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Structural Repair of Aircraft

Dr. Kishore BrahmaAXISCADES Engineering Pvt. Ltd.

Bangalore

Fatigue Durability28-30th May 2015

J N Tata Auditorium Indian Institute of Science

Bangalore

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Agenda

Introduction Lessons learnt F&DT goals for repair Regulations for repair Classification of repairs Type of damages Widespread fatigue damage (WFD) Origin of allowable damage Types of repairs Life extension of repair Conclusions

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Introduction

Design deficiency

Number of Flights

interval

NcritNdet

acrit

adet

CrackLength

Inappropriate maintenance Inappropriate repair

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Lessons LearntInappropriate Repair - 1

Boeing 747 of Japan Air Lines (JAL) crashed on 12 August 1985, killing all 520 people on board

The accident was caused by the failure of the rear pressure bulkhead, which allowed a build up of pressure in the fin and subsequent rupture of all aircraft control lines and hydraulic lines

The rear pressure bulkhead had been previously repaired following damage sustained in a tail scrape on landing at Osaka Airport in August 1978

However, the repair solution had undermined the importance of fatigue leading to fatigue cracking and eventual fracture of the rear pressure bulkhead

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Lessons LearntInappropriate Repair -2

On May 25, 2002, China Airlines flight CI-611, a Boeing 747-200 experienced an in-flight structural breakup and crashed

In 1980, the airplane experienced a tail-strike while landing in Hong Kong. A permanent repair was accomplished by installing external aluminium doubler. The damaged skin was not removed

Metallurgical examination of the recovered wreckage revealed a region of fatigue cracking with multiple-site fatigue damage (MSD) extending for about 2360 mm under the left edge of the doubler.

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Consequences

The accident showed that in-service repairs also have a significant influence on the damage tolerance of aircraft

Fatigue justifications were required for existing repairs of oldest aircraft

Damage tolerance certification of repairs has become mandatory.

In-service repairs subject to inspection programmes as per those for the original un-repaired structure

Introduction of regulations for repair

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F&DT Goals for Repair Design

The fatigue and damage tolerance goals for repair design are aligned as far as possible with the goals of the original aircraft structure

The above mentioned goals for repair design may influence the design and allowable of the original aircraft structure and the structural inspection program after reaching the inspection threshold

Necessary actions need to be taken depending on the type of repair categories - ‘permanent repairs’ and ‘temporary repairs’. • The temporary repairs have a limited fatigue life and must be replaced,

modified or upgraded before reaching the life limit• The permanent repairs may need inspections after reaching a calculated

threshold

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Regulations for Repairs

Certification: • FAR + AC 25.571: damage tolerance and fatigue evaluation of structure• AC 25.1529-1A: Structural repair evaluation

Operation:• FAR 25.1529 + AC 25.1529-1A: Instructions for Continued airworthiness of

Structural Repairs on Transport Airplanes• 14 CFR 91.410, 121.370, 125.248, 129.32 + AC 120 –73: Repair assessment of

pressurized fuselages

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Classification of repairs

Repairable

Repair by replacement

Damage

Non-repairable

Non-allowable

Allowable

Minor Repair

Major Repair

Minor – Simple repair with strength reduction within certification limit

Major – Complex repair with strength restoration

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Scratch • A scratch is a line of damage which causes a cross sectional area change

Gouge• A gouge is a damaged area which results in a cross sectional area change

producing a continuous sharp or smooth channel like groove in the material

Crack A crack is a partial fracture or complete break in the material

Dents• A dent is a damaged area which is pushed in, with respect to its usual contour.

There is no cross sectional area change in the material, area edges are smooth

Nicks• A nick is a small decrease of material due to a knock, etc…. at the edge of a

member or a skin

Erosion• Paint erosion is caused by the pressure and speed of the air at leading edges areas

of the wings or aircraft nose. This can be prevented by anti erosion tape

Damage Types

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WFD

Widespread fatigue damage (WFD), in a structure is characterized by the simultaneous presence of cracks at multiple structural details that are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement (i.e. to maintain its required residual strength after partial structural failure)

Multiple site damage (MSD), is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in the same structural element (i.e. fatigue cracks that may coalesce with or without other damage leading to a loss of required residual strength)

Multiple element damage (MED), is a source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in adjacent structural elements

Origin of allowable damage

12

+ 2.5g

- 1.0g

Load Factor

Flight envelop Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit loadUltimate load

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Origin of allowable damage(Contd.)

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Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit loadUltimate load

Remaining thickness

Does not sustain Ultimate load requirement

Origin of allowable damage(Contd.)

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Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit loadUltimate load

Remaining thickness

Does not sustain Ultimate load requirement

Requires restoration of minimum ultimate load capability of the

structure through repair

Design Margin

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Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit loadUltimate load

Design margin

Actual thickness Remaining thickness

Structure still able to sustain ultimate load

Allowable Design Optimization

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Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit load

Ultimate load

Design margin

Actual thickness

Flight load

Material thickness (Flight load)

Limit load

Ultimate load

Limit load

Ultimate load

Design margin

Actual thickness

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Type of Repairs

External Structure – Skin• Skin Lightning Strike Repair• External Skin Repairs (small, limited, unlimited, and conversion of temporary skin

repairs)• External Skin Repair (Longitudinal joint, Circumferential joint)• Internal Skin repairs (small, limited, and conversion of temporary skin repairs)• Door Surround Skin Repair• Bush Skin Repair• Plug Repair• Jacking point Repair

Internal Structure• Stringer Repair • Frame Repair• Cross-beam Repair

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Life Extension of Repair

Repair Principle

Limitations

Production Drawings and Modification Validity

Material and Fastener Properties

Stress data

Geometry data

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Wide Spread Fatigue Damage Evaluation

- Fatigue & Damage Tolerance

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Fuselage stringer repairs can be susceptible to WFD due to multiple element damage (MED) provided if multiple adjacent stringer repairs are subjected to similar stress level

The unfactored WFD life is calculated by using either• Deterministic approach or • Probabilistic approach

If the deterministic approach is used, a knock down factor is applied to the unfactored fatigue life

In order to preclude the occurrence of WFD, a dedicated inspection program is defined

The inspection starting point (ISP) marks the beginning of the inspection program

They are followed by repeated inspections until the structure modification point (SMP) Is reached at which the repair has to be modified or replaced

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Conclusions

Repairs influence the damage tolerance of aircraft significantly

Damage tolerance certification of repairs has become mandatory

Design margin is the basis for allowable damage limit

Reduction in design margin has lead to reduction in allowable margin

F&DT justification is the basis for life extension of repair

The effect of WFD on life extension studies is considered by precluding its

occurrence during its operational life

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Thank You