AGK Systems - Airframe design and material introduction Flashcards
Certification Standard for small / large planes
CS-23 Smaller a/c
CS-25 Larger a/c
Minor Failure
Conditions may be Probable
Major Failure
Conditions must be no more frequent than remote
Hazardous Failre
Conditions must be no more frequent than Extremely Remote
Catastrophic Failure
Conditions must bear Extremely Improbable
Safe Life Concept
A/C which can comfortably sustain safe operations within its designed life span - its safe life
Safe life principles…
Explicitly stated design life
No catastrophic structural damage
Safe life may be measured in…
Flights Hours
Pressurisation Cycles
Landings or,
Calendar Duration
Fail safe concept
Multiple load paths
Designed to provide redundancy
Critical areas are built with fail safe multiple loads
~Ensures in one path fails, an alternate load path can carry all the loads through the duplicate structure
‘Where loss of components could lead to a/c loss
Damage tolerance concept
Some excess weight can be avoided by designing structures to be ‘Damage Tolerant’
These structures have crack-arresting features which limit extent to which a crack can spread
Surrounding paths carry all the load until the crack is detected
Redundancy concept
Is the duplication of vital components of systems for use in the event of a failure
Very useful tool for reducing probability of a major fault affecting a/c and passenger safety
Stress
Strain
Internal force per unit area
~Tension
~Compression
~Torsion
~Shear
Strain is the deformation of a component
Dynamic vs Static Loads
Dynamic
~Change quickly, rapid build up of load can cause serve stresses on materials
Static
~Build up slowly
~E.g. when a/c parked, its weight imposes load on wing spars from the landing gear
What is a Cyclic Load?
During flight loads can change and alternate rapidly
Components experience changes from compression to tension
Lead to Metal Fatigue
Why are the wind tanks used on t/o
Because there centre tanks have fresh fuel so by using wing tanks, you know the fuel is useable as yule just flown on it
What is the design load limit?
2.5g
Ultimate load 3.75g (1.5g x 2.5g)
Fatigue Life
Based on cycles
Great mass will cause greater stress
~leads to fewer cycles before fatigue
Increase the weight by 1%, you increase the fatigue by 5%
Operational Factors affecting Fatigue life
Max Ramp Mass (Taxiing/Towing)
Max Take-off Mass
Max Zero Fuel Mass
Max Landing Mass
How to reduce Operational Factors effecting Fatigue Life
Reduce loads by:
~Avoiding coarse manoeuvring
~Avoiding Turbulence
~Not flying with excess fuel/payloads
These will increase the number of cycles before fatigue failure
Corrosion Factors effecting Fatigue Life?
Corrosion destroys the smooth surface of metal
Tis exposes the metals underlying sharp crystalline structure
Ehen corrosion occurs, it provides a natural starting point for fatigue cracks
What is Elasticity and Plasticity
Elasticity is the tendency of a material to return to its original shape after external load is removed
Plasticity is the tendency of a material to permanently deform when subjected to external load
~When. retrial is easily deformed, its said to have plastic properties and is susceptible to plastic deformation
Reinforcing fibres can be made of?
Glass
Boron
Carbon
Aramid (aka Kevlar)
Matrix is…
The material that supports the fibres, bonds them together and holds them in the desired orientation
Usually a type of thermosetting resin
What’s a core (composite materials)
A material sometimes used in conjunction with fibre mats to create a strong sandwich-type laminate
The stronger the component needs to be the … fibre mats are used
more
can make a component stronger one end than the other by using more/less fibre mats
Composite Materials - Advantages
Fibres can be aligned to provide the best load qualities
Make complex shapes
Lighter
High Strength
High Stiffness
Resistant to corrosion
GOOD STRENGTH TO WEIGHT RATIO
Composite Materials - Disadvantages
Hard to inspect for flaws
May absorb water, which can freeze and cause delamination
Can be expensive
Less damage tolerant
Requires immediate repair
Does not conduct electricity so requires electrical bonding
What do carbon composite a/c have to avoid lightening strikes?
It has an metallic mesh
as the composite material has a low conductivity
Corrosion defintion
Disintegration of an engineered material into its constituent atoms
Types of Corrosion?
Oxidisation
Chemical reaction of metal with air causing a break down of metal surface
Electrolytic
Corrosion resulting from electric current between two metals coupled in an electrolytic environment
Stress Corrosion
Interaction of fatigue & corrosion that occurs in materials under high tensile loads
What is Delamination?
Problem affecting composite structures
If structure is over stressed, the layers begin to separate
This process reduces the strength of the structure
Can be difficult to detect
What process do you use to detect Delamination?
Ultrasound
Infrared Thermal Images
Radiography
Two types of Maintenance
Hard Time Maintenance
On Condition Maintenance
Hard Condition Maintenance
Definitive time limit on component
Failure preventative be removal from service before safe life expires
Service inspected on the last service before expiry
Time period based on:
Calendar time
Landings
Operating cycles
Flight hours
Block times
E.g. Landing Gear
On Condition Maintenance
Periodic Inspection of component, checking against known standard or wear limit
Post check, the component either continues in service or is replaced
Components that fail the check must be overhauled
What is Condition Monitoring?
Process of monitoring a parameter of condition in a a/c system (E.g. vibration, temp etc) in order to identify changes with could lead to faults
It allows maintenance to be scheduled as the a/c sends data to maintenance team
~Maintenance could be moved forward if it looks like a component will fail sooner than the actual maintenance time slot
