11.2 Airframe Structures

Airworthiness Requirements for Structural Strength

• Aircraft structures must withstand fatigue from pressurization cycles due to flight at high altitudes.
• Aircraft structures use a stressed skin design, where the skin, bulkheads, frames, beams, and ribs all bear loads.

Principal Structural Elements (PSE)

• PSEs are critical parts that significantly carry flight, ground, or cabin pressurization loads.
• PSE integrity is essential for maintaining overall structural integrity.
• Examples of PSEs include wings, horizontal and vertical stabilizers, canards, forward wings, winglets/tip fins, and pressurized areas like primary structure fittings, pressure bulkheads, door frames, and cockpit window posts.

Flight Loads

• Aircraft strength requirements are based on maximum expected service loads and a maximum load with a safety factor.
• Flight limit loads represent the maximum loads expected during normal service.
• Ultimate limit loads are flight limit loads multiplied by a factor of safety.
• Criteria for design, standards, and structural requirements for large aircraft can be found in CS-25 Certification Specifications for Large Aeroplanes.

Design for End Loads

• Aircraft structural members resist stress and are designed to carry loads.
• A single structural member may experience a combination of stresses: tension, compression, torsion, shear, and bending.
• Tension resists pulling forces, while compression resists squeezing forces.
• Torsion is stress from a twisting effect (torque).
• Shear resists forces causing layers of material to slide relative to each other.
• Bending stresses combine compression and tension.
• Structural members are designed to carry end loads (tension or compression) rather than side loads (bending).
• Some structures prioritize strength, while others need different qualities depending on their functions.
• Cowlings and fairings need enough strength to maintain a smooth profile.
• Stress analysis is used to determine the loads imposed on each aircraft part.

Structural Classifications - Primary, Secondary, and Tertiary

• Aircraft structures are classified into primary, secondary, and tertiary categories to evaluate damage and implement appropriate repair procedures.
• Manufacturer manuals specify each structure's category and technicians must adhere to guidelines for that category.

Primary Structure

• Primary structure failure can cause loss of control, structural collapse, harm to occupants, power unit failure, unintended operation, or inability to perform a service.
• Examples include wing spars, engine mounts, fuselage frames, and primary structural members of the main floor.
• Principal Structural Elements (PSEs) within the primary structure, bear loads from flight, ground ops, and pressurization.
• Primary structures may be designated as Structurally Significant Items (SSI) and might require special inspections and have repair constraints.

Secondary Structure

• Secondary structures are non-primary components with intrinsic structural significance and strength exceeding design requirements.
• These structures are less likely to weaken without facing failure risks.
• Examples include wing ribs, fuselage stringers, and designated sections of the aircraft's skin.

Tertiary Structure

• Tertiary structure includes the remaining components of the aircraft’s framework such as lightly stressed elements added for diverse purposes.
• Examples include fairings, fillets, various support brackets, and similar items.

Fail-safe, Safe-life, Damage Tolerance Concepts

Structural Design Philosophies

• The goal is to produce high-quality aircraft using the best design, materials, and manufacturing techniques.
• Aircraft must safely transport passengers/freight for extended periods.
• The structure must withstand all environmental conditions and stresses.
• Provisions like fail-safe load transfer, damage tolerance, and fatigue indexing are incorporated into the design phase.
• Criteria for damage inspection and assessment are detailed in aircraft technical publications.

Fail-safe Concepts

• The fail-safe method relies upon duplication of structural members, to ensure that if one member fails, the other would assume the load of the failed member.
• Fail-safe structure relies on inspection intervals and replacement of affected parts.
• Fail-safe design is not restricted to structural components. Example - passenger cabin windows.
• The disadvantage of the fail-safe concept is that it adds additional weight and is now considered outdated.

Safe-life Concepts

• The safe-life concept estimates how long a structure can remain in service before fatigue damage occurs.
• Testing includes static, dynamic load, and fatigue tests taken to the point of fatigue damage and catastrophic failure.
• Safe-life is calculated as cycles or operating hours, typically one-third of the maximum calculated during testing.
• Components may have separate lifespans and scheduled inspections/replacements before expected failure.
• "Safe-life" calculations may not account for conditions like frequent short flights or corrosive environments.

Damage Tolerance Concepts

• Damage tolerance design requires structural evaluation using repeated load tests to ensure the structure can withstand expected service loads plus a safety factor.
• If damage occurs (cracking or partial failure), the structure can withstand reasonable loads until the damage is detected during routine inspections.
• Multiple load path design distributes loads across small members, so failure of one member does not cause significant degradation.
• Crack limiting joints limit crack spread and ensure cracks are detectable during normal inspections.
• Relies on thorough visual inspection, making inspection intervals and inspector diligence critical.

Zonal and Station Identification Systems

Introduction

• An aircraft has five main structural units: fuselage, wings, stabilizers, flight control surfaces, landing gears.
• Aircraft sections have defined zones, reference points, lines, and planes.
• Each section of the aircraft has a measurement system, including wings, ailerons, flaps, fuselage, horizontal stabilizers, vertical stabilizers, pylons, and nacelles.
• The aircraft is divided into specified zones and areas by reference planes or coordinates which helps to identify the location of components quickly and is useful for calculating the centre of gravity and the distribution of weight.
• The reference planes are vertical, horizontal, and longitudinal.
• Zones are primary areas of the aircraft.
• Coordinate systems vary for each primary aircraft assembly.

Zonal Identification System

• Zonal identification systems are standardized as part of the Air Transport Association (ATA) specification.
• Aircraft are split into major zones, major sub zones, and zones for locating assemblies, sub-assemblies, doors, and panels.
• Major zones are defined with a standard series of numbers. The eight major zones have a three-digit number, the first digit is a number from one to eight followed by two zeros.
• 100 Lower half of the fuselage
• 200 Upper half of the fuselage
• 300 Empennage which covers the aft fuselage and the horizontal and vertical stabilisers
• 400 Power plants and the struts
• 500 Left wing
• 600 Right wing
• 700 Landing gear and the landing gear doors
• 800 Doors
• Sub-major zones are identified by the second digit of the number code. The second digit is a number from one to six for smaller aircraft or one to nine for larger aircraft, and usually, sub-major zones on the right-hand side of the aircraft have even numbers and the sub-major zones on the left-hand side have odd numbers.
• Zones are identified by the third digit of the number code numbered from forward to aft, from inboard to outboard, and from bottom to top.
• If more subzones of a zone are necessary to identify doors and panels, there is a letter after the zone numbers.
• Identification of access panels and service doors: The first digit is for the major zone, the second digit is for the sub-major zone, and the third digit is for the zone, where the first letter shows the position of the panel or door from forward to aft, inboard to outboard and bottom to top and the second letter shows if the panel or door is on the left or the right side.

Station Identification System

• Points on an aircraft are identified by using a coordinate system for the aircraft fuselage, wings, vertical and horizontal stabilizer, and nacelles.
• Points are taken from a datum point and measured in either inches or millimetres.
• On the fuselage, the coordinates are identified by body or fuselage stations, water lines, and butt lines.

Body Stations or Fuselage Stations (BS or FS)

• Body station or fuselage station is a vertical plane at a right angle to the body centreline.
• Body station is measured by the distance from a point in front of the nose of the aircraft.

Buttock Lines (BL)

• Body buttock line or butt lines are vertical planes parallel to the body (fuselage) centreline plane.
• The body buttock line 0 is the body centreline.
• The abbreviation for the left body buttock lines is "L BBL" and for the right body buttock lines "R BBL".

Waterlines (WL)

• Body waterlines are horizontal planes at a right angle to the body stations and the body buttock lines.
• They are measured from a parallel imaginary plane, body waterline 0, below the aircraft fuselage or sometimes below the landing gear.

Wing Stations (WS)

• Wing reference plane or wing stations extend left and right and are measured in either inches or millimetres from either a butt line reference point or from wing rib 1.
• The horizontal and vertical stabilizer coordinates are equal to the wing coordinates.

Component Stations (CS)

• Various components, such as flying control surfaces have their own numbering system usually numbered inboard to outboard.
• Engine nacelles and pylons also have their own coordinate system with their own butt line datum taken from the longitudinal centreline and their own station coordinates lining up with bulkheads and frames.
• Numbering of aircraft stations can be found in chapter 6 of the Aircraft Maintenance Manual (AMM).

Structural Stresses

Introduction

• Aircraft structures include stringers, frames, bulkheads, ribs, and skins.
• Structures endure various loads during aircraft service.
• Design considers these loads, with stress analysis and testing conducted before service release.
• Major stress types affecting aircraft structure: Tension, Compression, Torsion, Shear, Stress, Bending, Hoop stress, Strain, Fatigue

Tension

• Tension resists forces pulling apart.
• Measured in newtons (SI) or pounds-force (Imperial).
• Tensile strength is calculated by dividing the load by the cross-sectional area.

Compression

• Compression resists crushing forces causing parts to shorten.

Torsion

• Torsion causes twisting, for example when the rudder is deflected.

Shear

• Shear resists forces causing one layer of material to slide over another and acts perpendicularly to the plane of the material being sheared.
• Shear strength is usually equal to or less than tensile or compressive strength.
• Riveted joints are designed to resist shear forces.

Stress

• Stress is created within a material subjected to a force, calculated as force/cross-sectional area, and measured in N/m², Pa, or lb/in².

Bending

• Bending stress is a combination of compression and tension.
• Compression occurs on one side of the fuselage, tension on the other.
• During flight, wing lift causes the upper surface to be in compression and the lower surface to be in tension.

Hoop Stress

• Pressurized cabin structure experiences stress acting axially on bulkheads and circumferentially on the skin, where circumferencial tensile stress is referred to as hoop stress.
• Hoop stress is twice the value of longitudinal stress for any given pressure.

Strain

• Strain is the ratio of deformation of a material caused by stress calculated by the change in length in relation to the original length.

Fatigue

• Fatigue is the effect of cyclic or alternating loads on structural components.
• Fatigue damage occurs when material weakens over time by vibration or alternating loads beginning with a tiny crack which propagates causing catastrophic failure of the component.
• A weak spot such as a gouge or scratch corner can be the starting point for fatigue damage.

Drains and Ventilation Provisions

• Effective drainage and ventilation prevent fluids from being trapped in crevices of aircraft structures.
• The lower pressurized fuselage is drained using valved drain holes.
• Fluids are directed to drain holes via longitudinal and cross-drain paths through stringers and frames.

Drain Holes

• Drainage provided by drain holes are located at various positions on the aircraft’s lower fuselage.
• "Bilge-like" valves prevent air loss during pressurization fitting on the aircraft.
• Valves are spring-loaded to open when the cabin is depressurized, draining accumulated condensation.
• Valves close when the aircraft is pressurized due to higher cabin pressure.
• Drain valves vary by manufacturer, refer to the Aircraft Maintenance Manual (AMM) for specific information.

Ventilation

• Cavities in the aircraft structure that may experience the presence of flammable vapor or water must be ventilated to permit the vapor to evaporate, where vent pipes provide an escape route for the vapor.
• Areas like engine nacelles may contain ram air inlets and exit points.
• Engineers must ensure ventilation openings are unobstructed.
• Ventilation requirements can be found in CS-25.
• Additional ventilation requirements exist for toilets and galleys, with extraction fans drawing air through the cabin.

System Installation Provisions and Requirements

• System installation provisions vary by aircraft design, airframe, major/minor assemblies, sub-assemblies, and the systems installed.
• Fittings, attachments, and space are provided based on system type, installation cost, usage, serviceability, operability, availability, and maintainability.
• Items needing regular maintenance (filters, fluid level checkers, lubrication points) must be easily accessible.
• System components must be located for easy engineer access.

System Installation Requirements

• Line Replaceable Units (LRU) must be quickly fitted and removed.
• Grouping various system components in a single bay enhances serviceability.
• Cables carrying control signals or information are in grouped looms, protected and secured.
• Dedicated or general-purpose computers are grouped and secured in protected sections of the fuselage that provides maximum security and accessibility.

Lightning Strike Protection Provision

• Aircraft use necessary protection to prevent catastrophic effects from lightning strikes.
• Metallic components must be correctly bonded to the airframe and earth system.
• External structure provides basic protection.
• Modern composite aircraft use metallic paint, conductive mesh, or coating.
• External structure shields internal areas and protects electrical systems from electromagnetic interference.

Lightning Strike Assessment

• If the aircraft is hit by lightning: Perform a general walk-around inspection to find the damaged areas and assess the severity of the damage.
• Lightning strikes result in direct or indirect damage to the aircraft: Direct Damage - The surface is burned, melted, or shows signs of metallic distortion at two or more attachment points, and Indirect Damage - Damage to the electrical system and equipment that was caused by large electrical transients on the wiring.
• System malfunction requires full inspection.
• Lightning strikes cause small (3 mm) circular melt/scorch marks, or larger holes (6 mm or greater).
• Other signs include burnt or discolored skins and rivets.

A lightning strike conditional inspection examines these areas:

• External surfaces
• Static dischargers
• Fuel system valves
• Integrated Drive Generator (IDG) and related wires
• Hydraulic fittings in the tail section
• Radio systems
• Navigation systems
• Bonding jumpers
• Common strike areas: fuselage nose section, trailing edges, and extremities of wings and stabilizers.
• Aircraft components can become strongly magnetized after a lightning strike creating a magnetic field and magnetizing the component.

Possible internal damage to the aircraft due to lightning strikes can be to the electrical power systems and external light wiring:

• Fuel valves
• Generators
• Power feeders
• Electrical distribution systems

Aircraft Bonding

• Bonding joins two electrical conductors to bring them to the same electrical potential, creating a low-resistance path between structural parts, connecting all the metal parts that are not carrying current during normal operations.
• Grounding connects conductive objects to a conductive structure or return path to safely complete a circuit.

Aircraft Bonding Requirements

• Aircraft electrical bonding connects components using conductive pathways to maintain the same electrical potential preventing static charges.
• Bonding provides a low-resistance return path for single-wire electrical systems, which also aids shielding.
• Proper bonding enhances safety and performance, protecting equipment and passengers.

Bonding minimizes:

• Radio and radar interference
• Fire hazards from sparks
• Damage from lightning strikes Bonding hardware should be selected based on mechanical strength, current capacity, and ease of installation made to flat surfaces.

Grounding

• Grounding connects an object to the primary structure for current return.
• Connects systems and shielded cable shields to the aircraft's metallic structure.
• Grounding points allow current flow, including fault current, without generating heat.

Grounding requirements:

• Grounds must be separated (AC, DC, and shields).
• No more than four terminals on one stud.
• Dual grounds in fuel-vapor areas.

Composite Materials

• Composite materials have high electrical resistance and require integration of a ground plate into the airframe by bonding an aluminum wire mesh into the composite structure during manufacture.
• Direct Bonding: Exposing the mesh (ground plate) and mounting the equipment directly onto the conductive path, and Indirect Bonding: achieved by exposing the mesh and installing a bonding wire and connector.

Bonding Resistance

• Non-conducting paint and anodizing films must be removed from the surfaces to which the bonding terminals are to be attached.
• Measurements should not exceed 0.025 Ω.
• Bonding values are detailed in the aircraft maintenance manual.

Airframe Construction Methods

Introduction

• Fuselage provides space for cargo, controls, passengers, and equipment.
• In single-engine aircraft it houses the powerplant, while in multi-engine aircraft engines may be in, attached to or suspended from the wing structure.

Fuselage Members

• Made of different parts: frames/formers, bulkheads, longerons, stringers, doublers, beams and floor structures, struts, ties, and types of reinforcement
• Frames/Formers (lateral members) give cross-sectional shape, and provide attachment points for wings, stabilizers, and major structural points.
• Bulkheads give shape to the fuselage, close off or partition an area like pressure bulkheads.
• Longerons are main longitudinal members which take primary bending loads and usually extend across several points of support.
• Stringers are smaller and lighter used as stiffeners that assist sheet materials to carry loads along their length.
• Doublers are additional sections of material used to reinforce areas of skin.
• Struts handle compression forces where Ties handle tension forces.
• Floorboards are of honeycomb composite construction screwed into floor beams.

Reinforcement

• Using the lightest workable materials means that some areas require reinforcement, the use of Reinforcement may be made with doublers, butt straps, cleats, gussets, fishplates, angles, and stiffeners.

Wing Construction

• Braced with longitudinal members called ‘spars’ and reinforced at regular intervals with shape-giving ribs.
• Spars consist of an upper and lower boom separated by a web and Ribs maintain the correct contour of the covering but also stress-bearing.
• Wings are often covered with a stressed skin.

Fuselage Construction

• Two general types of fuselage construction: Truss type & Monocoque type
• Truss is a rigid framework made up of members such as beams, struts, and bars to resist deformation by applied loads often covered with fabric.
• Monocoque uses formers, frame assemblies, and bulkheads to give shape to the fuselage, where the skin carries the primary stresses.
• Semi-monocoque (Stressed Skin) Construction reinforcing the skin with longitudinal members, where loads are shared between the skin and the framework.

Methods of Skinning

• Using sheet metal wrapped and formed to the required shape riveted or bonded to other structural members depending according to the load that the component is required to accept.
• Modern composite skins are either filament-wound into the frame or preformed and then fastened together using special fasteners and bonding techniques.

Anti-corrosive Protection

• Design regulations require suitable anti-corrosive protection against deterioration.
• Structures are made with a strength-to-weight ratio in mind therefore common materials like aluminum are widely used for their passivation qualities.
• Surface treatments such as painting, anodizing, and using sealants protect materials from exposure.
• Cladding or plating is a form of corrosion protection by use of a sacrificial layer.

Attachments

Wing Attachments

• Main spar and a rear spar make box section linked by formers called 'ribs' with an additional third false spar in some designs.
• Section covered with stressed skin housing integral fuel tanks.
• Wings attached to each other via "wing box", the strongest section of the fuselage with supportive spars and chambers designed to isolate impacts.

Empennage Attachments

• Horizontal stabilizer constructed equivalent as wing sections attaching to fuselage by utilizing a “box section”.
• Vertical stabilizer attached to the rear fuselage by machined fittings aligning with fuselage frames and longerons.

Engine Attachments

• Safety, structural weight, flutter, drag, control, maximum lift, propulsive efficiency, and maintainability are affected.
• They may be placed in the wings, on the wings, above the wings, or suspended on pylons, mounted on the aft fuselage, on top of the fuselage, or on the sides of the fuselage.
• Pylons are constructed with a frame and skin structure which are riveted and bonded together forming a torque box, firewall, fairings, strut drains, and engine attachment fittings.
• Nacelles are streamlined enclosures housing the engine and its components, often presenting a round or elliptical profile.

Structure Assembly Techniques

Introduction

• Aircraft structure components are manufactured separately and joined.
• Aluminum is a substrate for outer skins, which are lightweight while retaining strength and durability, although carbon composite is becoming a firm design favorite.
• Aluminum alloy metal parts are joined using rivets, bolts, and screws.
• ATA chapter 51 and the SRM contain instructions, details of assembly, and techniques for structural repair.
• Advanced materials mean that repair requires special tooling, techniques, specialist knowledge to carry out.
• Large aircraft are made in sections fastened together at production break areas.

Riveting

• Used for skins and aluminum structures/frames, with special fasteners in high-stress/vibration areas.
• Solid Rivets are metal pins with a formed head through parts followed by the end opposite the head hammered to hold components together.
• Because weight is an important factor when constructing aircraft, the solid-shank rivet is the most preferred method when joining metal parts together.
• Blind Rivets are tubular with a mandrel through the center inserted into a hole drilled through the parts joined using a specially designed tool used to draw the mandrel into the rivet expanding the blind end of the rivet.
• Blind rivets can be inserted and fully installed in a joint from only one side of a part or structure, for use when access to the joint is available from only one side.

Bolting

• Bolts are used in areas requiring high strength, where that is not necessary screws are substituted.
• Aircraft quality bolts are made from alloy steel, stainless/corrosion-resistant steel, aluminum alloys, and titanium. Steel and stainless are the most common.

Bonding

• Modern materials, particularly composites, use adhesive to form a permanent bond where special fasteners and bonding techniques are used.
• Bonding of stringers to skins for metal or composite construction, elevator, aileron, tab and spoiler honeycomb to skin is typical.
• Combining adhesive bonds and rivets gives advantages such as weight savings, strength, and durability.
• Sandwich structures increase strength and durability while minimizing weight using a honeycomb structure or foamed core.

Methods of Surface Protection

Surface Protection Requirements

• Surface protection treats and prevents corrosion, damage by fluids, and erosion of metallic structures.
• Composite structures are treated for lightning strike, ultraviolet ray, and erosion protection.
• Protection depends on the material, function, and location.
• External areas must be protected.

Types of Surface Protection

• The protection build-up is not the same in all areas of the aircraft structure therefore the three important groups of protective treatment include Pre-treatment, paint coatings, and special coatings.
• Pre-treatment, paint and primer are rarely applied directly to metal surfaces that have not had some form of anti-corrosive treatment or surface preparation designed to assist paint adhesion.
• Anodising is the general name applied to methods of treating metals forming oxide coatings to increase the performance of the surface.
• Chromating is a pre-treatment that produces a protective oxide film of a produce oxide film immersed in a potassium dichromate solution.
• The primary reason for applying paint to an aircraft is to protect the skin and structure from corrosion and to protect the corrosion resisting properties protected by the primer.
• Special coatings are applied to those areas which require special corrosion protection and are broken down into two types with a water repellent generally made from silicone-free materials organically bound with a mineral oil base to repel moisture and Heavy-duty corrosion preventive compound, grease-like coatings containing corrosion inhibitors that protect against corrosive agents.

Surface Cleaning

Introduction

• Surface cleaning removes dirt, oil, fuel, and fluids as well as dust deposits and accumulated dirt from the landing gear.
• Airborne dust, chemical vapors, and debris react with aircraft metals therefore it is recommended that each aircraft be thoroughly washed.
• The inside of the aircraft must be kept clean, and special attention must be given to areas where spillages can occur to prevent corrosion.

Aircraft Cleaning Methods

• Three methods of cleaning the exterior surfaces of an aircraft; Wet washing, Dry washing, and Polishing.
• The method of cleaning is determined by the type and extent of the soil on the aircraft.

Wet Washing Aircraft

• A wet wash removes oil, grease, and carbon deposits.

Dry Washing Aircraft

• A dry wash removes aircraft film, dust, and small accumulations of dirt and soil applied with spray, mops, or cloths.

Aircraft Polishing by Hand

• Polishing can restore the brightness/color to painted and unpainted surfaces usually performed after cleaning can also remove oxidation and corrosion.

Aircraft in Service

• Aircraft in service must be cleaned periodically as part of the routine maintenance aligned with a maintenance program, or it can be separately controlled.
• The cleaning solvents recommended for use on a particular aircraft are usually detailed in the relevant Maintenance Manual while unauthorized solvents or detergents are not allowed.

Normal Exterior Cleaning

• Before commencing cleaning operations, all panels and covers must be in place and openings sealed off.
• Any thick mud, grease, or oil must be removed by hand, by scraping using wood, or by soft plastic scrapers.
• Undiluted solvents must never be allowed to come into contact with acrylic windows.

Heavily Contaminated Areas (Exterior)

• If contamination is not removed, it can cause severe corrosion.

Snow and Ice

• Followed by chemical salts and other melting agents, contamination must be washed down with clean water as soon as possible after exposure.

Salt Air Operating Environments

• An Aircraft cleaning and protection programs can require an increased frequency of washing and lubrication procedures, and engine compressor washing as aircraft is more susceptible to salt deposits and salt contaminant.

Acrylic Windows

• After aircraft washing, the windows must be washed with soap or a mild detergent in warm water performed with an approved plastics polish.

Radioactive Contamination

• Regular monitoring of high-flying aircraft with a Geiger counter must be made so that normal regular cleaning keeps contamination within acceptable limits.

Interior Cleaning

• The inside of the aircraft must be kept clean, and special attention must be given to areas where spillages can occur.
• Dirt and grit are best removed by the use of a vacuum cleaner, but oil or grease stains must be removed with the aid of a cleaner recommended by the manufacturer.
• At intervals, floor panels must be removed, and an inspection of the underfloor skin and structure must be carried out and, if necessary, corrosion-prevention treatment must be restored.

Carriage of Livestock

• Requires a very thorough internal cleaning with particular attention being paid to the bilge areas in the cargo compartments with a recommended disinfectant.

Airframe Symmetry

Alignment and Symmetry

• Misalignment can seriously affect flying quality and handling so aircraft symmetry and alignment checks must be performed after:
• Any major structural repair
• Following severe conditions, such as heavy landing, extreme turbulence, overspeeding, or violent maneuvers
• The flight crew reports unusual flight characteristics
• Wrinkling or buckling of structural skins
• Loose or sheared fasteners
• Areas of badly fitting access and inspection panels
• Position/angle of main structural components is related to a longitudinal datum line and a lateral datum line so before main components can be checked, the aircraft must be jacked and leveled.
• Level checks are performed with small aircraft that have fixed pegs or blocks attached to the fuselage that are parallel with the datum lines using a spirit level and a straight edge are rested across pegs/blocks.
• An alternative means of checking the level for larger planes the grid plate which is a permanent fixture installed on the aircraft floor or supporting structure can be used by suspending a plumb bob from a predetermined position in the ceiling of the aircraft over the grid plate.

Alignment Checks

• Transit and plumb bobs, theodolite and sighting rods used to check alignment, using a suitable sequence to ensure that the checks are made at all positions specified, including:
• Wing dihedral angle
• Wing incidence angle
• Verticality of the fin
• Engine alignment
• Symmetry check
• Horizontal stabiliser incidence

Checking Dihedral

• The dihedral angle must be checked in the specified positions using the special boards provided by the aircraft manufacturer or with a straight edge and an inclinometer.

Checking Incidence

• Incidence is usually checked at least two specified positions on the surface of the wing using an incidence board
• Which have stops at the forward edge and others are equipped with location pegs which fit into some specified parts of the structure.

Checking Fin Verticality

• After the rigging of the horizontal stabilizer has been checked, the verticality of the vertical stabilizer relative to the lateral datum can be checked from a given point on either side of the top of the fin to a given point on the left and right horizontal stabilizers.

Checking Engine Alignment

• Generally, the check entails a measurement from the center line of the mounting to the longitudinal center line of the fuselage at the point specified in the applicable manual.

Symmetry Check

• The principle is illustrated while the precise figures, tolerances, and checkpoints for a particular aircraft are found in the applicable Service or Maintenance Manual.
• On small aircraft, measurements between points are taken using a steel tape. to minimize error, a spring scale is used with tape to obtain equal tension, in large planes chalk is used on the floor to mark dimensions

Effects of Misalignment

• Correct alignment guarantees the designer's intent concerning performance, stability, and safety. and Guarantees that the designer's intent concerning performance, stability, and safety is upheld
• Misaligned aircraft can dramatically change flight characteristics and impair the safe control and operation of the aircraft as intended by the designer
• All aircraft performance data is based on the aircraft being properly aligned. Rate of climb, stall, and slow speed performances are all affected by a correct alignment.
• The ability of the aircraft to be predictable is based on its proper assembly and alignment criteria as determined by the designer