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of aircraft damage because the background
cockpit sounds can reveal problem areas of the aircraft
during the time leading up to the accident.
Non-speech data from the CAM can be analyzed with
sound spectrum analysis to detect whirl flutter, as well
as possibly differentiating a bomb explosion from cabin
decompression. Spectrum analysis can also be used to
confirm that the clicks and thumps recorded by the
CAM are simply cockpit controls, and the
sound of the aircraft moving through the air.
Pan Am Flight 103 disintegrated over Lockerbie, Scotland
in 1989 due to a bomb explosion.
Aircraft Accident Investigation 25
Speech information recorded by the CVR can be analyzed
with spectrum analysis in order to match the recorded
voices to the appropriate person.
To further understand sound spectrum analysis, you
must first understand the physics of sound.
Sound is the vibration of any substance. Sound is processed
in the form of waves. A wave is a disturbance
that travels through a medium. The most common medium
that sound waves travel through is air, but it may
also travel through substances such as water, metal, or
wood. The amplitude of a sound is the height of the
wave. Loud sounds will have higher waves than softer
waves, resulting in higher amplitude. Sounds are generally
measured in cycles, or frequencies.
Sound may be represented graphically as a waveform,
spectral plot, sonogram, or spectrograph (spectrogram).
Spectrographs are the graphical representations used
commonly in sound spectrum analysis be cause it presents
sounds in a three-dimensional form and it shows a
clearer visual of how the amplitudes of various components
of a sound change.
Sound spectrum analysis is performed with the aid of a
personal computer and specialized spectral analysis
software. The audio information recorded from the
CVR is loaded to the software program, which displays
the information in a graphical representation. Each
channel from the CVR can be separated to analyze each
section of audio information if necessary.
Spectrographs can display data in color and in black
and white.
As previously mentioned aircraft damage can be assessed
effectively with the use of a sound spectrum
analysis. The National Transportation Safety Board
(NTSB)’s Sound Spectrum Group has assisted with
many major accident investigations by analyzing the
sounds obtained from the CVR and CAM. Such accidents
that the sound spectrum group have worked on
include American Airlines, Flight 587, in Belle Harbor,
New York, and Egypt Air, Flight 990, off the coast of
Nantucket, Massachusetts.
American Airlines Flight 587
American Airlines, Flight 587, crashed shortly after
take off from John F. Kennedy International Airport on
November 12, 2001. The aircraft encountered wake
turbulence forces from the aircraft that departed just
before flight 587, and the vertical tail of the aircraft
separated from it and landed over two miles from the
main site of impact.
The NTSB’s Sound Spectrum Group examined the
CVR to document any signals of airframe vibration or
flutter. In order to examine this, the team had to analyze
the sound of the aircraft while it moved through
the air. The airframe will vibrate at a resonant frequency
during normal flight. An airframe vibration of
the aircraft might change the constant vibration or
change the normal steady background noise recorded on
the CVR. The team found that the vibration of the aircraft
remained relatively constant, and the only change
in vibration occurred during the retraction of the landing
gear, flaps, and slats.
An engine from Flight 587.
Another technique was used to examine airframe vibration,
which involved a low pass filter applied to the
Aircraft Accident Investigation 26
CVR recording. A signal processor calculated the frequency
content of the low pass signal that was passed
through it. Neither of the two methods identified airframe
vibrations or flutter associated with flight 587.
The final examination by the Sound Spectrum Group
was to document unknown or unusual sounds in the
cockpit or from the aircraft. There were many sounds
recorded including thumps, clicks, squeaks, rattles, etc.
These sounds were later identified as the movements of
items in the cockpit during the wake turbulence. The
team did not identify any sounds that could be associated
with the tail separation of the aircraft.
Egypt Air Flight 990
Landing in LAX earlier during the day of the accident.
In order to examine the phrases spoken, the sound spectrum
　
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