Explainer / What are black boxes?

Iran is examining the black box from a Ukrainian jet that crashed on January 8 in a fireball shortly after taking off from Tehran.

Recording information

Aircrafts are usually equipped with two black boxes that record information about a flight. Both recorders are installed to help reconstruct the events leading to an aircraft accident.

One of these, the Cockpit Voice Recorder (CVR), records radio transmissions and sounds in the cockpit, such as the pilot’s voices and engine noises. The other, Flight Data Recorder (FDR), monitors parameters such as altitude, airspeed and heading.

Both recorders are installed in the most crash survivable part of the aircraft, usually the tail section.

They are not actually black, but high-visibility orange. Experts disagree about how the nickname originated, but it has become synonymous with the quest for answers after crashes.

Many historians attribute their invention to Australian scientist David Warren in the 1950s.

What are the parts of a black box?

They weigh about 4.5 kg and contain four main elements…

* A metal chassis designed to hold the recorder and facilitate recording and playback.

* An underwater locator beacon to assist in locating in the event of an overwater accident. The device called a “pinger” is activated when the recorder is immersed in water.

* The core housing or ‘Crash Survivable Memory Unit’, made of stainless steel or titanium, and designed to withstand equivalent to 3,400 times that of gravity.

* Inside that housing, circuit boards holding fingernail-sized recording chips.

The CVR records the flight crew’s voices as well as other sounds inside the cockpit. The recorder’s “cockpit area microphone” is usually located on the overhead instrument panel between the two pilots. Communications with Air Traffic Control, automated radio weather briefings and conversation between the pilots and ground or cabin crew are recorded.

The FDR onboard the aircraft records many different operating conditions of the flight. It monitors many important parameters such as time, altitude, airspeed, heading and aircraft attitude. In addition, some FDRs can record the status of more than 1,000 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms.

How is the data analysed?

The chip on the flight data recorder of a different model of a Boeing 737 that crashed in October 2018 contained 1,790 parameters, spread over 19 flights.

The CVR contains two hours of recordings, more than enough to cover the brief Ukraine International Airlines flight.

Technicians peel away protective material and carefully clean connections to make sure they do not accidentally erase data. The audio or data file must be downloaded and copied.

Iranian and western experts say Iran has the equipment to extract the data if the recorders are in good order, but not if they are damaged, which requires special equipment and protocols only available in major western or Russian agencies.

The data itself means nothing at first. It must be decoded from raw files before being turned into graphs. Western agencies sometimes use “spectral analysis” - a way of examining sounds that allows scientists to pick out barely audible alarms or the first fleeting crack of an explosion.

Who has access to the data?

The lead investigator, in this case Iran, decides who has access to the data. Iran has said it will work closely with Ukraine, which said that Tehran was cooperating with it.

The US has called for an independent investigation, however. In most jurisdictions, only the senior investigator and a handful of people actually hear the voice recordings, which are transcribed, and translated if necessary, then sealed.

How much time is required for the analysis?

Investigators prefer to work methodically, but political and media pressure will be intense. Depending on any damage to the boxes and the type of accident, some investigators say they can get a very basic picture in days or even hours. But they stress this is not always the case, and rarely tells the whole story.

Deeper investigations take a year or more to complete. An Iranian official has said the probe may take two years.

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