How black boxes work and help investigate plane crashes?

David MG

Plane crashes often leave a lot of unanswered questions. This makes investigators turn to the airplane’s black boxes for answers. Since its widespread use after the post-World War II era, these recording devices that store flight data and pilots’ conversations have evolved to help investigators shed some light on the accidents in the air and on the land. 

Why a black box?

There are several versions of why flight recorders are called black boxes. The name could relate to the World War II era’s first electronic modules that were installed on military aircraft. Because of photographic film used for recording, such boxes should not let light in, as it could damage the tape. So they were painted black to prevent reflection. According to the other version, the name may come from early engineering design philosophies, where boxes that contained electronic components were termed as black boxes. 

The first flight recorder (FDR) was introduced by Professor James J. Ryan, a graduate of the University of Iowa and University of Pittsburgh engineering schools. 

During WWII, the U.S. Army Air Corps and Civil Aeronautics Board started working on the development of survivable flight data recorders. The company’s mechanical division hired James J. Ryan. 

His 16-pound hatbox-sized recorder was divided into two compartments, with the recording equipment (tiny electric motor with a thin sheet of 2-inch-wide aluminum foil) positioned above and measuring devices (the altimeter, the accelerometer, and the airspeed indicator) below. It could operate for 300 hours without servicing and could be easily mounted in any aircraft’s tail section. Ryan’s recorder was released in 1953 and later sold to Lockheed Aircraft Company, which kept the device’s basic features. By 1957, all planes over 12,500 pounds were required to have FDRs. 

David Warren, a research scientist from the Aeronautical Research Laboratory* (ARL) of Melbourne, introduced his cockpit voice recorder (CVR) in 1958. Warren was a member of the research group, which was involved in the accident investigation related to the mysterious crash of the world’s first jet-powered commercial aircraft, the Comet. Warren’s CVR consisted of wire, used as the recording mechanism, which was placed in a titanium box. After the plane crash at Mackay, Queensland in 1960, it was recommended to install cockpit voice recorders on all airliners. Australia was the first country in the world to make cockpit-voice recording compulsory.

How Black Boxes are made? 

Flight recorders cost between $10,000 and $15,000 each. They are made to withstand loads and remain intact in a wide variety of emergency scenarios. 

Before being approved for use, they are tested. Recorders must withstand impact on a concrete wall at a speed of 750 km/h, a pressure of 2.25 tons for at least five minutes, a temperature of up to 1100 degrees Celsius for at least an hour, and be found at a depth of up to 6000 meters underwater.

Data in modern CVR and FDR recorders is stored on stacked memory boards inside the crash-survivable memory unit (CSMU), a large cylinder that bolts onto the flat portion of the recorder. The memory boards can accommodate two hours of audio data for CVRs and 25 hours of flight data for FDRs. The boxes that store recorders are painted bright orange or red to be easily detectable. 

Types of the black boxes

In addition to conversations between crews and dispatchers, CVR also saves ambient sounds. The records from CVR have an exact timestamp reference. Each of the parameters is recorded several times per second. The recording is carried out in cycles: new data overwrites the oldest. The cycle duration is 17-25 hours to ensure that there is enough storage for any flight. 

FDR stores information on such flight parameters as airspeed, magnetic heading, speed, fuel flow, horizontal stabilizer. The recording device receives all of this information from various sensors. According to the Federal Aviation Administration (FAA) requirement, the flight data recorder should record at least 88 parameters on aircraft manufactured after August 19, 2002. Solid-state recorders can track more parameters than magnetic tape and store up to 25 hours of flight data.

How do black boxes work?

Several microphones built into the cockpit listen to flight crew conversation track any ambient noise in the cockpit. Each microphone is connected to the cockpit voice recorder (CVR). CVR records all sounds in the cockpit, including conversations of the pilots, instructions given by the on-board computer, communication with ground services, conversations with flight attendants, as well as announcements that the crew makes for passengers. Also, the device registers all rustles, sounds of switches, and a running engine. CVR automatically starts recording in flight and on the ground when at least one engine is running. 

All private conversations of pilots are also recorded and stored in a black box ‒therefore, the analysis of these records and their decryption should be carried out under full control. GDPR rules and personal data protection should also be taken into account. The content of the conversations can only be used to find out the causes of the accident or technical failure.

Solid-state storage CVRs can record the last 120 minutes of conversations ‒ older information is automatically erased. Previous generation recorder models generally stored only the last 30 minutes of conversations. Also, in modern planes pilots can pause recording or even delete it. 

Pilots cannot get direct access to FDR. On older aircraft models, however, they had to manually turn on such devices before the flight began. In modern aviation, this happens automatically. Data from FDR helps experts to establish the causes of the accident or a serious technical malfunction and identify and eliminate potential sources of malfunctions in the future. 

Black boxes are usually placed near the tail, which is statistically less likely to be damaged in accidents. To be quickly detectable in the sea, after contact with salt water black boxes automatically turn on a beacon ‒ the signal which can be caught within a radius of two kilometers. Since the signal radius is small, investigators still need to know the approximate location of the wreckage of the aircraft. 

After the crash 

After the black box is located, it is transported to the computer labs where investigators download the data from the recorders and try to recreate the events of the accident. 

From a solid-state recorder, if it is not damaged, investigators can extract stored data through USB or Ethernet ports very quickly. However, the black boxes are usually dented or burned. It might take from weeks to months to retrieve data from the damaged recorder. The memory boards are removed, cleaned up, have a new memory interface cable installed, and connected to a working recorder. 

The recordings stored on a CVR are interpreted by a team of experts. In the U.S., it  consists of representatives from the airline and airplane manufacturer, an NTSB transportation-safety specialist, an NTSB air-safety investigator, also a language specialist from the FBI and, if needed, an interpreter. The interpretation process may take up to several weeks. 

Aircraft malfunctions are statistically rare. However, when a plane accident occurs, black boxes help to recreate the events that led to the crash. Besides, there is still plenty of room for improvement. For instance, the U.S. The National Transportation Safety Board has been working on video implementation into black box systems. As this might violate pilots’ privacy, the project is still on hold. Since 2002, some legislators have offered to introduce a flight recorder that automatically ejects itself from the plane during an incident.

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