This document discusses various concepts related to aircraft structural design and airworthiness requirements. It describes how aircraft structure is divided into primary, secondary, and tertiary categories based on their importance. Primary structure, if failed, could cause loss of control or structural collapse. Examples provided stress the importance of withstanding forces like tension, compression, shear, bending, and torsion to ensure structural integrity and safety. Station identification systems are also covered to precisely locate structural components through methods like station numbering and zoning.

1. AIRFRAME STRUCTURES- GENERAL
CONCEPTS

2. AIRWORTHINESS REQUIREMENTS
FOR STRUCTURAL STRENGTH :
• An aircraft is airworthy "when it meets its type design and is in a
condition for safe operation" [FAA]
• The goal is to only allow aircraft meeting established minimum
standards to fly in an attempt to safeguard aircrews and the general
public.
• Other requirements for weights, ventilation, factors of safety and
even door operation are included.

3. • EASA Part 25, also known as Certification Standards 25 (CS25), states
the requirement for structural airworthiness for aircraft with
maximum total weight above 5 700 kg.

4. STRUCTURAL CLASSIFICATION :
• Aircraft structure is divided into three categories for the purposes of
assessing damage and the application of repair protocol that are
suitable for the structure under consideration.
• Manufacturer manuals designate which category a structure falls
under and the technician is required to repair and maintain that
structure in accordance with rules specified for the category under
which it falls.

5. •Categories:
oPrimary structure
oSecondary structure
oTertiary structure

6. •PRIMARY STRUCTURE :
Primary structure is any portion of the aircraft structure that, if it fails,
on the ground or in flight, would likely cause any of the following:
 A loss of control of the aircraft.
 Catastrophic structural collapse.
 Injury to occupants.
Power unit failure.
 Unintentional operation.
 Inability to operate a service.

7.  Some examples of primary structure are wings spars, engine
mounts, fuselage frames, and main floor structural members.
These elements are those which carry flight, ground and
pressurization loads.
Primary structure may also be represented as a structurally
significant item or SSI. These elements are specified in a
supplemental structural inspection document.
Due to their structural importance, they may require special
inspection and have specific repair limitations.

8. • SECONDARY STRUCTURE :
 Secondary structure is all non primary structure portions of the aircraft
which have integral structural importance and strength exceeding design
requirements.
• These structures weakening without risk of failure such as those described
for primary structure.
• Prominent examples of secondary structure are wing ribs, fuselage
stringers and specified sections of the aircraft skin.

9. • TERTIARY STRUCTURE :
 Tertiary structure is the remaining structure.
 Tertiary structures are lightly stressed structures that are
fitted to the aircraft for various reasons.
 Fairings, fillets, various support brackets, etc. are examples
of tertiary structure.

10. DAMAGE TOLERANT CONCEPTS :
FAIL SAFE :
• It is designed so that the aircraft may continue to operate
safely until the defect is detected in a scheduled
maintenance check.
• Manufacturer testing and fatigue analysis is used when
developing fail safe structural elements.

11. SAFE LIFE :
• Safe life structural elements are those which have a very low
risk of unacceptable degradation or failure for a stated
amount of time.
• The fatigue capability of the structure is learned through
testing.
• Stress handle capacity decide through testing.
• The affects of corrosion, wear and fatigue are considered
when operating under the safe life design principle.

12. DAMAGE TOLERANCE ?
• The damage tolerance approach is based on the principle that while
cracks due to fatigue and corrosion will develop in the aircraft
structure, the process can be understood and controlled.
• A key element is the development of a comprehensive programme of
inspections to detect cracks before they can affect flight safety.
• By distributing loads over a larger area and designing multiple load
paths for carrying loads, a structure can be damage tolerant.

13. • Damage tolerance means that the structure has been evaluated to
ensure that should serious fatigue, corrosion, or accidental damage
occur within the operational life of the aero plane, the remaining
structure can withstand reasonable loads without failure or excessive
structural deformation until the damage is detected.

14. STATION IDENTIFICATION AND ZONAL LOCATION
SYSTEMS :
STATION NUMBERING :
• Even on small, light aircraft, a method of precisely locating each
structural component is required.
• Various numbering systems are used to facilitate the location of
specific wing frames, fuselage bulkheads, or any other structural
members on an aircraft.

15. • Most manufacturers use some system of station marking. For
example, the nose of the aircraft may be designated "zero
station," and all other stations are located at measured
distances in inches behind the zero station.
• Thus, when a blueprint reads "fuselage frame station 137,"
that particular frame station can be located 137 inches
behind the nose of the aircraft.

16. • To locate structures to the right or left of the center line of an aircraft,
a similar method is employed. Many manufacturers consider the
center line of the aircraft to be a zero station from which
measurements can be taken to the right or left to locate an airframe
member.
• Fuselage stations (Fus. Sta. or FS) are numbered in inches from a
reference or zero point known as the reference datum. [Figure ]
• The reference datum is an imaginary vertical plane at or near the
nose of the aircraft from which all fore and aft distances are
measured.
• The distance to a given point is measured in inches parallel to a
center line extending through the aircraft from the nose through the
center of the tail cone.

18. • Buttock line or butt line (BL) is a vertical reference plane down the
center of the aircraft from which measurements left or right can be
made. [Figure ]

19. Water line (WL) is the measurement of height in inches perpendicular
from a horizontal plane usually located at the ground, cabin floor, or
some other easily referenced location. [Figure 3]
Figure 3. Water line diagram

20. • Aileron station (AS) is measured outboard from, and parallel to, the
inboard edge of the aileron, perpendicular to the rear beam of the
wing.
• Flap station (KS) is measured perpendicular to the rear beam of the
wing and parallel to, and outboard from, the inboard edge of the flap.
• Nacelle station (NC or Nac. Sta.) is measured either forward of or
behind the front spar of the wing and perpendicular to a designated
water line.

21. Wing stations are often referenced off the butt line, which bisects the center of the fuselage
longitudinally. Horizontal stabilizer stations referenced to the butt line and engine nacelle stations are
also shown

22. Another method
 Zone method
• it used to facilitate the location of aircraft components on air
transport aircraft.
• This involves dividing the aircraft into zones.
• These large areas or major zones are further divided into sequentially
numbered zones and subzones.
• The digits of the zone number are reserved and indexed to indicate
the location and type of system of which the component is a part.

23. Zone vice distribution :

24. •Force?
•Pressure?
•Stress?
•Strain?

25. STRUCTURAL STRESS
• LEARNING OBJECTIVE: Identify the five basic stresses
acting on an aircraft.
• Stress is a material's internal resistance, or
counterforce, that opposes deformation.
• The primary factors to consider in aircraft structures
are strength, weight, and reliability.
• These factors determine the requirements to be met
by any material used to construct or repair the
aircraft.

26. • Airframes must be strong and light in weight. An aircraft built
so heavy that it couldn't support more than a few hundred
pounds of additional weight would be useless.
• All materials used to construct an aircraft must be reliable.
Reliability minimizes the possibility of dangerous and
unexpected failures.

27. • Many forces and structural stresses act on an aircraft when it is
flying and when it is static.
• When it is static, the force of gravity produces weight, which is
supported by the landing gear. The landing gear absorbs the
forces imposed on the aircraft by takeoffs and landings.
• During flight, any maneuver that causes acceleration or
deceleration increases the forces and stresses on the wings and
fuselage.
• The degree of deformation of a material is strain.

28.  There are five major stresses to which all aircraft are subjected:
• Tension
• Shear
• Compression
• Bending
• Torsion

29.  Tension :
• Tension is defined as pull.
• It is the stress of stretching an object or pulling at its ends.
• Tension is the resistance to pulling apart or stretching
produced by two forces pulling in opposite directions along
the same straight line.
• For example, an elevator control cable is in additional
tension when the pilot moves the control column.
• The tensile strength of a material is measured in pounds per
square inch (psi) and is calculated by dividing the load (in
pounds) required to pull the material apart by its cross
sectional area (in square inches).

30. COMPRESSION :
• If forces acting on an aircraft move toward each other to
squeeze the material, the stress is called compression.
• Compression is the opposite of tension. Tension is pull, and
compression is push.
• Compression is the resistance to crushing produced by two
forces pushing toward each other in the same straight line.
• For example, when an airplane is on the ground, the landing
gear struts are under a constant compression stress.

31. SHEAR :
• Cutting a piece of paper with scissors is an example of a
shearing action.
• In an aircraft structure, is a stress exerted when two pieces
of fastened material tend to separate.
• Shear stress is the outcome of sliding one part over the other
in opposite directions.
• Aircraft parts, especially screws, bolts, and rivets, are often
subject to a shearing force.
• Usually, the shearing strength of a material is either equal to
or less than its tensile or compressive strength.

32. • Bending : it is a combination of tension and compression.
• For example, when bending a piece of tubing, the upper portion
stretches (tension) and the lower portion crushes together
(compression). The wing spars of an aircraft in flight are subject to
bending stresses.

33. TORSION :
• Torsional stresses result from a twisting force.
• When you wring out a chamois skin, you are putting it under
torsion.
• Torsion is produced in an engine crankshaft while the engine
is running. Forces that produce torsional stress also produce
torque.

34. VARYING STRESS :
• All structural members of an aircraft are subject to one or
more stresses.
• Sometimes a structural member has alternate stresses; for
example, it is under compression one instant and under
tension the next.
• The strength of aircraft materials must be great enough to
withstand maximum force of varying stresses

36. SPECIFIC ACTION OF STRESSES :
• The fuselage of an aircraft is subject the fives types of stress—
torsion, bending, tension, shear, and compression.
• Torsional stress in a fuselage is created in several ways.
• For example, torsional stress is encountered in engine torque
on turboprop aircraft. Engine torque tends to rotate the
aircraft in the direction opposite to the direction the propeller
is turning. This force creates a torsional stress in the fuselage.
• Torsional stress on the fuselage is created by the action of the
ailerons when the aircraft is maneuvered.

37. • When an aircraft is on the ground, there is a bending force on the
fuselage. This force occurs because of the weight of the aircraft.
Bending increases when the aircraft makes a carrier landing. This
bending action creates a tension stress on the lower skin of the
fuselage and a compression stress on the top skin.

38. DRAINAGE AND VENTILATION PROVISIONS :
• The collection of water and other fluids in the many cavities found on
an aircraft can lead to corrosion and could present a fire hazard.
Drainage and ventilation are used to address this issue.
• There are two types of drains, internal and external.
• External drains have openings to the exterior of the aircraft.
• They are found on the wings, empennage and fuselage as well as
engine nacelles. An external drain dumps the fluid overboard.
• In unpressurized aircraft the drains may remain open at all times.

39. • Drain valves are used in pressurized sections of aircraft so that they
may remain sealed during pressurization.
• Typically located along the aircraft keel, some external drains use the
pressurizing air to hold the valve closed.
• A rubber flapper type valve, a plunger type valve or a normally open
spring loaded valve are closed by pressurization air.
• When depressurized, such as when the aircraft is on the ground, the
drain valves open.
• Galley and lavatory drain masts must be heated to prevent ice
formation and blockage caused by cold temperatures at high altitude.

40. • A drain mast is nothing more than an airfoil shaped projection
designed to guide the fluid overboard away from the skin of the
aircraft.
• Internal drain paths are required to direct fluid to the external drain
sites. Tubes, channels, dams and internal drain holes are all common.
The design of structural members often includes considerations that
prevent :fluids from being trapped.

41. VENTILATION :
• Any cavity in the aircraft structure that may experience the presence
of a flammable vapor or water must be ventilated to permit the vapor
to evaporate.
• If necessary, vent pipes are used provide an escape route for the
vapor. Some highly susceptible areas, such as an engine nacelle, may
even contain ram air inlets and exit points to enable a full flow of
fresh air through the cavity.
• The technician should ensure that all openings designed for
ventilation are unobstructed.

42. SYSTEM INSTALLATION PROVISIONS :
• In addition to designing functioning support systems for operation of
the aircraft, design engineers must also make the system components
fit into the aircraft.
• Depending on the system and components, provisions for access and
servicing must also be: addressed. Items that receive regular
maintenance such as filters, fluid level checks, bearing lubrication,
etc. must be located so that technicians can easily access them.
• Line replaceable units (LRU's) must be able to be quickly uninstalled
and installed. Aircraft maintenance is a significant expense for the
operator.

43. • Anything that can be done to locate system components for easy
access for maintenance saves time and lowers the cost of operating
the aircraft.
• for example, may have its several key components mounted next to
each other in an air conditioning bay.
• The hydraulic reservoir, pumps and filters may all be located in a
different bay or in the wheel well area. Avionics and electronics are
frequently mounted in an avionics bay.
• Not only are the "black boxes" easily accessible but environmental
conditions can be better controlled than if the units were spread
throughout the aircraft.

44. CONSTRUCTION METHODS :
• FUSELAGE :
Types :
1 ) truss type
2 ) monocoque
3 ) semi monocoque

48. METHODS OF SURFACE PROTECTION :
• The manufacturer's maintenance manual details the surface
protection compounds that must be applied by the technician for all
of the various areas of the aircraft.
• Different areas on the aircraft may be prone to different contaminants
and the recommended treatments are designed accordingly.
• Do not assume that a product is suitable for treatment of an area of
the aircraft structure without consulting the manufacturer's data.

49. Methods:
 ANODIZING :
 One of the most common for aluminum based alloys is anodizing.
Anodizing is an electrolytic treatment that coats the metal with a
hard, waterproof and airtight, oxide film.
Anodizing usually contains a dye. Various colors are used. This
permits easy identification that a part has be anodized.
The oxide film acts as an isolator. When attaching a bonding lead, the
film must carefully be removed to ensure electrical conductivity.

50. Anodizing provides an excellent base for many finishes as well as for
bonding adhesives.
Ex. Acrylic lacquers, and polyurethane paints adhere well to
anodized parts and provide good resistance to chemical attack and
wear.

51.  CHROMATING :
An alternative to anodizing used for surface protection on
magnesium and zinc alloy parts is chromate.
When chromated, parts are generally immersed in a potassium
bichromate solution.
The chromate coating protects the surface from corrosive elements
and has a yellowish appearance on magnesium alloys.
 Products are available to obtain a chromate coating on a part in the
field.
 Alocrom 1200 is one such product.

52.  CLADDING :
Cladding a material with another, non corrosive material is a popular
means of material surface protection. This is done as the raw material
is formed into the product material.
 Sheet aluminum, for example, may be clad to protect the corrosive
copper or zinc aluminum alloy from which many aluminums products
are made.
Alclad is a process of cladding aluminum in which a pure aluminum
skin is rolled onto the face of an alloy aluminum sheet.
 Pure aluminum forms a stable aluminum oxide surface when
exposed to air that protects the pure aluminum itself and the material
that has been clad.

53.  PAINTING :
Many aircraft structural elements and parts are painted to protect
them from corrosion.
 The paint acts as a barrier so that the agents of corrosion cannot
reach the material being protected. To be effective, paint must be
applied to a clean dry surface.
 It must be compatible with the material composition so that a good
bond is formed and it adheres when it is applied. Material surface
treatments such as paint primer and alodine are used before painting
because they bond strongly to the base material as well as to the
paint.

55. EXTERIOR AIRCRAFT CLEANING :
• Aircraft are cleaned before major inspections. Typically a high
pressure water or steam is sprayed in conjunction with cleaning
agents to clean the exterior of the aircraft.
• While a clean aircraft aids in corrosion prevention, the cleaning
process may put water and agent where it is not desirable and, thus,
it may even cause corrosion.
• Areas into which the cleaning spray should not enter must be covered
or sealed from its entrance. Pitot tubes and static ports are such areas
as well as tires and brake assemblies.
• ( precision ???)

56. • The manufacturer's maintenance manual gives detailed instructions
on cleaning procedures. Areas to be protected and the proper
cleaning agents to use must be noted.
• A cleaning agent that is suitable for one area of the aircraft may not
be for another.
• Aircraft are generally washed outside in an area with adequate and
environmentally responsible drainage.
• Washing with cleaning agents should not be performed in high
temperatures where the agent may dry before being rinsed off.
• Use the ratio of agent to water that is recommended. Use of the
wrong agent may cause the agent to attack materials.

57. • Hydrogen embrittdement occurs when certain agents soak into an
aircraft metal. Minute cracks form and stress corrosion develops.
• Engine and wheel well areas may require a special washing technique
or cleaning agents due to dirt, oil, grease and exhaust debris buildup.
• Be aware that some cleaning procedures are followed by greasing
various locations that may have had grease washed out during the
cleaning process.