This presentation is an examination of structural repair of aircraft. It details the goals, regulations and classification of repairs for different types of aircraft damage. The paper that this presentation is based on was presented by Dr. Kishore Brahma of the AXISCADES Engineering Core Group at the International Conference & Exhibition on Fatigue, Durability & Fracture Mechanics (FatigueDurabilityIndia2015) in Bangalore from 28-30th May 2015.

1. Confidentiality Notice:
All data and information contained in the materials presented with regard to AXISCADES or its Clients is confidential and proprietary. In the event that it is required to divulge confidential information or make
representations regarding AXISCADES‘s Clients, you will need to obtain necessary written approval from AXISCADES for such AXISCADES Clients. You will divulge any confidential information or make any
representations regarding AXISCADES‘s Clients only after obtaining necessary written approvals from AXISCADES or its Clients.
Dr. Kishore Brahma
AXISCADES Engineering Pvt. Ltd.
Bangalore
Fatigue Durability
28-30th May 2015
J N Tata Auditorium
Indian Institute of Science
Bangalore

2. 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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3. Introduction
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Design deficiency
Number of Flights
interval
NcritNdet
acrit
adet
Crack
Length
Inappropriate maintenance Inappropriate repair

4. Lessons Learnt
Inappropriate Repair - 1
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 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

5. Lessons Learnt
Inappropriate Repair -2
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 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.

6. Consequences
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 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

7. 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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8. Regulations for Repairs
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 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

9. Classification of repairs
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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

10.  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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11. 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
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12. Origin of allowable damage
12
+ 2.5g
- 1.0g
Load Factor
Flight envelop Flight load
Material thickness (Flight load)
Limit load
Ultimate load
Limit load
Ultimate load
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13. Origin of allowable damage
(Contd.)
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Flight load
Material thickness (Flight load)
Limit load
Ultimate load
Limit load
Ultimate load
Remaining thickness
Does not sustain
Ultimate load
requirement

14. Origin of allowable damage
(Contd.)
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Flight load
Material thickness (Flight load)
Limit load
Ultimate load
Limit load
Ultimate load
Remaining thickness
Does not sustain
Ultimate load
requirement
Requires restoration
of minimum ultimate
load capability of the
structure through
repair

15. Design Margin
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Flight load
Material thickness (Flight load)
Limit load
Ultimate load
Limit load
Ultimate load
Design
margin
Actual
thickness
Remaining thickness
Structure still able to
sustain ultimate load

16. 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

17. 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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18. Life Extension of Repair
 Repair Principle
 Limitations
 Production Drawings and Modification Validity
 Material and Fastener Properties
 Stress data
 Geometry data
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19. 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

20. 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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21. Thank You
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