Black Boxes
How Black Boxes Work
After
the crash, they didn't find a single body for five days.
 |
| The cockpit voice recorder from the downed Alaska Airlines Flight 261, held by the robotic arm of the remotely piloted vehicle that retrieved it. |
It
didn't happen in the early days of the airline industry; it occurred in 2009,
on a fancy modern aircraft controlled by a competent company.
Airplane
accidents are statistical rarities. But when they happen, they're often fatal,
and people want answers as to why their loved ones died.
There
are usually 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.
Following
any airplane accident in the U.S., safety investigators from the National
Transportation Safety Board (NTSB) immediately begin searching for the
aircraft’s 's black boxes.
It
took investigators nearly two years to find the FDR from Flight 447.
The
box had not only survived impact, but also being submerged under nearly 13,000
feet of salty, corrosive seawater.
In
the end, the data proved that pilot error had contributed to a stall that
eventually caused the crash.
These
recording devices, which cost between $10,000 and $15,000 each, reveal details
of the events immediately preceding the accident.
In
this article, we will look at the two types of black boxes, how they survive
crashes, and how they are retrieved and analyzed.
Black Boxes Begin
The widespread use of aviation recorders didn't begin
until the post-World War II era.
|
| An older model flight data recorder from EgyptAir 990, which crashed in 1999. |
Older
black boxes used magnetic tape, a technology that was first
introduced in the 1960's. 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. These days, black boxes use solid-state memory boards,
which came along in the 1990s.
Solid-state
recorders are considered much more reliable than their magnetic-tape
counterparts.
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).
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 such as acceleration, airspeed,
altitude, flap settings, outside temperature, engine performance, and cabin
temperature and pressure.
Magnetic-tape
recorders can track about 100 parameters, while solid-state recorders can track
a lot more.
For
instance, in the Boeing 787, the units can log a whopping 146,000 parameters,
resulting in several terabytes of data for every single flight.
That
incredible load of data is a double-edge sword; it's great for monitoring the
aircraft, but it can overwhelm engineers and maintenance personnel.
To
manage all of that data, they need sophisticated data management software.
Whether
the system is an older version or fully modern, 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.
.
Cockpit Voice Recorders
.
Cockpit Voice Recorders
In almost every commercial aircraft, there are several
microphones built into the cockpit that listen to flight crew conversation.
 |
| The cockpit voice recorder and the flight data recorder that were retrieved from Comair Flight 5191 in 2006. |
There
may be up to four microphones in the plane's cockpit, each connected to the
cockpit voice recorder (CVR).
Microphones
send audio to the CVR, which digitizes and stores the signals.
In
the cockpit, there is also a device called the associated control unit,
which provides pre-amplification for audio going to the CVR.
The
four microphones are place in the pilot's headset, co-pilot's headset, headset
of a third crew member (if there is a third crew member) and near the center of
the cockpit, to 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 use solid-state storage can record two hours of audio. Similar to the
magnetic-tape recorders, solid-state recorders also record over old material.
Flight Data Recorders
The flight data recorder (FDR) is designed to record
the operating data from the plane's systems.
 |
| One of the black boxes recovered from Air France Airbus A330, which crashed into the Atlantic in 2009. |
So
whenever the pilot flips a switch or twiddles a knob, the FDR records each
action.
In
the U.S., 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 hundreds or even thousands more.
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:
·
Time
·
Pressure altitude
·
Airspeed
·
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 recorded by the FDR gives investigators one more clue
about the cause of an accident.
Built to Survive
Airplane crashes are violent affairs.
 |
| The flight recorders from Continental Airlines flight 1404, which slid off the runway during takeoff in Denver, Colo. In 2008. |
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, jarring 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 data.
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's a thin layer of
aluminum around the stack of memory cards.
·
High-temperature
insulation -- This dry-silica material
is 1 inch (2.54 centimeters) 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 centimeters) thick. Titanium can be used to create this outer
armor as well.
These
hardened housings are incredibly important.
Without
adequate protection, all of the flight data would be destroyed.
So
to make sure that data stays safe, engineers attack their black boxes with full
fury to see if their products can withstand extreme abuse.
Testing a CSMU
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 sample data onto the memory boards
inside the CSMU.
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 sequence:
·
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-kilogram) weight with a 0.25-inch
(0.64-centimeter) steel pin protruding from the bottom onto the CSMU from a
height of 10 feet (3 meters). This pin, with 500 pounds behind it, impacts the
CSMU cylinder's most vulnerable axis.
·
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 Celsius) for one hour. The FAA
requires that all solid-state recorders be able to survive at least one hour at
this temperature.
·
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 accounted for.
Black
boxes are usually sold directly to and installed by the airplane 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.
After a Crash
Although they are called "black boxes,"
aviation recorders are actually painted bright orange.
 |
| The Brazilian Navy shows recovered debris of an Air France aircraft lost in mid-flight over the Atlantic Ocean. |
These
are especially helpful when a plane lands in the water.
There
are two possible origins of the term black box: Some believe it's
because early recorders were painted black, while others think it refers to the
charring that occurs in post-accident fires.
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
carrying handle, this cylinder is actually a beacon.
If
a plane crashes into the water, the 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, the beacon is activated.
The
beacon sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep
as 14,000 feet (4,267 meters).
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 U.S. when investigators locate a black box, it's 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 drying out.
Retrieving Information
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.
 |
| The cockpit voice recorder from Russian Sukhoi Superjet 100, which crashed into the side of a mountain in 2012. |
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 through USB or Ethernet ports.
Very
often, recorders retrieved from wreckage are dented or burned. In these cases,
the memory boards are removed, cleaned up and have a new memory interface cable
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 representatives from the airline and airplane
manufacturer, an NTSB transportation-safety specialist and an NTSB air-safety
investigator.
This
group may also include a language specialist from the FBI 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.
The Future of Black Boxes
There are all sorts of potential improvements on the
horizon for black box technology.
Most
obviously, current systems don't record any video of cockpit activity.
 |
| Not just for airplanes: The black box pictured here came from a race car that was involved in a crash in Australia in 2006. |
The
NTSB continues to insist that there's no such thing as having too much
information when investigating plane crashes. At present, video recording is
still on hold.
But
the technology is more than ready. Airbus, for example, installs a Vision
1000 system in all of its helicopters.
The
Vision 1000 camera is mounted behind the pilot's head, where it records video
of the pilot's actions and the cockpit area, as well as the view beyond the
windshield, at four frames per second.
It
weighs about a half a pound and needs only power and a GPS connection for
activation.
Video
isn't the only improvement that's found resistance from the status quo.
Since
2002, some legislators have pushed for the Save Aviation and Flight Enhancement
Act, which would require not one, but two flight recorders, including one that
automatically ejects itself from the plane during an incident.
Such
self-ejecting recorders are easier to locate are less likely to suffer
catastrophic damage. So far, though, the law has not passed Congress.
Black
boxes aren't just for planes. They're now integrated into many types of
vehicles.
You
may even have one in your car, though you don't know about it.
About
90 percent of new cars have event data recorders (EDRs) that track the same
kind of data as airplane black boxes.
The
EDR is ostensibly designed to maintain and monitor the car's safety system, but
accident investigators can and do use EDR data to better understand wrecks ...
and sometimes, to assign blame after an accident.
When
it comes to black boxes mounted to airplanes, it's entirely possible that
they'll go by the wayside.
Instead
of recording to a box, airplanes may soon simply stream all of their essential
data directly to a ground-based station.
These
systems already exist. For example, AeroMechanical Services' FlyhtStream
air-to-ground system sends flight data to a home base via satellite.
Such
systems eliminate the desperate search for a box that may have been destroyed
in a crash, and may be more dependable, too.
For
the moment, though, black boxes are still very much a necessity each and every
day as thousands of planes take to the skies, flying millions of people all
over the world.
Author's Note: How Black Boxes Work
I have a recurring nightmare about zooming through the
skies in a doomed jet.
Each
time, the plane leaves the runway during takeoff and then violently accelerates
straight up into the sky.
I
never get to the end of the dream. Perhaps that's a good thing.
Happily,
aircraft malfunctions are exceedingly rare – statistically speaking, your car
is a whole lot more dangerous.
But
when planes do fall from the sky, it's a relief to have some idea why ... otherwise,
engineers and family members would be left agonizing, wondering why innocent
people died in such an awful manner.
I
hope I'm never part of an accident scene where a black box is necessary.
Unless, of course, it's just in my dreams. – NC
Nathan Chandler, Contributing Writer
Nathan Chandler is a freelance writer and photographer based in Lincoln, Neb. He earned his B.A. from the University of Northern Iowa, and has researched and written about consumer tech for more than 10 years.
Nathan Chandler is a freelance writer and photographer based in Lincoln, Neb. He earned his B.A. from the University of Northern Iowa, and has researched and written about consumer tech for more than 10 years.
https://science.howstuffworks.com/transport/flight/modern/black-box.htm
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