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:

* 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 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.

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 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
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-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 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 C) 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.

"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."