The US Airways pilot who last month landed a passenger jet in New York's Hudson River told air safety investigators he ditched the plane in the water to avert the "catastrophic" consequences of falling short of a runway in a built-up area. The reason given for the emergency landing of Flight 1549 — since dubbed the "miracle on the Hudson — was supported only after the discovery of the black boxes confirmed that the plane lost power to both of its engines simultaneously.

In this case, all 155 passengers aboard the Airbus A320 escaped safely. Unfortunately, that is not the rule. In all cases, the airplane's cockpit voice recorder (CVR) and flight data recorder (FDR) — commonly known as the black boxes — are among the crucial components necessary to determine the cause of the crash.

The CVR and FDR are often the sole survivors of airplane accidents. As such, the recording devices reveal otherwise-irretrievable details of the events immediately preceding the accident.

How They Work
The CVR records radio transmissions and sounds in the cockpit, such as the pilot and flight crew's voices as well as engine noises, according to the National Transportation Safety Board (NTSB). "From these sounds, parameters such as engine revolutions per minute (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 recorded," the NTSB says.

The newer CVRs use solid-state technology, so they don't have moving parts and are thus more resistant to shock, vibration and moisture.

The second crucial device, the FDR, records a number of a flight's operating conditions, such as altitude, airspeed and heading. Under current U.S. federal regulations, a newly manufactured aircraft must monitor a minimum of 88 important parameters such as time, altitude, airspeed, heading and aircraft attitude. Some FDRs monitor many more in-flight characteristics. "The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms," according to the NTSB. Generally, each parameter is recorded a few times per second, though some units store data at a much higher frequency if the data begins to change quickly.

Solid-state digital CVR units can record two hours of data, and FDRs record 25 hours of data, according to the NTSB. Solid-state recorders record new material to replace old material. All data collected by the airplane's sensors are sent to the flight-data acquisition unit (FDAU) at the front of the aircraft; HowStuffWorks.com describes the FDAU as "the middle manager" of the entire data-recording process, taking the information from the sensors and sending it to the black boxes.

Both recorders, powered by one of two power generators that draw power from the plane's engines, are installed in the most crash-survivable part of the aircraft, usually the tail section.

How They Survive Crashes
The recorder chassis consists of the two recording devices and a crash-survivable memory unit (CSMU) that stores the data. In a typical accident, the rest of the recorder chassis is destroyed, but the CSMU, a large cylinder that bolts to the flat portion of the recorder, is built to survive extreme heat and tons of pressure.

Generally, the device is fixed in the rear of the aircraft, assuming the typical crashing plane goes in nose first, the forward part of the airframe absorbs most of the impact. The entire front acts like a crush zone and the rear feels the least impact.

In addition to its strategic location, the CSMU uses three layers of materials to insulate and protect the information-storing memory boards: aluminum around the stack of memory cards; high-temperature insulation to keep the memory boards protected in post-accident fires; and a stainless-steel (or titanium) shell containing the insulation material. (Source: HowStuffWorks.com)

How They are Retrieved
Following any airplane accident in the United States, safety investigators from the NTSB immediately begin searching for the aircraft's black boxes.

Each recorder is equipped with a third essential black box tool, the Underwater Locator Beacon (ULB). In the event of an accident over water, the ULB will assist in locating the black boxes. When the recorder is immersed in water, a device called a "pinger" is activated, transmitting an ultrasonic pulse that is readily detectable by sonar and acoustical locating equipment, according to the NTSB, which notes the beacon "can transmit from depths down to 14,000 feet." Once the beacon begins "pinging," it does so once per second for 30 days.

Following an accident, both recorders are removed from the accident site immediately and transported to the computer labs at the NTSB headquarters in Washington, D.C. for processing. Special care is taken in transporting these devices to avoid any (additional) damage to the recording medium. In cases of water accidents, recorders are placed in a cooler of water to keep them from drying out.

"Using sophisticated computer and audio equipment, the information stored on the recorders is extracted and translated into an understandable format," the NTSB says. At this point, a team of experts is usually brought in to interpret the stored recordings.

It may take weeks to interpret just 30 minutes of words and sounds recorded by the CVR. Despite the painstaking process, today the black boxes have been universally adopted as a means to investigate accidents and prevent future recurrence.

Resources

Cockpit Voice Recorders (CVR) and Flight Data Recorders (FDR)
National Transportation Safety Board

Cockpit Voice Recorder Handbook for Aviation Accident Investigations
NTSB Office of Research and Engineering Office of Aviation Safety

Flight Data Recorder Handbook for Aviation Accident Investigations
NTSB Office of Research and Engineering Office of Aviation Safety

How Black Boxes Work
by Kevin Bonsor
HowStuffWorks.com

Airline Black Boxes Get a New Role: Reassurance
by Christine Negroni
International Herald Tribune, Dec. 16, 2008

How Airplane Black Boxes Survive Crashes
by April Holladay
WonderQuest, Feb. 16, 2008 (last updated)

Cockpit Tape Reveals Engine Loss and a 'Mayday'
by Matthew L. Wald and Al Baker
The New York Times, Jan. 18, 2009

1549 to Tower: 'We're Gonna End Up in the Hudson'
by Matthew L. Wald and Al Baker
The New York Times, Jan. 17, 2009