Aircraft structural maintenance (LA)

Airframe of fixed wing aircraft consist of

1.      Fuselage

2.      Wings

3.      Stabilizers

4.      Flight control surfaces

5.      Landing gear

Airframe of helicopter

1.      Fuselage

2.      Main rotor

3.      Tail rotor

4.      Landing gear

Determination of load acting on every parts of the aircraft is known as stress analysis.

Five major stresses acting on aircraft

1.      Tension

2.      Shear

3.      Compression

4.      Bending

5.      Torsion

Stress is an internal force of a substance which opposes or resists deformation.

Strain is the deformation of a material or substance.

Tension is the stress that resists a force that tends to pull apart.

Compression is the stress that resists a crushing force. Compressive strength measures in psi

Torsion is the stress that produces twisting. Toshional strength of a material is its resistance to twisting or torque.

Shear is the stress that resisists the force tending to cause one layer of a material to slide over an adjacent layer.

Shearing strength of a material is either equal to or less than its tensile or compressive strength. Aircraft parts especially screws, bolts and rivets are often subject to a shearing force.

Bending force is a combination of compression and tension.

Fuselage construction classified in to two, truss type and mono coque type

Truss type: rigid frame work (beams, struts and bars)

Truss type constructed using steel tubing can carry both tension and compression loads.

Monocoque construction depends largely on the strength of the skin or covering to carry primary stresses.

Monocoque design classified into three Monocoque, semi, and reinforced shell.

True Monocoque construction uses formers frame assemblies and bulk heads to give shape skin carries primary loads/ stresses.

Problem involved in mono coque construction is maintaining enough strength while keeping the weight within allowable limits.

Semimonocoque fuselage is constructed primarily of the alloys of aluminum and magnesium, although steel and titanium are found in areas of high temperatures.

Primary bending loads in the Semimonocoque type are taken by the longerons. Vertical structural members are bulk heads, frames and formers

Stringers are smaller and lighter than longerons and serve as fittings, chiefly used for giving shape and attachment of the skin.

Stringers and longerons prevent tension and compression from bending the fuselage.

Stringers are usually one piece aluminum alloy construction and are manufactured in variety of shapes by casting extrusion or forming.

A gusset is a type of connecting bracket

The bracing between longerons is often referred to as web members. Installed vertically/ diagonally.

Quick access to the accessories and other equipment carried in the fuselage is provided by numerous access doors inspection plates landing wheel wells and other openings.

Servicing diagrams showing the arrangements of equipments and locations of access doors are supplied by the manufacturer in AMM.

Fuselage stations are numbered inches from a reference or zero point known as the reference datum. The reference datum is an imaginary vertical plane at or near the nose of the aircraft from which all horizontal distances are measured.

Buttock line or butt line is a width measurement left or right of and parallel to the vertical center line.

Waterline is a measurement of height in inches perpendicular from a horizontal plane located a fixed number of inches below the bottom of the aircraft fuselage.