(Updates Jan 12 story with Washington crash)

By Tim Hepher

Jan 31 (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 ( BA ) factory, according to

aircraft data on Flightradar24.