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the tension load. There is little if any plastic deformation.
Under-stress failures
The following issues are common to aircraft accidents
involving the under-stress of certain parts
• Fatigue cracking
• Corrosion
• Wear
• Creep (the permanent elongation of a metal part
due to combination of stress and high temperature)
Composites
Construction techniques
• A composite is any non-homogenous material
• the composite most commonly found in structural
applications on aircraft is called carbon fiber reinforced
plastic. This may be found alone or sandwiched
around a metallic or non-metallic honeycomb
structure
Properties / Failures
• Composites do not develop fatigue cracks; they
develop delaminations, which can be hard to find.
• When they fail, they do not fail in a ductile or brittle
manner; they delaminate
Questions to ask while examining parts
• Was the manner of failure consistent with the way
this part was stressed in flight?
• If this part did fail inflight, would that explain the
accident?
AIRCRAFT SYSTEMS
Systems overview
Common factors to all systems
• Supply: involves a source of energy or fluid that
needs to be moved somewhere else (fluid, fuel,
etc.)
• Power: something that moves the supply through
the system (i.e. pump)
• Control: most systems can be controlled, to some
extent, by the cockpit; the control often consists of
an input signal identifying what is desired and a
feedback signal identifying what happened
• Protection: most aircraft systems incorporate protection
devices to prevent the system from destroying
itself (i.e. pressure regulators, fuses, circuit
breakers, etc.)
• Distribution: this provides a means for the systems
medium (i.e. fuel) to be distributed
• Application: the purpose of the system
Component Examinations
The following methods are commonly used when examining
aircraft systems components
• Photograph it – get pictures of what the part looked
like before examining it
• X-ray it – before taking the component apart, consider
an x-ray; this is non-destructive and will provide
a means of examining items that normally
would not be available to inspect even if taken
apart
• Test the part – if possible, add pressure or electricity
to see if the part actually works
• Tear-down analysis – open the part (take apart) for
further examination
• Documentation – write down what has been done
to the part as well as any conclusions about that
part
Specific Systems
Mechanical systems
These usually are associated with pilot controls that are
tied to stick, column, or pedal movements that often
involve mechanical items such as cables, pulleys, rods,
etc.
Cable Systems
Cables are a popular method of transferring mechanical
force somewhere else. They are usually tied into flight
control systems and propulsion control systems
Hydraulic Systems
Hydraulic systems use fluids that enable the function
of:
• Flaps
• Landing gear on larger aircraft
• Certain flight controls
• Brakes
• Other
Aircraft Accident Investigation 18
Pneumatic Systems
Pneumatic systems usually use a form of compressed
gas to power systems such as:
• aircraft pressurization
• air conditioning systems
Fuel Systems
When looking at fuel systems, consider the following
parts for examination:
• Fuel vent systems
• Fuel return lines
• Fuel pumps
• Fuel system contaminants
• Fuel system filters
Electrical System
These systems tend to be slightly more complicated.
Areas to loom at might include:
• circuit breakers
• emergency power sources
• electrical wiring
Combination systems
Several common combination systems found on aircraft
include:
• electromechanical systems
• hydromechanical systems
• pneumomechanical systems
Protection Systems
Common protection systems include:
• Fire protection
• Ice protection
• Anti-skid systems
• Other
Investigation questions about systems
When examining aircraft systems, the investigator
should consider items such as:
• continuity
• integrity
• condition
• system function
• influence on the rest of the aircraft
• influence on the accident causation
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