Journal of Accident Investigation

A Mathematical Cross-Correlation for Time-Alignment of
Cockpit Voice Recorder and Flight Data Recorder Data
Joseph A. Gregor, National Transportation Safety Board
ABSTRACT
A new method is described for performing timing correlations between flight data
recorder and cockpit voice recorder information. This method involves the use of the crosscorrelation
function to “search” the typically larger FDR data file for a best match to the
event pattern present in the CVR data file. The results of this search give a first-order
estimate of the time differential between identical events as recorded on both units. A simple
curve fit may then be employed to obtain a general conversion from time as represented in
the CVR, tcvr, and time as represented in the FDR, tfdr.
INTRODUCTION
Investigators are often called upon to utilize electronic data acquired from multiple
sources in the course of the data collection and analysis phase of an accident investigation.
The two most common sources of electronic information available in the event of a major
aviation accident are the flight data recorder (FDR) and the cockpit voice recorder (CVR)
although many other sources of data are present on modern-day aircraft, and these other
sources are being utilized with increasing frequency. Currently, most on-board data sources
have one negative characteristic in common: they typically operate on an independent
time-base. In addition, virtually all recorded data are currently acquired and stored without a
time-stamp. As a result, there is usually little or no independent information on the relative
or absolute time at which an electronically “recorded” event occurred. All that is known
is the relative time between the beginning of each recording, and the time at which the
event occurred. A key first step in the accident investigation process is the synchronization
of all collected electronic data to a common time-base, followed by a determination of the
relationship between this elapsed “ship-time” and an appropriate local or global clock (local
time or UTC 1 ). This task is typically accomplished by matching the electronic signature of
1 Coordinated Universal Time - equivalent to mean solar time at the prime meridian (0° longitude).
NTSB JOURNAL OF ACCIDENT INVESTIGATION, SPRING 2006; VOLUME 2, ISSUE 1 43