Definitions
AD – Airworthiness directive
AMC – Acceptable means of compliance
DOA – Design organisation approval
MPD – Maintenance planning document
OEM – Original equipment manufacturer
RDAS – Repair design approval sheet
RDAF – Repair design approval form
RDT – Repair deviation record
SB – Service Bulletin
SIL – Service information letter
Factors like environmental deterioration, improper maintenance practices, fatigue stress and operational conditions can lead to aircraft structural damages. Unidentified damages or improper repair/assessment practices could lead to catastrophic failure of aircraft. As a precautionary measure, OEM and regulatory authorities recommend to follow inspections and maintenance procedures via Maintenance Planning Document (MPD), Airworthiness Directives (AD), Service Bulletins (SB) or Service Information Letter (SIL).
Common damages are:
•Scratch – scratch is a line of damage of any depth and length I n the material which causes a cross-sectional area change. A sharp object usually causes it.
•Gouge – gouge is a damage area of any size which results in a cross-sectional area change. It is usually caused by contact with a relatively sharp object which produces a continuous, sharp or smooth channel-like groove in the material.
•Mark – mark is a damaged area of all sizes where a concentration of scratches, nicks, chips, burrs or gouges etc. is shown. You must prepare the damage as an area and not as a series of individual scratches, gouges etc.
•Crack – crack is a partial fracture or complete break in the material.
•Dent – 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.
•Nick – small decrease of material due to a knock etc. at the edge of a member or skin.
•Distortion – any twisting, bending or permanent strain which results in misalignment or change of shape. May be caused by impact from a foreign object, but usually results from vibration or movement of adjacent attached components. This group includes bending, buckling, deformation, imbalance, misalignment, pinching, and twisting.
•Corrosion – destruction of metal by chemical or electro-chemical effect. Rust is oxidation or corrosion of metallic alloy part containing ferrous materials.
•Crease – damaged area which is pushed in or folded back on itself. The edges are sharp or well specified lines or ridges.
•Abrasion – abrasion is a damage area of all sizes which causes change in a crosssectional area because of scuffing, rubbing, scraping or other surface erosion. It is usually rough and irregular.
•Debonding – debonding is when a separation of materials occurs due to an adhesive failure.
•Delamination – is when the separation of plies occurs in a multi-laminate material. This can be caused by the material being hit – Impact Delamination, or when there is a resin failure for any other reason.
•Fretting – surface damage at the interface between elements of the joints resulting from very small angular or linear movements. Evidence of fretting is usually the production of fine black powder staining.
•Indentation – indentations are pressure marks which typically show regular smooth deformation of the surface without change of the cross sectional area.
•Burn mark – localized indication of excessive heating ( paint and/or surface black discoloration) after lightning strike. Scorch mark, pitting or local melt thru in addition to paint and/or surface black discoloration can be seen on metallic skins or fasteners. On composite structures, in addition to paint discoloration, puncturing or delamination/disbonding can occur. The size limit for visual inspection is the limit of pain discoloration.
•Penetration – is an area of a component's surface that has been pierced through or into its interior.
Damage can happen at anytime and following procedure can help for its rectification.
Steps to assess Aircraft Structural Damage
Damage Identification Phase: When a damage is identified, the inspector should record the following information: damage location, damage size (dimension), damage type and details of any secondary damage if applicable.
Note: Identification section in manuals will help during accomplishment of this phase (e.g. Chapter 51 in SRM and subsection of any applicable chapter)
Damage Assessment Phase: Damage has to be assessed in accordance with approved data for rectification solutions with details from the identification phase. To determine the damage category inspector should:
•remove all unwanted material from the surface of the damaged component and/or surface
•cut out all broken, bent, heated or damaged areas of the component and/or surface
•remove all loose rivets
In all forms of damage, particularly where shock has been sustained, secondary damage is likely to exist. Therefore, a close examination of the structure surrounding the initial damage must be made. Damage caused by transmission of force may be located some distance from the impact, resulting in structure deformation, drawn rivets or bolt holes.
If misalignment or twisting of the airplane structure is suspected, alignment and/or levelling checks must be carried out.
Damage Categorisation Phase: After cleaning and investigating the damage and surrounding area, the damage must be classified into one of the following categories, also taking into account the location of the damage.
1.Repairable Damage – the damage must be classified either as 'Allowable Damage' or as damage which requires a repair.
a. Allowable Damage – minor damage which does not affect the structural integrity or decrease the function of a component.
b. Non allowable Damage – damage which exceeds the 'Allowable Damage' limits and requires a structural repair. A structural repair restores the structural integrity and function of the component to meet airworthiness requirements. For example this can involve cutting out the damaged area, installing a reinforcing piece (either by bonding or bolting to the original structure). These specific repairs are to be found in each chapter of this manual.
2.Non repairable Damage – damage to structural components which cannot be repaired and where replacement of the complete component is recommended as a repair is not practical or economical.
Note: Allowable damage section in manuals will help us in this phase.
Repair Phase: If the damage is not within allowable limits, this has to be rectified in accordance with applicable repair solution.
Note: Repair section in manuals will help during accomplishment of this phase.
Replacement Phase: If the damage is beyond economical repair, the damaged part has to be replaced.
Note: IPC will help us to find suitable replacement including alternative part.
Approved data: Structural Repair Manual (SRM) or Aircraft/Engine Maintenance Manual (AMM or EMM), Repair drawings, and AMC like DOA holder or OEM repair/assessment procedures with its approval documents.
Note: Airbus issues RDAS or from now RDAF. Boeing issues RDR for Minor repair and FAA issues Form 8100-9 for Major Repairs.
How to record a damage or repair
Each damage/repair in aircraft should be listed out in Dent and Buckle Chart and/or Repair status list. The following information must be recorded:
•aircraft type, series, model and weight variant
•manufacturer serial number, aircraft registration and serial number (if on a transferable component)
•type of damage (dent, scratch, nick, crack, corrosion etc.)
•action taken including references
•Location of repair – general (e.g. fuselage nose section lower LH Skin or LH Outboard Flap L/E etc.), detailed (e.g. between FR XX and FR YY, Stringer A and Stringer B or 200 mm (7.87 in) from STA XXX etc.) and dimensions to existing structural details (distance to stringer, frames etc.)
•Dimensions of the repair – length, width or diameter, orientation and any additional dimensions defining the damage or repair geometry, size of cut-out if applicable, depth of dent, etc…
•Proximity to adjacent repairs, original doublers, modifications or production joints
•Time of repair embodiment (e.g. date, number of flight cycles (CSN) and flight hours (TSN) at the time of installation)
•Allowable damage limit or repair design
•Type of repair, inspection requirements and/or removal time limit
•Any additional comments of deviations.
Classification of damage/repair:
The aircraft damages/repair which affects weight, balance, strength, performance and powerplant operations or flight characteristics are classified as Major, and others are Minor.
Repair or Damage Category:
A – Permanent Repair/allowable damage with no supplemental inspection.
B – Permanent Repair/allowable damage with an additional inspection.
C – Temporary Repair/allowable damage with or without additional inspection.
Note: Both B and C category repairs might be reported to Airlines planning department for further maintenance/inspection.
Aircraft repairs or damages influences the damage tolerance ability of structures. Hence, assessment and proper recording of damage is vital in aircraft maintenance. Always privileges can be availed in best practices. Following up all recommended maintenance activities by OEM and regulatory authority will help to prevent aircraft structure from significant damage occurrence.
Proper recording of structural damage and repairs will help in smooth Lease transition and cancels unwanted reassessment.