Journal of Accident Investigation

A MATHEMATICAL CROSS-CORRELATION FOR TIME-ALIGNMENT OF COCKPIT VOICE RECORDER AND FLIGHT DATA RECORDER DATA
Figure 9. Scatter plot showing the results of a cross-correlation between FDR on-key/off-key data and corresponding
CVR on-key/off-key data. Times are normalized to remove the calculated shift between CVR and FDR times, and
normalized so that the data begin at time t=0.
CASE STUDY INVOLVING A SLOPE ≠ 1
On May 2, 2002, an Atlantic Coast Airlines Dornier 328,
tail number 429FJ, experienced a smoke-in-the-cockpit
emergency while en route from Greensboro, South Carolina,
to LaGuardia International Airport, New York. Data were
recovered from two flight recorders on-board the aircraft
at the time of the mishap: a Fairchild Model FA-2100-403
solid-state CVR, and an L-3 Communications Fairchild
F-2100 FDR. Attempts at correlating the CVR and FDR data
proved resistant to the standard technique normally employed
in such cases. This technique involved engineers manually
applying a piecewise linear fit to the data. Since both units
were solid-state, the assumption was made that a one “piece” fit
should suffice to characterize the entire data set. If this failed
to work, an additional assumption could be made that one of
the units experienced a momentary power failure, thus causing
a discontinuity in the time-base for that unit. In this case, a
two-piece linear fit could be applied – one covering the period
before the power upset, and one covering the period after the
upset. If this did not satisfy, one could assume two power upset
events, requiring a 3-piece linear fit, and so on. In the case of
the Dornier 328, the number of “pieces” required to effect a
fit across the entire data set threw into question the validity
of the entire procedure. The cross-correlation technique was
developed as an independent method of identifying the most
likely match between these two data sets – independent of any
assumptions on their behavior, save that they should correlate
with one another to some degree, since they represented two
records of the same event.
Data from the Dornier 328 CVR were cross-correlated with
data from the FDR, resulting in the correlation peak illustrated
in figure 10. Note that, while it is far less prominent than the
peak found in figure 6 for a typical CVR/FDR combination
(one yielding to standard correlation techniques), there is still
a clearly identifiable peak. This illustrates the robust nature of
the cross-correlation technique. The broadening and flattening
of the correlation peak are indications that, while a unique
best-match has been found at the specified location, this match
is not perfect.
NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1 1