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• Where and how did the fire originate
• Where did the fire go (spread)?
• What did the fire involve?
• What was the fire environment?
• What were the results of the fire?
Variables effecting fires
Aircraft Accident Investigation 16
• Time of exposure to the fire
• Temperature of the fire
• Behavior of the flames
• Burning characteristics of aircraft materials
• Thickness of aircraft materials
• Containment – was there any?
• Suppression activities (fire extinguishing agents,
ARFF, etc.)
Sources of fuel
Here is a list of some common sources of fuels contributing
to aircraft fires:
• Aircraft fuel
• Oil
• Hydraulic fluids
• Battery gases
• Cargo
• Waste material
Sources of ignition
Here is a list of common ignition sources of aircraft
fires:
• hot engine section parts
• engine exhaust
• electrical arc
• overhead equipment
• bleed air system
• static discharge
• lightning
• hot brakes / wheels
• friction sparks
• aircraft heaters
• APU
• Inflight galleys
• Ovens / hot-cups
• Smoking materials
Inflight fire vs. Post-impact fire
There are two types of evidence that indicate if a fire
occurred in-flight or post-flight
1. Indirect evidence - these are just clues that aid in
indicating if there might have been an inflight fire:
• extinguishing system actuated
• oxygen masks dropped
• deactivated electrical circuits
2. Direct evidence
• inflight fire effects: if a fire occurs inflight and is
contained be the aircraft structure, it will be indistinguishable
from a ground or post impact fire
unless there is some internal forced ventilation system
that changes the characteristics of the fire.
Most inflight fires, though, eventually burn through
the structure and are exposed to the slipstream.
This adds oxygen to the fire which raises the temperature
of the fire substantially thus melting materials
that would not normally burn in a ground fire
(ground fires usually reach temperatures around
2000°F while inflight fires reach temperatures of
around 3000°F)
STRUCTURAL INVESTIGATION
Types of structural failures
Overstress
The part should have failed (more stress was placed on
the part than it was designed to withstand)
• Pilot induced: aerobatics, over reaction to turbulence,
improper recovery techniques, any other
operation outside of the aircraft’s operating envelope
• Weather induced overstress: excessive gust loading
(turbulence), wind shear
• Wake turbulence induced overstress: downwash,
wingtip vortices
Under-stress
The part should not have failed
• Faulty manufacture: the part did not meet the design
specifications.
• Faulty repair / modification
• Reduction of load bearing capacity: over time,
metal parts may corrode or develop fatigue cracks.
The result of either of these is that the part can no
longer sustain the specified load.
Failures
This Boeing 737, Aloha 243, experienced a catastrophic
failure in flight. Metal fatigue caused a crack to
form in the front section of the fuselage which led to a
rapid decompression in flight along with the tearing
away of a large portion of the fuselage.
Aircraft Accident Investigation 17
Overload failures
The following failures are often associated with an
overstress type of failure
• Ductile material: the most obvious feature of tension
fracture in ductile material is the gross plastic
deformation in the area surrounding the fracture.
The more ductile the material, the more dramatic
will be the necking down of the material on either
side of the fracture
• Brittle material: brittle tension load failures tend to
have their fracture surface oriented 90 degrees to
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
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