Journal of Accident Investigation

JOSEPH GREGOR
a common event or series of events recorded on each electronic
data source to obtain a common elapsed time-base. One of
these events, occurring at a known local or UTC time, is then
used to translate from elapsed time to local time or UTC.
THE DATA REDUCTION PROCESS
FDR information is typically recorded in digital form as a
continuous series of interleaved samples representing data
obtained from various sensors located aboard the aircraft.
These sensors are used to measure various flight and aircraft
systems parameters such as airspeed, altitude, pitch, roll, and
yaw. Sampled integer data are interleaved and organized into
groups – or “subframes” – and identified by a subframe reference
number (SRN). A subframe is similar to the type structure in
an object-oriented programming language. The parameter data
within each subframe are akin to the individual fields within the
structure. A typical FDR can record 64, 128, or 2 6 samples
per subframe, and store anywhere from 2 to 100 hours of flight
data. Newer installations will be capable of storing even larger
subframes. In a typical installation, each subframe represents
data acquired over a one-second timeframe. Some parameters
are sampled only once per second, so that one sample of data
from this source will appear within each subframe. Some
parameters are sampled several times per second, yielding
several samples within each subframe. A few parameters are
sampled at a rate less than once per second, and these data will
only be present in a certain subset of the available subframes.
Individual parameters will always appear in the same relative
order within each subframe. This order, together with the SRN
number, 2 may be used to determine the acquisition time of the
data represented by any parameter.
CVR information may be recorded in either analog or digital
form, depending on the model of recorder employed. If in analog
form, the information is digitally sampled in the laboratory as
step one of the data reduction process. CVR information is
eventually expressed in the form of digital WAV 3 files containing
audio information sampled at a rate of ≥ 22,050 Sa/s. 4 A
typical CVR recording contains three to four tracks of audio
from independent sound transducers (microphones). Older
units employ magnetic tape as the storage medium, recording
30 minutes of audio simultaneously from each transducer in a
closed-loop system wherein older information is overwritten
by new information. Newer units employ semiconductor flash
memory. These units typically include two tracks of audio
2 A number representing the number of subframes since the beginning
of the FDR recording. The current standard creates and stores one
subframe of data every second.
3 A file format developed by Microsoft and used extensively in Microsoft
Windows for the recording, storage, and playback of audio sound.
4 Sa/s = samples per second.
information recorded for 2 hours on a simulated closed-loop
system, in addition to four tracks of higher fidelity 30-minute
recordings representing the last half-hour of operation.
Once the CVR and FDR information is expressed in digital
form, it is important to know which sample within the FDR
data corresponds to a given sample in the CVR data. Both
units operate in independent, uncorrelated time-bases and
the data is generally not time stamped. As a result, there is no
straightforward way to correlate the data between these two
units. The method generally employed is to compare a sample
from each source known to coincide with an identical recorded
event. The event most easily exploited in this way is an
activation of the microphone switch for initiation of an external
radio transmission. This so-called ‘mic-key’ event is recorded as
a Boolean variable in the FDR, sampled at a rate of 1 Sa/s. This
means that within each subframe there will be one datapoint
indicating the state of the microphone switch at the time the
sample was taken. The corresponding radio transmission will
appear as an audio signal on one or more tracks of the CVR
recording. Often, an electronic artifact (transient) coinciding
with the activation of the microphone key will also be present in
the recorded audio. The sample number for this “event” within
the digitized CVR data may be compared with the corresponding
FDR SRN to determine the offset time between these two units.
Assuming that both time-bases operate at the same rate, all
that remains is to identify an external event recorded on either
unit for which the time of occurrence is known. One example
would be an external radio transmission recorded on both the
CVR and by an external FAA facility. The local or UTC time
assigned to the ATC recording of this transmission may then
be used to convert from CVR / FDR time to the appropriate
global time.
Several additional issues present themselves when making
any real-world attempt at obtaining a mapping between the
recorded CVR and FDR data. First, in the case of a tape-based
analog CVR, variations in tape speed may cause the time-base
within the unit to operate at an effective data rate different than
that within the FDR. In fact, the effective CVR data rate may
vary over the course of the recording if power fluctuations or
mechanical difficulties within the tape drive mechanism occur.
Second, pilots typically make many external radio transmissions
during the course of a flight. This can lead to a large number
of recorded microphone keying events. Depending on the
integrity of the FDR data, there may be some uncertainty in
the position of the accident flight (the SRN range) within what
can be an extremely large data file. Both factors may lead to
Air Traffic Control.
44 NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1