Design Philosophy

Design Philosophy

Class Objectives • Clear understanding of design, analysis, and validation requirements for aircraft structures • Exposure to Structures Engineering processes and tools

Design and Analysis of Aircraft Structures

4-2

Lifetime Safety Cycle

• Manufacturer • Operator • Civil aviation authorities Boeing design requirements Regulatory requirements

Design • Operate • Maintain

Produce

Validate and certify

Delivery

In-service operation

Continuous feedback of information

Design and Analysis of Aircraft Structures

4-3

Safety Wheel Design requirements and objectives

Typical safety subject

Final drawings

Federal Aviation Regulation standards

Design and Analysis of Aircraft Structures

4-4

Sources of Design Criteria

Airlines

Boeing

Regulatory agencies (including FAA) Design and Analysis of Aircraft Structures

4-5

Safety Requires Diligent Performance by All Participants Manufacturers Design analysis, test and continued verification

Regulatory actions

Maintenance and inspection

Airworthiness authorities

Operators

Design and Analysis of Aircraft Structures

4-6

Principal Guidance Documents

Design and Analysis of Aircraft Structures

4-7

Structural Design Criteria Consist of Ten Major Elements Design loads Environment/ Discrete events

Material/Fastener Stiffness and flutter

Structures Design Criteria

Maintainability • Repairability • Inspectability

Producibility Crashworthiness

Static strength

Durability • Fatigue • Corrosion

Damage Tolerance/ Residual Strength/ Fail Safety/Safe Life Design and Analysis of Aircraft Structures

4-8

Loads Are the Foundation of Airplane Design

Design loads

Ultimate loads

Limit loads

Operating loads

Design and Analysis of Aircraft Structures

4-9

Center of Gravity/Gross Weight Envelopes

Design and Analysis of Aircraft Structures

4-10

Design Loads Are Based on Load Factors

3

C

L max Flaps up

C

n = 2.5

A

n = 2.0

L max 2 Flaps down Limit 1 load factor - n V 0

-1

D

V

S

H

F Airspeed – knots

n = -1.0

V

V

C

D E

F

Design and Analysis of Aircraft Structures

4-11

Flight Profile

Design and Analysis of Aircraft Structures

4-12

Operating Loads Consist of Random Cycles

Air

Ground

Ground

Design and Analysis of Aircraft Structures

4-13

Boeing Structural Aluminum Alloy Improvements

Design and Analysis of Aircraft Structures

4-14

Material/Process Properties Checklist - Metals Materials/Processes

Producibility

Static

Material specification

Forming

Tension

Process specification

Machining

Compression

Corrosion property

Trimming

Shear

Repair specification

Joining

Bearing

Assembly

Buckling

Chemical processing

Crippling

Real time process

Joint

Inspection

Environmental factors

Safety

Damage Tolerance and Fatigue Fatigue crack growth rate

KIC Residual strength

Stress corrosion

KA KISCC

Incidental damage Fatigue stength

Open hole Joints

Disposal Cleaning

Air Environment

Finish Fatigue factors

Design and Analysis of Aircraft Structures

Environment

4-15

Material/Process Properties Checklist - Composites Materials/Processes

Reliability

Static

Damage Tolerance and Fatigue

Material specification

Layup

Laminate

Damage tolerance

Process specification

Cure

Part specific layup

Repair specification

Handling

Joint

Finishing

Interlaminar shear

Notch

Machining

Crippling

Delamination

Joining

Environment factors

Assembly

Sandwich

Delamination Damage growth

Residual strength

Real time process control

Impact

Impact Notch Durability

Chemical safety

Post impact

Inspection

Open hole

Disposal

Bearing Environment factors

Design and Analysis of Aircraft Structures

4-16

Aircraft Must Be Free From Flutter and Service Vibration Design requirement • Aircraft is designed to be flutter free up to 1.15 times maximum design dive speed envelope (Vd/Md) up to Mach 1. Analytical approach • Unsteady aerodynamics and flutter finite element component and airplane analyses are conducted. Validation • Analysis is verified by windtunnel models, ground vibration, and flight tests up to Va/Md. Design and Analysis of Aircraft Structures

4-17

Structure Must Have Adequate Static Strength Design requirements • Structure must remain elastic up to limit loads • Structure must carry ultimate loads. Analytical approach • Margins of safety are computed for all members based on maximum stresses and structural allowables to verify designs. Validation • Design is validated by limit loads, ultimate loads, and destruction tests. Design and Analysis of Aircraft Structures

4-18

Design and Analysis of Aircraft Structures

4-19

Static Margins of Safety (MS) Are Computed Based on Maximum Applied and Allowable Stresses and Structural Allowables

Allowable stress or strain; material or structural allowables

Maximum applied stress or strain; developed from finite-element analysis or traditional procedures

F -1 MS = f max

A

B

Design and Analysis of Aircraft Structures

Typical

4-20

Aircraft Are Designed for 30 Years of Service Design requirements • Structure must meet or exceed the design service objective with minimum service corrosion or cracking Analytical approach • Margins of safety are computed for all members based on maximum and allowable operating stresses Validation • Panel, component, and full scale airplane testing Design and Analysis of Aircraft Structures

4-21

Economic and Market Conditions Result in Use of Airplanes Beyond Original Economic Design Life Objectives Boeing Commercial Jet Fleet Summary October 31, 2004 Data

747

767

727

777 707 1955

737

720 1960

1965

757 1970

1975

1980

Minimum service design objectives

Model 707 720 727 737 747 757 767 777 737NG

Total airplanes Flights 20,000 735 30,000 153 60,000 1,822 75,000 4,585 20,000 1,336 50,000 1,040 50,000 916 40,000 493 75,000 1,489

Hours 60,000 60,000 50,000 51,000 60,000 50,000 50,000 60,000 51,000

1985

1990

1995

2000

High time airplanes Flights 39,800 45,000 87,700 97,300 39,100 35,400 40,300 18,000 16,600

Design and Analysis of Aircraft Structures

Hours 98,700 69,300 93,700 99,700 119,000 74,200 79,100 38,100 27,500 4-22

Configuration Capability Must Meet Operating Requirement Short, medium, long operating flight profiles

Standardized damage model based on rating system

Life goal and desired reliability

Good detail Standardized load conditions Quantitative scale

Options 1

Based on test or service experience

2

Calculated detail quality based on • Material • Fastener fit • Load transfer • Stress concentration

Actual quality

Damage analysis Required quality

Margin

Design and Analysis of Aircraft Structures

4-23

Fatigue Margins of Safety Are Computed Based on the Fatigue Allowables and Maximum GAG Stresses

Air

Ground

Ground

Allowable ground-air-ground stress

Fmax -1 MS = f max Actual ground-air-ground stress

S alt

N

Design and Analysis of Aircraft Structures

4-24

10-Year Comparison of Service Bulletin LaborHours (727, 737, and 757)

Design and Analysis of Aircraft Structures

4-25

Comparison of 767 and 777 Fatigue Test Findings

Design and Analysis of Aircraft Structures

4-26

Aircraft Are Designed for Corrosion Prevention Design requirement • The design objective is to be free from significant corrosion during the operational life of the airplane. Maintenance • Specified preventive maintenance must be performed. Validation • Operator feedback is used to improve prevention measures. Design and Analysis of Aircraft Structures

4-27

Design Features for Corrosion Prevention Good access Material selection

Corrosion inhibitors

Finishes

Sealants Drainage Design and Analysis of Aircraft Structures

4-28

Comet Accident

Design and Analysis of Aircraft Structures

4-29

707-300 Horizontal Stabilizer Rear Spar Failure

Fatigue failure initiated at rear spar upper chord Rear spar attachment

Fail-Safe mid chord B

A

Up

Up Fwd Fwd

A

Up

B Section A-A

Design and Analysis of Aircraft Structures

Section B-B

Outbd

4-30

Safety Is the Most Important Goal

Design and Analysis of Aircraft Structures

4-31

FAR 25.571 Amendments Related to Fail Safety and Damage Tolerance Amendment Level and Date 25-0 (12/24/64)

Title

Summary of Changes to section 25.571(b) or (c)

Fatigue evaluation of flight structure

25-45 (12/1/78)

Damage-tolerance and fatigue evaluation of structure

25-96 (4/30/98)

Damage-tolerance and fatigue evaluation of structure

(c) Fail safe strength “It must be shown by analysis, tested, or both, that catastrophic failure or excessive deformation, that could adversely affect the flight characteristics of the airplane, are not probable after fatigue or obvious partial failoure of a single PSE. (b) Damage-tolerance (fail-safe) evaluation. “The evaluation must include a determination of the probably locations and modes of damage due to fatigue, corrosion, or accidental damage. The residual strength evaluation must show that the remaining structure is able to withstand loads corresponding to…” (b) Damage-tolerance evaluation. Initial flaw of maximum probable size from manufacturing defect or service induced damage used to set inspection thresholds; sufficient full scale fatigue test evidence must demonstrate that WFD will not occur within DSO (no airplane may be operated beyond cycles equal to ½ the cycles on fatigue test article until testing is completed).

Design and Analysis of Aircraft Structures

4-32

Monolithic Structure is Used to Improve Producibility Cast Framework D357.0-T6 Aluminum Per BMS 7-330

Outer skin 2024-T3 Clad

737-600/700/800 Airstair Door Design and Analysis of Aircraft Structures

4-33

Two-Bay Crack in the Wing Lower Surface

Design and Analysis of Aircraft Structures

4-34

Example of Safe Fuselage Decompression

Design and Analysis of Aircraft Structures

4-35

Example of Save Wing Penetrations

Design and Analysis of Aircraft Structures

4-36

Damage Tolerance Regulation Comparison

Analysis

FAR 25.571 (before 1978)

FAR 25.571 (after 1978)

Residual strength

• Single element of obvious failure

• Multiple active cracks

Crack growth

• No analysis required

• Extensive analysis required

Inspection program

• Based on service history • FAA air carrier approval

• Related to structural damage characteristics and past service history • Initial FAA engineering and air carrier approval

Design and Analysis of Aircraft Structures

4-37

Safety is Maintained by Damage-Tolerant / Fail-Safe Structures Damage detection and restoration

Ultimate

Structural strength

Ultimate load capability required after damage detection

Fail-safe requirement NDI detection period

Visual detection period

Operating loads

Damage size Allowable damage Visual NDI

Damage detection thresholds

Service time Design and Analysis of Aircraft Structures

4-38

Structure Must Be Damage Tolerant Design requirement • Structure must have capability to withstand damage until detected and repaired. Analytical approach • Damage tolerance is verified by analytical assessment of damage growth, residual strength, and surveillance. Validation • Damage tolerance is validated by panel and component tests. – – – –

Residual strength Crack growth Qualification Inspection program Design and Analysis of Aircraft Structures

4-39

Damage Detection Evaluation

Design and Analysis of Aircraft Structures

4-40

Structural Classification and Damage Tolerance Requirements Required design attributes

Structural category Other structure

Secondary structure

c Damage obvious or malfunction evident

d Primary structure (Structurally significant items or principal structural elements)

Damage detection by plannd inspection

f

Structural classification examples

Design for loss of component or safe separation

Continued safe flight

Flap track canoe fairings (safe separation or safe loss or segment)

Design for faioure or partial failure of a principal structural element with continued structural integrity

• Residual strength

Wing fuel leaks

Inspection program matched to structural characteristics

• Residual strength • Crack growth • Inspection program

All primary structure not included in categories and

Design for conservative ratigue life (damage tolerant design is impractical)

Fatigue analysis verified by test

Landing gear structure

e Safe life design

Analysis requirements

Design and Analysis of Aircraft Structures

4-41

Residual Strength Versus Damage Size or Notch Length

Residual strength

Ultimate strength

Limit strength

70% limit strength

Damage size or notch length

BVID

VID

Desirable source Large accidental damage

Design and Analysis of Aircraft Structures

4-42

Probabilistic Life Cycle Design ($$) • Static/Ultimate Strength

Service Feedback

• Durability/Safe Life

Maneuver Gust Thermal Payload Environment Fit-up stress Sensors Etc.

• Fail Safety/Damage Tolerance Probabilistic Risk Assessment • Certification Modulus Strength Ult. strength Toughness S-N/DaDT Corrosion Damage Tolerance, Shimming Etc.

Load

Fleet Readiness • Inspection and Repair

• Health Monitoring

• Statistical Quality Control • Initial Defects Quantification

Manufacturing ($$)

Operation/ Maintenance ($$)

Design and Analysis of Aircraft Structures

4-43

Local Versus Widespread MSD or MED Local damage

Crack initiation

Crack extension

Llocal Multiple site damage (MSD)

Maximum Lskin allowable skin damage Multiple element damage (MED)

Maximum allowable damage Design and Analysis of Aircraft Structures

4-44

Widespread Fatigue Damage Detection

Structural strength

1 Threshold

WFD detection Local damage period detection period

Ultimate 2

Required residual strength Critical

Normal inspection programs

Special Inspections or actions

Service period, flight cycles Design and Analysis of Aircraft Structures

4-45

Aging Fleet Programs

Design and Analysis of Aircraft Structures

4-46

Landing Gear is an Example of Safe-Life Structure

Design and Analysis of Aircraft Structures

4-47

Airplane Designed to Survive Minor Crashes

Rear spar

Landing gear beam

Shear pins

Obstacle

Design and Analysis of Aircraft Structures

4-48

Strut Design and Structural Fuses

Design and Analysis of Aircraft Structures

4-49

Floor Structure is Often Designed by Crash Conditions

Design and Analysis of Aircraft Structures

4-50

KBE Evolution and Implementation History

Design and Analysis of Aircraft Structures

4-51

External Criteria That Affect the Design of the Structure

Bird strike

Tire burst

Engine blade loss

Tire tread

Design and Analysis of Aircraft Structures

4-52

Fan Bladeout Test

Design and Analysis of Aircraft Structures

4-53

Summary • Regulatory requirements have evolved over the years based on significant service and test experience • Validation and certification approach is primarily analytical supported by test evidence • Supporting evidence includes testing through a “building block” approach • The environmental effects are characterised by test and are accounted for in the analysis • Process and tools are continually improved to enhance accuracy and reduce design cost and cycle time

Design and Analysis of Aircraft Structures

4-54

Appendix

Design and Analysis of Aircraft Structures

4-55