Explainer-How black boxes hold vital clues to air disasters

By Tim Hepher

(Reuters) - Investigators have recovered the so-called black boxes from an American Airlines Bombardier CRJ-700 regional jet, which collided with a U.S. Army UH-60 Black Hawk helicopter and crashed into the Potomac River on Wednesday, killing 67 people.

Lead investigator Brice Banning said on Thursday that the helicopter also contained "some form of recording devices" that would be read either by the National Transportation Safety Board or by the Defense Department under existing agreements.

WHAT ARE BLACK BOXES?

They are not actually black but high-visibility orange. Experts disagree how the nickname originated but it has become synonymous with the quest for answers when planes crash.

Many historians attribute their invention to Australian scientist David Warren in the 1950s. Earliest devices recorded limited data on wire or foil. Later devices switched to magnetic tape. Modern ones use computer chips inside hard casings.

There are two recorders: a Cockpit Voice Recorder (CVR) for pilot voices or cockpit sounds, and a separate Flight Data Recorder (FDR). Some devices combine both functions.

WHAT IS THEIR ROLE?

They are mandatory on civil flights and the aim is to preserve clues from cockpit sounds and data to help prevent future accidents, but not to determine wrongdoing or liability.

In broad terms, investigators say the FDR helps them analyse what happened and the CVR can - though not always - start to explain why. But experts caution that no two probes are the same and virtually all accidents involve multiple factors.

The disappearance in 2014 of Malaysian Airlines MH370 triggered debate about whether data should be streamed instead.

HOW BIG ARE THEY?

They weigh about 10 pounds (4.5 kilos) and contain four main parts: 

* a chassis or interface designed to fix the device and facilitate recording and playback

* an underwater locator beacon 

* the core housing or 'Crash Survivable Memory Unit' made of stainless steel or titanium able to withstand a force equivalent to 3,400 times the feeling of gravity.

* the recording chip on a circuit board.

HOW ARE RECORDERS HANDLED?

After contact with water, they must first be thoroughly dried and the connections cleaned to ensure data is not erased accidentally. Audio and data files are downloaded and copied.

The data itself means little at first. It must be decoded from raw files before being turned into graphs and synchronised with other data, like air traffic control transmissions.

Lab experts sometimes use "spectral analysis," a way of deciphering fleeting sounds or barely audible alarms.

HOW MUCH INFORMATION IS AVAILABLE?

FDRs must record at least 88 essential parameters but modern systems can typically track 1,000 or more additional signals.

The CVR usually contains two hours of recordings on a loop and this is being extended to 25 hours.

Implementing such regulatory changes can take years, a delay highlighted by the crash last month of a Jeju Air Boeing 737.

The recorders in that accident, in which 179 people died, did not capture the last four minutes of flight, officials say.

A spate of accidents in the 1990s in which recorders had stopped working when power was lost led the NTSB to recommend enough backup power to provide 10 minutes of extra recording.

The change was finally adopted for new planes delivered from 2010, but only came into effect eight months after the 737-800 involved in the Jeju crash left the Boeing factory, according to aircraft data on Flightradar24.

(Reporting by Tim Hepher; Editing by Ros Russell, William Maclean)