On January 31, 2000,
Alaska Airlines Flight 261 departed Puerto Vallarta,
Mexico, heading for Seattle, WA, with a short stop scheduled
in San Francisco, CA. Approximately one hour and 45 minutes
into the flight, a problem was reported with the plane's
stabilizer trim. After a 10-minute battle to keep the
plane airborne, it plunged into the Pacific Ocean off the
coast of California. All 88 people onboard were killed.
Photo courtesy U.S.
Department of Defense
cockpit voice recorder from the downed Alaska Airlines
Flight 261, held by the robotic arm of the remotely
piloted vehicle that retrieved
With any airplane crash, there are many unanswered
questions as to what brought the plane down. Investigators
turn to the airplane's flight data recorder (FDR) and
cockpit voice recorder (CVR), also known as "black
boxes," for answers. In Flight 261, the FDR contained 48
parameters of flight data, and the CVR recorded a little more
than 30 minutes of conversation and other audible cockpit
Following any airplane accident in the United States,
safety investigators from the National
Transportation Safety Board (NTSB) immediately begin
searching for the aircraft's black boxes. These
recording devices, which cost between $10,000 an $15,000 each,
reveal details of the events immediately preceding the
accident. In this edition of HowStuffWorks,
we will look at the two types of black boxes, how they survive
crashes, and how they are retrieved and analyzed.
Recording and Storage
Brothers pioneered the use of a device to record propeller
rotations, according to documents provided by L-3
Communications. However, the widespread use of aviation
recorders didn't begin until the post-World War II era. Since
then, the recording medium of black boxes has evolved in order
to record much more information about an aircraft's operation.
Most of the black boxes in use today use magnetic
tape, which was first introduced in the 1960s, or
solid-state memory boards, which came along in the
1990s. Magnetic tape works like any tape
recorder. The Mylar tape is pulled across an electromagnetic
head, which leaves a bit of data on the tape. Black-box
manufacturers are no longer making magnetic tape recorders as
airlines begin a full transition to solid-state technology.
Solid-state recorders are considered much more reliable
than their magnetic-tape counterparts, according to Ron
Crotty, a spokesperson for Honeywell,
a black-box manufacturer. Solid state uses stacked
arrays of memory
chips, so they don't have moving parts. With no moving
parts, there are fewer maintenance issues and a decreased
chance of something breaking during a crash.
Data from both the CVR and FDR is stored on stacked
memory boards inside the crash-survivable memory
unit (CSMU). In recorders made by L-3 Communications, the
CSMU is a cylindrical compartment on the recorder. The stacked
memory boards are about 1.75 inches (4.45 cm) in diameter and
1 inch (2.54 cm) tall.
The memory boards have enough digital storage space to
accommodate two hours of audio data for CVRs and 25 hours of
flight data for FDRs.
Airplanes are equipped with sensors that gather data. There
are sensors that detect acceleration, airspeed, altitude, flap
settings, outside temperature, cabin temperature and pressure,
engine performance and more. Magnetic-tape recorders can track
about 100 parameters, while solid-state recorders can track
more than 700 in larger aircraft.
All of the data collected by the airplane's sensors is sent
to the flight-data acquisition unit (FDAU) at the front
of the aircraft. This device often is found in the
electronic equipment bay under the cockpit. The
flight-data acquisition unit is the middle manager of the
entire data-recording process. It takes the information from
the sensors and sends it on to the black boxes.
Both black boxes are powered by one of two power generators
that draw their power from the plane's
engines. One generator is a 28-volt DC power source, and
the other is a 115-volt, 400-hertz (Hz) AC power source. These
are standard aircraft power supplies, according to Frank
Doran, director of engineering for L-3
Communications Aviation Recorders.
Cockpit Voice Recorders
In almost every
commercial aircraft, there are several microphones
built into the cockpit to track the conversations of the
flight crew. These microphones are also designed to track any
ambient noise in the cockpit, such as switches being thrown or
any knocks or thuds. There may be up to four microphones in
the plane's cockpit, each connected to the cockpit voice
Any sounds in the cockpit are picked up by these
microphones and sent to the CVR, where the recordings are
digitized and stored. There is also another device in the
cockpit, called the associated control unit, that
provides pre-amplification for audio going to the CVR. Here
are the positions of the four microphones:
- Pilot's headset
- Co-pilot's headset
- Headset of a third crew member (if there is a third crew
- Near the center of the cockpit, where it can pick up
audio alerts and other sounds
Most magnetic-tape CVRs store the last 30 minutes of sound.
They use a continuous loop of tape that completes a cycle
every 30 minutes. As new material is recorded, the oldest
material is replaced. CVRs that used solid-state storage can
record two hours of audio. Similar to the magnetic-tape
recorders, solid-state recorders also record over old
Final Words of Flight
CVR recordings can hold important clues to the
cause of an accident. In the case of Alaska Airlines Flight
261, the conversations between the captain and his first
officer pointed NTSB investigators to the plane's stabilizer.
This is an excerpt taken from the official NTSB transcript of
Flight 261, which crashed on January 31, 2000, off the coast
of California. This excerpt contains an exchange between
Captain Ted Thompson and First Officer William
Tansky and the Los Angeles Route Traffic Control
Center, Alaska two-sixty-one. We are, uh, in a dive
here, and I've lost control, vertical
Yea, we got it back under control here.|
Whatever we did is no good. Don't do that
Yea, no, it went down. It went full nose
Uh, it's a lot worse than it was?|
Yea. Yea. We're in much worse shape now.|
address: Folks, we have had a flight-control problem
up front here, we're working on it.|
||Flight 261 to
LAX-CTR: L.A., Alaska two-sixty-one. We're with you,
we're at twenty-two-five [22,500 feet]. We have a jammed
stabilizer and we're maintaining altitude with
Alaska two-sixty-one, L.A center. Roger, um, you're
cleared to Los Angeles Airport via present
attendant: Okay, we had like a big bang back
I think the [stabilizer] trim is broke.|
Okay, we are inverted, and now we gotta get
Push, push, push...push the blue side up.
Okay, now let's kick rudder. Left rudder, left
can't reach it.|
Okay. Right rudder, right rudder.|
Are we flying? We're flying, we're flying. Tell 'em what
Oh, yeah. Let me get...|
Gotta get it over again. At least upside down we're
Ah, here we go.|
here to read the full transcript (PDF) of Flight 261.
Flight Data Recorders
The flight data
recorder (FDR) is designed to record the operating data
from the plane's systems. There are sensors that are wired
from various areas on the plane to the flight-data acquisition
unit, which is wired to the FDR. When a switch is turned on or
off, that operation is recorded by the FDR.
In the United States, the Federal Aviation
Administration (FAA) requires that commercial airlines
record a minimum of 11 to 29 parameters, depending on the size
of the aircraft. Magnetic-tape recorders have the potential to
record up to 100 parameters. Solid-state FDRs can record more
than 700 parameters. On July 17, 1997, the FAA issued a Code
of Federal Regulations that requires the recording of at least
88 parameters on aircraft manufactured after August 19, 2002.
Here are a few of the parameters recorded by most FDRs:
- Pressure altitude
- Vertical acceleration
- Magnetic heading
- Control-column position
- Rudder-pedal position
- Control-wheel position
- Horizontal stabilizer
- Fuel flow
Solid-state recorders can track more parameters than
magnetic tape because they allow for a faster data flow.
Solid-state FDRs can store up to 25 hours of flight data. Each
additional parameter that is recorded by the FDR gives
investigators one more clue about the cause of an accident.
Built to Survive
In many airline accidents,
the only devices that survive are the crash-survivable
memory units (CSMUs) of the flight data recorders and
cockpit voice recorders. Typically, the rest of the recorders'
chassis and inner components are mangled. The CSMU is a large
cylinder that bolts onto the flat portion of the recorder.
This device is engineered to withstand extreme heat, violent
crashes and tons of pressure. In older magnetic-tape
recorders, the CSMU is inside a rectangular box.
Using three layers of materials, the CSMU in a solid-state
black box insulates and protects the stack of memory boards
that store the digitized information. We will talk more about
the memory and electronics in the next section. Here's a
closer look at the materials that provide a barrier for the
memory boards, starting at the innermost barrier and working
our way outward:
- Aluminum housing - There is a thin layer of
aluminum around the stack of memory cards.
- High-temperature insulation - This dry-silica
material is 1 inch (2.54 cm) thick and provides
high-temperature thermal protection. This is what keeps the
memory boards safe during post-accident fires.
- Stainless-steel shell- The high-temperature
insulation material is contained within a stainless-steel
cast shell that is about 0.25 inches (0.64 cm) thick.
Titanium can be used to create this outer armor as well.
To ensure the quality and survivability of black boxes,
manufacturers thoroughly test the CSMUs. Remember, only the
CSMU has to survive a crash -- if accident investigators have
that, they can retrieve the information they need. In order to
test the unit, engineers load data onto the memory boards
inside the CSMU. L-3 Communications uses a random pattern to
put data onto every memory board. This pattern is reviewed on
readout to determine if any of the data has been damaged by
crash impact, fires or pressure.
There are several tests that make up the crash-survival
- Crash impact - Researchers shoot the CSMU down an
air cannon to create an impact of 3,400 Gs (1 G is the force
of Earth's gravity, which determines how much something
weighs). At 3,400 Gs, the CSMU hits an aluminum, honeycomb
target at a force equal to 3,400 times its weight. This
impact force is equal to or in excess of what a recorder
might experience in an actual crash.
- Pin drop - To test the unit's penetration
resistance, researchers drop a 500-pound (227-kg) weight
with a 0.25-inch steel pin protruding from the bottom onto
the CSMU from a height of 10 feet (3 m). This pin, with
500-pounds behind it, impacts the CSMU cylinder's most
- Static crush - For five minutes, researchers
apply 5,000 pounds per square-inch (psi) of crush force to
each of the unit's six major axis points.
- Fire test - Researchers place the unit into a
propane-source fireball, cooking it using three burners. The
unit sits inside the fire at 2,000 degrees Fahrenheit (1,100
C) for one hour. The FAA requires that all solid-state
recorders be able to survive at least one hour at this
- Deep-sea submersion - The CSMU is placed into a
pressurized tank of salt water for 24 hours.
- Salt-water submersion - The CSMU must survive in
a salt water tank for 30 days.
- Fluid immersion - Various CSMU components are
placed into a variety of aviation fluids, including jet
fuel, lubricants and fire-extinguisher chemicals.
During the fire test, the memory interface cable
that attaches the memory boards to the circuit board is burned
away. After the unit cools down, researchers take it apart and
pull the memory module out. They restack the memory boards,
install a new memory interface cable and attach the unit to a
readout system to verify that all of the preloaded data is
Black boxes are usually sold directly to and installed by
manufacturers. Both black boxes are installed in the tail of
the plane -- putting them in the back of the aircraft
increases their chances of survival. The precise location of
the recorders depends on the individual plane. Sometimes they
are located in the ceiling of the galley, in the aft cargo
hold or in the tail cone that covers the rear of the aircraft.
"Typically, the tail of the aircraft is the last portion of
the aircraft to impact," Doran said. "The whole front portion
of the airplane provides a crush zone, which assists in the
deceleration of tail components, including the recorders, and
enhances the likelihood that the crash-protected memory of the
recorder will survive."
After a Crash
Although they are called
"black boxes," aviation recorders are actually painted bright
orange. This distinct color, along with the strips of
reflective tape attached to the recorders' exteriors, help
investigators locate the black boxes following an accident.
These are especially helpful when a plane lands in the water.
There are two possible origins of the term "black box": Some
believe it is because early recorders were painted black,
while others think it refers to the charring that occurs in
In addition to the paint and reflective tape,
black boxes are equipped with an underwater locator
beacon (ULB). If you look at the picture of a black box,
you will almost always see a small, cylindrical object
attached to one end of the device. While it doubles as a
handle for carrying the black box, this cylinder is actually a
If a plane crashes into the water, this beacon sends out an
ultrasonic pulse that cannot be heard by human ears but
is readily detectable by sonar and acoustical locating
equipment. There is a submergence sensor on the side of
the beacon that looks like a bull's-eye. When water touches
this sensor, it activates the beacon.
The beacon sends out pulses at 37.5 kilohertz (kHz) and can
transmit sound as deep as 14,000 feet (4,267 m). Once the
beacon begins "pinging," it pings once per second for 30 days.
This beacon is powered by a battery
that has a shelf life of six years. In rare instances, the
beacon may get snapped off during a high-impact collision.
In the United States, when investigators locate a black box
it is transported to the computer labs at the National
Transportation Safety Board (NTSB). Special care is taken
in transporting these devices in order to avoid any (further)
damage to the recording medium. In cases of water accidents,
recorders are placed in a cooler of water to keep them from
Photo courtesy U.S.
Department of Defense
U.S. Navy Lieutenant Junior Grade Jason S.
Hall (right) watches as FBI Agent Duback (left) tags the
cockpit voice recorder from EgyptAir Flight 990 on
"What they are trying to do is preserve the state of the
recorder until they have it in a location where it can all be
properly handled," Doran said. "By keeping the recorder in a
bucket of water, usually it's a cooler, what they are doing is
just keeping it in the same environment from which it was
retrieved until it gets to a place where it can be adequately
After finding the black boxes,
investigators take the recorders to a lab where they can
download the data from the recorders and attempt to recreate
the events of the accident. This process can take weeks or
months to complete. In the United States, black-box
manufacturers supply the NTSB with the readout systems and
software needed to do a full analysis of the recorders' stored
If the FDR is not damaged, investigators can simply play it
back on the recorder by connecting it to a readout system.
With solid-state recorders, investigators can extract stored
data in a matter of minutes. Very often, recorders retrieved
from wreckage are dented or burned. In these cases, the memory
boards are removed, cleaned up and a new memory interface
cable is installed. Then the memory board is connected to a
working recorder. This recorder has special software to
facilitate the retrieval of data without the possibility of
overwriting any of it.
A team of experts is usually brought in to interpret the
recordings stored on a CVR. This group typically includes a
representative from the airline, a
representative from the airplane manufacturer, an NTSB
transportation-safety specialist and an NTSB air-safety
investigator. This group may also include a language
specialist from the Federal
Bureau of Investigation and, if needed, an interpreter.
This board attempts to interpret 30 minutes of words and
sounds recorded by the CVR. This can be a painstaking process
and may take weeks to complete.
Both the FDR and CVR are invaluable tools for any aircraft
investigation. These are often the lone survivors of airplane
accidents, and as such provide important clues to the cause
that would be impossible to obtain any other way. As
technology evolves, black boxes will continue to play a
tremendous role in accident investigations.
Lots More Information!