AIRFRAME DESIGN AND MATERIALS

Question 1

For which a/cc Certification Standard 23 (CS-23) applies?

Answer 1

• Sets standards for Normal, Utility, Aerobatic & Commuter aircraft

Question 2

Which a/cc Certification Standard 25 (CS-25) applies?

Answer 2

• Applies to turbine powered large aircraft

Question 3

Cs25 certification standards?

Answer 3

• Minor Failure Conditions may be Probable
• Major Failure Conditions must be no more frequent than Remote
• Hazardous Failure Conditions must be no more frequent than Extremely Remote
• Catastrophic Failure Conditions must be Extremely Improbable

Question 4

What is the safe life of an a/c?

Answer 4

an aircraft which can comfortably sustain safe

operations within its designed life span

Question 5

Safe life may be measured in:

Answer 5

• Flight Hours
• Pressurisation Cycles
• Landings, or
• Calendar Duration

Question 6

What is multiple Load paths ?

Answer 6

Multiple Load Paths ensure that if one path fails, an alternate load path can carry
all the loads through the duplicate structure

Question 7

What is the damage tolerance design concept?

Answer 7

Some excess weight can be avoided by designing structures to be ‘Damage Tolerant’

• Damage tolerant structures have crack-arresting features which limit extent to which a crack can spread.
• Surrounding load paths carry all the load until the crack is detected

Question 8

Differences between Dynamic loads,static loads and cyclic loads?

Answer 8

Dynamic Loads:
• Loads imposed by manoeuvring
• Can change VERY quickly
• Rapid build up of load can impose severe stresses on materials

Static loads
Build up slowly or remain constant
• e.g. When parked, the aircraft’s weight imposes load on the
wings spars from the landing gear

Cyclic loads
During flight loads can change and alternate rapidly
• Components experience changes from compression to tension
• This cyclic application of loads can lead to Metal Fatigue

Question 9

What are the spar lines?

Answer 9

Spar lines are lateral lines that extends from one side to another,like on the wings ,stabilizers

Question 10

What value of design load factor is the ultimate load factor?

Answer 10

1.5 times design load

Question 11

What is the fatigue life ?

Answer 11

The number of cycles required for a metal to fail at a certain stress

Question 12

What is the fatigue strenght?

Answer 12

The Stress at which failure occurs for a given number of cycles is the Fatigue
Strength

Question 13

Classification of structures

Answer 13

i. Primary Critical, Extremely Remote, Catastrophic
ii. Secondary Major, Not to such a critical degree, limit operation
iii. Tertiary Minor, Failure would not be catastrophic

Question 14

How does mass influence fatigue life or also the life cycle?

Answer 14

Increasing by 1% the mass the fatigue life increases by 5 % so it ridiuses the cycles

Question 15

Corrosion factor on fatigue life?

Answer 15

Corrosion destroys the smooth surface of metal
• This exposes the metals underlying sharp crystalline structure
• When corrosion occurs is provides a natural starting point for fatigue cracks

Question 16

What are the material properties thasn a/c should have?

Answer 16

High Strength 
 High Stiffness 
Good Corrosion Resistance 
Good Fatigue Resistance 
Ability to withstand High Temperatures
 Easy for Manufacturing 
Cheap 
Lightweight 
Electrical Conductor

Question 17

Components of composite materials?

Answer 17

Fiber ,matrix ,core

Question 18

Properties of Glass fiber reinforced,graphite fiber reinforced,kevlar reinforced

Answer 18

• Glass Fibre Reinforced
GFRP Glass Fibre Reinforced Plastic. Used in non-load bearing ducts and panels

• Graphite Reinforced
Stronger than fibreglass, but will cause corrosion to Aluminium Alloys when in direct contact

• Kevlar Reinforced
KFRP High tensile strength, low density, resistant to impact damage

Question 19

What is the core made of ?

Answer 19

• Made from very light

material such as foam, or aluminium honeycomb

Question 20

What the matrix is made of?

Answer 20

Resin

Question 21

Advantages of composites?

Answer 21

The Fibres  can aligned to provide the best load qualities 
 Make complex Shapes 
 Lighter 
High strength 
 High stiffness 
 Resistant to corrosion

Question 22

Disadvantages of composites?

Answer 22

Hard to inspect for flaws
May absorb moisture which can freeze and cause delamination
Can be expensive
Less damage tolerant
Requires immediate repair
Does not conduct electricity so requires electrical bonding

Question 23

What is corrosion?

Answer 23

Corrosion is defined as the disintegration of an engineered material into
its constituent atoms

Question 24

3 main types of corrosion?

Answer 24

Oxidisation
Electrolytic
Stress corrosion

Question 25

What is Delamination?

Answer 25

Affecting only composites, and if the structure is overstressed layers can detach and create bubbles between them and reduce the strenght of the structure

Question 26

Two tyoes of maintanance?

Answer 26

Hard Time Maintenance:

• Definitive time limit on component
• Failure preventative be removal from service before safe life expires
• Component may be overhauled or discarded
• Time period based on:
• Calendar time • Landings • Operating cycles Landing Gear would be a example • Flight hours of a ‘Hard Time’ item
• Block times
——-
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