Preliminary data about the aircraft collision over Washington DC show readings about the altitudes of the passenger plane and army helicopter, investigators say.

There were no survivors from the collision, which happened when an American Airlines flight carrying 64 passengers hit an army Black Hawk helicopter that apparently flew into the jet’s path.

Data from the jet’s flight recorder showed its altitude as 325 feet (99 metres), plus or minus 25 feet, when the crash happened on Wednesday night, local time, according to the National Transportation Safety Board (NTSB).

However, data in the control tower showed the Black Hawk helicopter at 200 feet at the time.

The roughly 100-foot discrepancy has yet to be explained.

Investigators hope to reconcile the altitude differences with data from the helicopter’s black box, which has taken more time to retrieve because it was waterlogged.

The US Army has released the name of the third soldier who died, Captain Rebecca Lobach, who was on board the helicopter with two other soldiers.

Here is what we know about black boxes.

What are black boxes?

The cockpit voice recorder and the flight data recorder are tools that help investigators reconstruct the events that have led up to a plane crash.

An NTSB investigator works on a black box. (Reuters: NTSB)

They are orange in colour to make them easier to find in wreckage and are usually installed a plane’s tail section.

The recorders are also equipped with beacons that activate when immersed in water and can transmit from depths of 14,000 feet.

While the battery that powers the beacon will run down after about one month, there is no definitive shelf life for the data itself, according to NTSB investigators.

Technicians will keep black boxes submerged in fresh water to wash away the corrosive salt if the devices have been submerged in seawater.

The electronics and memory are checked, necessary repairs are made and the chips are then scrutinised under a microscope.

What does the cockpit voice recorder do?

The cockpit voice recorder collects radio transmissions and sounds such as the pilot’s voices and engine noises, according to the NTSB’s website.

The American Airlines CRJ700 jet that crashed into the Potomac River. (AP: Nicholas Toto/610 Aviation)

Investigators may pay close attention to the engine noise, stall warnings and other clicks and pops.

From those sounds, investigators can often determine engine speed and the failure of some systems.

Investigators are also listening to conversations between the pilots and crew and communications with air traffic control.

Experts make a meticulous transcript of the voice recording, which can take up to a week.

What is the flight data recorder?

The flight data recorder monitors a plane’s altitude, airspeed and heading.

Those factors are among at least 88 parameters that newly built planes must monitor.

Wreckage from the crash floats in the Potomac River in Washington DC. (AP: Petty Officer 2nd Class Taylor Bacon/US Coast Guard)

Some recorders can collect the status of more than 1,000 other characteristics, from a wing’s flap position to the smoke alarms.

The NTSB says it can generate a computer animated video reconstruction of a flight from the information collected.

In 2014, NTSB investigators said these recorders carried 25 hours of information, including about previous flights, which could sometimes provide hints about the cause of a mechanical failure on a later flight.

While an initial assessment of the data is provided to investigators within 24 hours, the analysis will continue for weeks more.

Where does the black box originate from?

At least two people have been credited with creating devices that record what happens on an aeroplane.

One is French aviation engineer François Hussenot, who came up with the idea for the cockpit voice recorder in the 1930s, according to the website for European plane-maker Airbus.

Some have suggested the name black box stems from Mr Hussenot’s device because it used film and “ran continuously in a light-tight box, hence the name ‘black box'”, according to Airbus.

In the 1950s, Australian scientist David Warren came up with the idea for the cockpit voice recorder.

Mr Warren had been investigating the crash of the world’s first commercial jet airliner, the Comet.

He thought it would be helpful for airline accident investigators to have a recording of voices in the cockpit, the Australian Department of Defence said in a statement after his death.

Mr Warren designed and constructed a prototype in 1956 and the device was installed in commercial airliners worldwide several years later.

ABC/wires