13.2 Basic Structures Flashcards
What are the 4 common certification specifications for aircraft types?
CS–23 (Normal, utility, aerobatics and commuter)
CS–25 (large aeroplanes)
CS–27 (small rotorcraft)
CS–29 (large rotorcraft)
What is the limit load condition?
Maximum loads anticipated during normal operation of the aircraft
How do negative load limits usually compare to positive load limits?
Usually less than
What is the ultimate load?
The load that a component must withstand with a positive margin from the limit load
How long must a structure be able to support ultimate loads without failure?
At least 3 seconds
Which areas of aircraft structure will normally be coloured:
Red
Yellow
Green
Red – Primary structure
Yellow – Secondary structure
Green – Tertiary structure
What conditions will signify when a structure is primary?
Catastrophic structural collapse or loss of control
Power unit failure
Injury to occupants
Unintentional operation of or inability to operate essential equipment or services
What is a Principle Structural Element (PSE) defined by?
Components which contribute significantly to carrying flight, ground and pressurisation loads
Failure could result in catastrophic failure of the aeroplane
What is a fail safe structure described as?
A structure having multiple load paths, providing redundancy
How is the inspection cycle determined in damage tolerance and fail safe?
If a crack of detectable length is missed on first inspection, it will be picked up in the second inspection before it becomes too dangerous
What is ‘notching’ and what is it sometimes referred to as?
Abrupt change in cross–sectional area
stress raiser
Why are additional strengthening frames and doublers often used at fuselage cut–outs such as windows and doorways?
Absorb the stress normally carried by the skin and stringers
Transferred from the door by its hinges and stops
Why is a doubler usually tapered or chamfered?
Doubler is stronger than the original skin
It can transfer loads at its edges greater than the skin was designed to take
What are the three indicators which signify zones?
Major zone
Major sub–zone
Zone
What are the major zones?
100 – Lower half of the fuselage to the rear pressure bulkhead
200 – Upper half of the fuselage to the rear pressure bulkhead
300 – Empennage, including fuselage aft of the rear pressure bulkhead
400 – Power plants and struts or pylons
500 – Left wing
600 – Right wing
700 – Landing gear and landing gear doors
800 – Doors
900 – Reserved for uncommon differences between aircraft types not covered by standard series numbers
What is the difference between the wing buttock lines and the wing station lines?
Stations run perpendicular to the leading edge
Buttock lines run parallel to the Body Buttock Line
What are ties?
Generally, thin solid members used to take tension
What is circumferential stress also known as?
Hoop stress
What is hoop stress?
Fuselage structure subjected to a stress that is caused by the difference in pressure between the inside and outside of the pressurised cabin area
What is the relative proportion between longitudinal stress and hoop stress?
Longitudinal stress is always less than the hoop stress
Hoop stress is the same pressure over a larger surface area
What is cyclic fatigue caused by?
Repeated fluctuating loads
What causes fretting fatigue?
Small scale rubbing movements and abrasion of adjacent parts
What causes thermal fatigue?
Stress fluctuations caused by thermal expansions and contractions
In aviation, how is fatigue testing carried out on aircraft?
Carried out on an aircraft structure taken from the production line
How is air loss prevented from certain types of drain valve used on pressurised aircraft?
Pressurised air within the cabin keeps the valves closed
What type of material for vent pipes is used for lead/acid batteries?
Plastic
What type of material for vent pipes is used for alkaline batteries?
Stainless steel
What is the maximum permitted concentration of carbon monoxide in the cabin?
50 ppm
1 part in 20,000
What is the maximum permitted concentration of carbon dioxide in the cabin?
0.5% by volume
What operates the ground crew call horn in the nosewheel bay with regards to equipment cooling?
Thermal switches in the Electrical and Electronic compartment which sense over temperature on the ground
What does the venturi component in a toilet and galley ventilation system do?
Limits the outflow of cabin air when aircraft is pressurised
What is a piccolo tube?
Tube with many small holes
What are NACA air intakes designed for?
To admit or collect the required mass or pressure of air
What is the purpose of the blowout or vent panel fitted between the side of the cabin floor and cargo compartment?
Prevents distortion of the floor if a rapid depressurisation occurs
At what value of differential pressure is a blowout panel or vent panel designed to operate?
1 lb/in²
What dangers are caused by differences of potential or lightning strikes?
Damage to the aircraft structure
Fire hazard
Radio interference
Electrical shocks
Corrosion – (electrochemical action)
When should a Bonding Test be carried out?
When it is called for in the AMM
After refitting of major components eg tailplanes, fin, control surfaces
If bonding jumpers or leads are replaced
After reports of electric shock from the structure
After replacement or modification to electrical systems
What stress is: a) the skin and b) the framework onto which the skin is attached, designed to take?
a) Skin – tension
b) Framework – Compressive (and bending)
What is the difference between frames and formers?
Frames – heaviest vertical member, carry concentrated loads, used to attach other units
Formers – lighter vertical member, give shape to the fuselage
What are bulkheads?
Complete diaphragms which separate one section of the fuselage from another
What is the difference between longerons and stringers?
Longerons – carry primary bending loads
Stringers – smaller and lighter, give shape and rigidity to the skin
What is a cleat?
Alloy bracket used to attach the stringer to the frames and formers
Also potentially to the skin
How are floor beams fitted?
Transversely
What is the key difference between longerons/stringers and intercostals?
Intercostals do not pass through frames or formers
How are crack stoppers placed?
Normally at right angles to the path of an anticipated crack which will reduce the rate of further propagation
What is a fish plate used for?
Local strengthening at the intersection between a rib and a stringer or a frame and a stringer
What is the purpose of lightening holes?
Flanged holes increase stiffness and decrease weight
Easier routing of control cables, pipelines and electrical cables
What are the alternative names for a production break?
Transportation joint
Primary splice
What is the purpose of a production break?
Makes building simpler
Allows future versions to be stretched by the addition of an extra section
Wherever possible, what is the force encountered by bolts and rivets?
Shear
What is the grip length of a bolt?
Plain shank length
Approximately equal to the thickness of the materials being fastened together, plus the thickness of any washer under the bolt
Where is a close tolerance bolt used?
Joint is subject to frequent load reversal or severe vibration
What are torque load values normally determined by?
Friction
Type of thread
Material
Lubrication
Finish
What is the relative thickness of pure aluminium in alclad sheets?
5%
What is anodising known as?
Surface conversion treatment
How is a component anodised?
Suspended in an electrolyte and made the anode of the electrical circuit which also has a cathode
Why are freshly formed anodic films sealed?
They are porous so the coating is sealed to improve corrosion resistance
What will happen if the sealing of anodised parts is carried out by immersing in hot water?
Avoids the colour change caused by the dichromate solution
What is the resulting colour when a part is sealed by immersing in a hot solution of potassium or sodium dichromate and water?
Yellow/green – sulphuric acid films
Dark green – thick hard films
What is the basic principle of electroplating?
Make the part to be plated the cathode of an electrical circuit and the plating metal the anode, while both are immersed in a suitable electrolyte
At what relative humidity must primers be applied?
Above 30% and below 75%
What may happen very often, where an epoxy primer has been used?
The topcoat may be stripped off leaving the primer intact
What should the temperature and relative humidity of paint shops be kept at?
15°C – 25°C
30% – 75%
What is rigging generally defined as?
Assembly and relative alignment or adjustment of the various components of an aircraft
What is the suitable sequence for checking the rigging of an aircraft?
Wing dihedral angle
Wing incidence angle
Engine mounting alignment
Tailplane dihedral angle
Tailplane incidence angle
Fin verticality
Symmetry check
How is a wing and tailplane incidence check carried out?
Using special incidence boards, supplied by aircraft manufacturer, placed at specified positions on the wing and tailplane
What methods are there for checking fin verticality?
Measuring distance between point on top of fin and given point on the port and starboard tailplanes
Plumb bob
Manufacturer’s fixture
Sighting rods and theodolite
What might be the effect on the tailplane incidence if the aircraft has experienced hogging (bow about the horizontal plane)?
Incidence increases
What is the opposite of hogging (bow about the horizontal plane)?
Sagging
Where are the actual symmetry measurements relating to a given aircraft found?
Aircraft log book
If using steel tape to measure distances between points for measuring symmetry, what should also be used?
Spring balance
5lb usually being sufficient
In what position must the aircraft be in when using plumb bobs to check symmetry?
Rigging position
When would cut countersinking and dimpling be used?
Countersinking – 20 SWG and under is dimpled
18 SWG and thicker is cut countersunk
What is the NACA method of riveting?
Flush riveting
Rivet installed from the inside so that shank protrudes from countersunk hole
Shank fills the countersunk hole and milling machine shaves the formed shanks flush with the skin
Where might rivets be in a small amount of tension?
Negative pressure – top surface of wings
Positive internal pressure – Skin of pressurised fuselages
