mod 11.2 airframe structures

airworthiness requirements for large powered turbine requirements is

CS-25

SC -23 is for

aeroplanes with MTOW less than 5670kg and propellers

what is the requirement for safe life

major structural elements replaced after a number of cycles

fail safe design common on most jet aeroplanes means

even if there is a failure there is multiple load paths remaining

bonded and bolted fittings are often called

back to back fittings

fail safe principal requires that damage is detected during

inspection

obvious damage is considered to be part of

safe life inspections

cracks usually develop in several elements at the same time taking alternate load paths weaker this is called

multiple site cracking

damage tolerance requires damage is detected and repaired before

the strength is below its minimum

damage tolerance also considers

eviromental change and accidental damage (discreet)

what are the three classes of structure

primary secondary tertiary

primary structure if damage can lead to

loss of life or failure of aircraft

examples of primary structure

spars, lengerons, engine mounts, stressed skin, ( red in manuals)

example of secondary structure is

flooring (yellow in manuals)

secondary structure often gives the

aerodynamic shape in aeroplane construction

tertiary sttructure are lightly stressed parts such as

wheel covers minor component brackets, faring

identification of structure is normally

primary- red or white

secondary- yellow or hatched

tertiary- green or stripped

what are the three planes of an aircraft

lateral vertical longitudinal

the horizontal planes are called (based on a cross section)

waterlines or z stations

the plane that cuts aeroplanes vertically are known as ( based on a cross section)

buttock lines or x stations

the plane that cuts the aeroplane vertically are known as( based on a side view)

body stations or y stations

what are the three zones an aeroplane is divided into

major zone

major sub zone

unit zone

zone 100

fuse lower section

zone 200

fuse top section

zone 300

stab

zone 400

nacelles

zone 500

left wing

zone 600

right wing

zone 700

landing gear

zone 800

doors

zones are identified by how many digit number

3

what concept is used to design modern aircraft structures

semi - monocoque (load carrying shell)

open sections that allow inspection of stringers and avoid moisture accumulation such as

z or j stations

fail safe designs are achieved using

tear stoppers

spanwise wing and stab skin splices

fatigue failure life of a structural member is usually defined as the time to

initiate a crack that will tend to reduce the ultimate strength of the member

overall fatigue life of the aircraft is the time

repair of the structure is no longer feasible

what scatter factors are used

2 but up to 4

primary structure for transport aircraft are designed based on test results of how many hrs

120,000

if tested for 120,000 hrs and a scatter factor of 2 is used a structure will remain crack free for

60,000 hrs

doublers are used to

reduce stress concentrations around splices cut outs doors windows ect and tear stoppers at frames and longerons

2024-t3 ali has excellent

fatigue strength fracture toughness and notch sensitivity

7075-t6 ali has… and is used for

the highest strength with acceptable toughness…. floor beams stabs spar caps in control surfaes

parts in which residual stress could present what material is used

7075-t73

7075-t73 has superior

stress corrosion resistance and exfoliation corrosion resistance and good fracture toughness

thick sections of 7075-t73 are used during the heat treatment becouse of

possible residual stresses induced in the heat treatment

a typical design stress load limit would be

54,000psi and a desitiy of alli is 101 lbs/in3

minimum thickness for high speed transport wings is due to lighting strikes is about

0.06 inches

minimum thickness on fuse to allow for countersunk rivets is

0.05

for low speed aircraft that loads are low minimum skin can be as low as

0.016 - outerwings and tail cone

0.03 around tanks

control cables are routed through the structure by the use of

grommets, fairleads and pressure seals

places in the aircraft structure that fluids may be expected what is provided

drain ports

for unintended pressure differences and the accumulation of hazardous gasses the structure is supplied with

a means of ventilation

drain valves along the lowest point of the pressure cabin close when

the cabin is pressurised

external drain ports to drain fluid overboard are always

open ( technically heated to)

sometimes a levelling compound is used to prevent

fluid from collecting in cavities and directs fluid to the drain

a lighting strike can be up to how many amps

200,000

direct effects of a lighting strike are

vaporise metal control cables and weld hinges and explode fuel vapours if current arcs around fasteners

indirect effects of a lighting strikes are

potential differences induce transient voltages and destroy unprotected electronics

lighting stirke protection have 3 goals

provide paths so it stays on the exterior

eliminate gaps to prevent arcing especially fuel vapours

protect wiring

some areas are have poor continuity and it is aided by the use of

bonding leads and conductive strips

since composites dont conduct electricity lighting and static protection has to

be built into the structure

to prevent arcing and sparking in metal fasteners in composite structures are

incapsulated with plastic caps or polysulfide coating