Cockpit Voice Recorders (CVR) and Flight Data Recorders
Large commercial aircraft and some smaller commercial,
corporate, and private aircraft are required by the FAA to be equipped
with two "black boxes" that record information about a flight. Both recorders
are installed to help reconstruct the events leading to an aircraft accident.
One of these, the Cockpit Voice Recorder (CVR), records radio transmissions
and sounds in the cockpit, such as the pilot's voices and engine noises.
The other, the Flight Data Recorder (FDR), monitors parameters
such as altitude, airspeed and heading. The older analog units use one-quarter
inch magnetic tape as a storage medium and the newer ones use digital
technology and memory chips. Both recorders are installed in the most
crash survivable part of the aircraft, usually the tail section.
Each recorder is equipped with an Underwater Locator
Beacon (ULB) to assist in locating in the event of an overwater accident.
The device called a "pinger", is activated when the recorder is immersed
in water. It transmits an acoustical signal on 37.5 KHz that can be
detected with a special receiver. The beacon can transmit from depths
down to 14,000 feet.
Following an accident, both recorders are immediately
removed from the accident site and transported to NTSB headquarters
in Washington D.C. for processing. Using sophisticated computer and
audio equipment, the information stored on the recorders is extracted
and translated into an understandable format. The Investigator-in-Charge
uses this information as one of many tools to help the Safety Board
determine the Probable Cause of the accident.
The Cockpit Voice Recorder
The CVR records the flight crew's voices, as well as
other sounds inside the cockpit. The recorder's "cockpit area microphone"
is usually located on the overhead instrument panel between the two
pilots. Sounds of interest to an investigator could be engine noise,
stall warnings, landing gear extension and retraction, and other clicks
and pops. From these sounds, parameters such as engine rpm, system failures,
speed, and the time at which certain events occur can often be determined.
Communications with Air Traffic Control, automated radio weather briefings,
and conversation between the pilots and ground or cabin crew are also
A CVR committee usually consisting of members from the
NTSB, FAA, operator of the aircraft, manufacturer of the airplane, manufacturer
of the engines, and the pilots union, is formed to listen to the recording.
This committee creates a written transcript of the tape to be used during
the investigation. FAA air traffic control tapes with their associated
time codes are used to help determine the local standard time of one
or more events during the accident sequence. These times are applied
to the transcript using a computer process which provides a local time
for every event on the transcript. More precise timing for critical
events can be obtained using a digital spectrum analyzer. This transcript
contains all pertinent portions of the recording and can be released
to the public at the time of the Safety Board's public hearing.
The CVR recordings are treated differently than the
other factual information obtained in an accident investigation. Due
to the highly sensitive nature of the verbal communications inside the
cockpit, Congress has required that the Safety Board not release any
part of a CVR tape recording. Because of this sensitivity, a high degree
of security is provided for the CVR tape and its transcript. The content
and timing of release of the written transcript are strictly regulated:
under federal law, transcripts of pertinent portions of cockpit voice
recordings are released at a Safety Board public hearing on the accident
or, if no hearing is held, when a majority of the factual reports are
Time recorded .......................... 30 min continuous,
2 hours for solid state digital units
Number of channels ................. 4
Impact tolerance ...................... 3400 Gs /6.5ms
Fire resistance .......................... 1100 deg C /30 min
Water pressure resistance ........ submerged 20,000 ft
Underwater locator beacon ...... 37.5 KHz
Battery: 6yr shelf life
30 day operation
The original article can be found
at the NTSB