With the crashes of AirAsia flight QZ8501 and Malaysia Airlines MH17, along with the disappearance of flight MH370, there is again focus on airline "black boxes".

Here are some things you might not know about black boxes:

1. They're not black

Black boxes are the same colour as the Golden Gate Bridge in San Francisco ... kind of. They are a tone of what's known as international orange, which is a set of three colours used in aerospace and engineering to distinguish objects from their surroundings. The Golden Gate Bridge is a darker shade, while the international orange used for black boxes is much brighter.

The tone of international orange used to paint the Golden Gate Bridge is most closely matched by Pantone colour 180. ( AFP: Justin Sullivan )

2. A 'black box' comes in two parts

The "black box" is made up of two separate pieces of equipment: the flight data recorder (FDR) and a cockpit voice recorder (CVR). They are compulsory on any commercial flight or corporate jet, and are usually kept in the tail of an aircraft, where they are more likely to survive a crash. FDRs record things like airspeed, altitude, vertical acceleration and fuel flow. Early versions used wire string to encode the data; these days they use solid-state memory boards. Solid-state recorders in large aircraft can track more than 700 parameters.

The black boxes from the Asiana plane that crashed short of the runway at San Francisco airport on July 6, 2013. ( Twitter: @NTSB )

3. They were invented by an Australian

Dr David Warren's own father was killed in a Bass Strait plane crash in 1934, when David was just nine years old. In the early 1950s, Dr Warren had an idea for a unit that could record flight data and cockpit conversations, to help analysts piece together the events that led to an accident. He wrote a memo for the Aeronautical Research Centre in Melbourne called "A Device for Assisting Investigation into Aircraft Accidents", and in 1956 produced a prototype flight recorder called the "ARL Flight Memory Unit". His invention did not get much attention until five years later, and the units were eventually manufactured in the UK and US. However, Australia was the first country to make the technology compulsory.

4. Experts don't call them "black boxes"

The term "black box" is favoured by the media, but most people in the know don't call them that. There are several theories for the original of the name "black box", ranging from early designs being perfectly dark inside, to a journalist's description of a "wonderful black box", to charring that happens in post-accident fires.

From The Conversation:

Black boxes are normally referred to by aviation experts as electronic flight data recorders. Their role is to keep detailed track of on-flight information, recording all flight data such as altitude, position and speed as well as all pilot conversations. It is common for many civil airliners to have multiple devices to carry out these tasks so that information can be gathered more easily in the event of a failure. In most instances, they are used to help in the diagnosis of what may have been the likely cause of an accident.

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5. Only 2 hours of cockpit conversations are kept

Digital recorders have enough storage for 25 hours of flight data but only two hours of cockpit voice recording, which is recorded over itself in a loop. The CVRs track the crew's interactions with each other and air traffic control, but also background noise that can give vital clues to investigators. Earlier magnetic tape versions could only record 30 minutes of cockpit conversations and noise, which was also recorded in a loop.

The magnetic tape flight data recorder from Alaska Airlines Flight 261 was retrieved off the coast of California after the plane crashed in 2000. ( AFP: Manny Ceneta )

6. It can take a long time to find one

A US Navy Towed Pinger Locator (TPL) has been sent to help find MH370's black box. ( Supplied: US Navy )

Black boxes are fitted with an underwater locator beacon that starts emitting a pulse if its sensor touches water. They work to a depth of just over four kilometres, and can "ping" once a second for 30 days before the battery runs out, meaning MH370's black box stopped pinging around April 7, 2014. After Air France flight 447 crashed into the Atlantic Ocean, it took search teams two years to find and raise the black boxes. They provided valuable information about what actually happened prior to the crash.

The US Navy sent a Towed Pinger Locator (pictured above) to help with the search for MH370's flight data recorders. The locator is used for finding emergency relocation pingers on downed Navy and commercial aircraft at a maximum depth of 20,000 feet anywhere in the world. Because flight MH17 crashed over land, finding it should be more straightforward.

7. They're virtually indestructible...

FDRs are usually double-wrapped in titanium or stainless steel, and must be able to withstand atrocious conditions. The crucial part that contains the memory boards, the CSMU, is shot out of an air cannon to create an impact of 3,400 Gs and then smashed against a target. It is subjected to a 227kg weight with a pin attached to it, which is dropped onto the unit from a height of three metres. Researchers try to crush it, destroy it in an hour of 1,100 degree Celsius fire, submerge it in a pressurised salt water tank, and immerse it in jet fuel.

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8. ... But they're not as powerful as your phone

In the aftermath of MH370, experts say it might be time to update methods of collecting flight data. Passengers are able to text, stream and surf the internet but the data recorders on board are not communicating in real time with the rest of the world. However, the bandwidth needed to stream huge amounts of data from large aircraft is not currently feasible. Aviation author Stephen Trimble writes in the Guardian that Boeing has applied for a patent on a system that will transmit a subset of data including the plane's location: