of Aileron Override Unit for jamming (eg every 20,000 FH)

• Actuator Inspections: Detailed inspection of Horizontal Stabilizer Actuator Gimbals or checking the actuator’s locking mechanism and torque limiter (eg every 7,500 FH)

• Fuselage/structure: Major repairs to pressurized fuselages or spar repairs/modifications must be reported and done per approved data

Definition - Safe Life (still in use)

• Structural elements have been evaluated to be able to withstand repeated loads of variable magnitude during its service life without detecting cracks

  • Some parts are not inspectable

  • Some parts have critical crack lengths less than the detectable crack length

Definition - Fail Safe (no longer in use*)

• Following fatigue failure or obvious partial failure of a single, Principle Structure Element (PSE), a catastrophic failure of the aircraft is not probable

  • A redundant load path is provided

  • No longer used in certification

*still used for aircraft design

Definition - Structures Damage Tolerance Inspection

• Should serious fatigue, corrosion, or damage occur within operational life, that remaining structure can withstand reasonable loads without failure or deformation until damage is detected.

  • Reliance on inspections.

Structures Damage Tolerance Evaluation

• All large Transport Category aircraft now require damage tolerant design

• All PSEs are evaluated for damage tolerance, except where impractical in which case safe life evaluation is used and a lifed item is identified (Airworthiness Limitation).

Structures Damage Tolerance Considerations

• Probable location & modes of damage, due to fatigue, corrosion, or accidental damage

• Crack propagation (from detectable to critical length)

• Effect of faults and/or failure on adjacent structure

• Damage at multiple sites due to prior fatigue

• Residual strength

Principle Structural Elements

• Failure of a PSE could be catastrophic

• MSG (Maintenance Steering Group) structural analysis documentation identifies PSEs

• MSG analysis process considers typical fatigue loading, environmental and accidental damage scenarios, and accounts for potential differences and extremes

• Analysis must be supported by tests

• Engineering works with MRB Structural working group to integrate PSEs as part of Structural Significant Items (SSI) analysis

• High degree of collaboration between Engineering and Maintenance

• All PSEs are evaluated for damage tolerance except where not practical

Structures Fatigue Evaluation

• When impractical to evaluate PSEs for damage tolerance, fatigue evaluation is required

• Demonstrate the effectiveness of the provisions to preclude the possibility of widespread fatigue damage occurring within the design service goal of the airplane “with sufficient full scale test evidence”

• Requires a considerable amount of engineering judgement and early agreement between applicant and TCCA

Definition - Airworthiness Limitations (AWL)

• A mandatory discard task for life limited components

• A mandatory inspection task designed to examine structural integrity or system safety of the product (Damage Tolerance Inspection, CMRs resulting from SSA)

• AWL - A condition required for Type Certification

Definition - Maintenance Review Board (MRB)

The section of the Airworthiness Authority responsible for the approval of scheduling recommendations made by industry on the minimum maintenance requirements of new aircraft types.

Note: The recommendations referred to in this definition are those contained in an MRB report.

MRB as a Process

The process of developing and approving the minimum scheduled maintenance requirements (tasks and intervals) for new types of Transport Category aircraft.

Definition - Maintenance Steering Group (MSG)

A decision logic process, developed by the Maintenance Steering Group of the Air Transport Association of America (ATA), for determining an aircraft’s initial minimum scheduled maintenance requirements. The process applies Reliability Centred Maintenance (RCM) concepts.

MSG-3 Analysis

1. Structures - identifies SSIs, design philosophy, assess susceptibility and frequency of potential damage, determine inspection method and frequency.

2. Zonal Analysis

3. Systems/Powerplant/Avionics

Integration Phase

• Certification Maintenance Coordination Committee (CMCC)

• Disposition of Certification Maintenance Requirements (CMRs)

• Airworthiness Limitations of PSEs and Lifed Items

CMCC Process

• All Candidate CMRs are evaluated

• Equivalent, compatible MRB tasks, or CMR status

• Where no equivalent MRB or AFM tasks exist, all remaining Candidate CMRs become “one star” or “two star” CMRs

The CMCC

• A Committee made up by the manufacturer

• Includes manufacturers, operator representatives designated by the ISC (Industry Steering Committee) Chairperson, regulatory representatives, and the MRB Chairperson

• Functions as an advisory committee to the applicant

• Proposes CMRs or changes to MRB tasks or intervals

Dispositioning of all CCMRs

• Is most effective when engineering and maintenance collaborate

• Includes selection of “true” CMRs

• Requires selection of “one star” or “two star” category

• Requires listing of CMRs in a separate document, which becomes an appendix to the MRB report

Integration - Maintenance Schedule into the ICAs

• The ICAs consist of a number of documents

• They should be in a format that is understandable and useable

• The Maintenance Schedule should be in place in the “principal manual” (usually the Maintenance Manual)

• A number of different standards are available for formatting various manuals

  • ATA 100

  • iSpec 2200 (replaced ATA 100)

  • GAMA spec2 (small A/C)

• Placement of information should be consistent with the standards chosen

Integration - AWLs into the ICAs

• AWLs (Life limits, DT inspections, CMRs) must be placed in a distinct and separate section of the “principal manual” (usually the Maintenance Manual)

• Some manufacturers may place the AWLs in other documents (Maintenance Planning data, Maintenance Requirements Manual, Airworthiness Limitations Manual, etc.)

Maintenance Schedule Changes

• Changes can be expected to tasks and intervals based on in-service experience gained by operators and manufacturers

• Changes to MRB tasks and intervals require TC Maintenance and Manufacturing Branch approval

• Changes to AWLs require TC Engineering approval

ICA Manuals - Sample Content

The following are required for certification:

• Aeroplane/rotorcraft manual

• Engine maintenance manual

• Engine overhaul manual

• Propeller maintenance manual

• Propeller overhaul manual

• System wiring diagram

• Component overhaul/maintenance manuals

• Non-Destructive Testing and Inspection manual or section

• Maintenance Review Board (MRB) Report (Transport Category Aircraft)

Extra - Supplemental ICAs (Design Change Projects)

• Same requirements for changes to a type design as for initial type certification

• Guidance provided by MSI 53 “Review of Supplemental Instructions for Continued Airworthiness”

• Not all design change projects will require supplemental ICAs (more and more are)

• Compliance with regulatory standards must be shown