Black Box Flight Recorders and Cockpit Voice Recorders

The idea of Flight Data Recorders or more commonly known as 'Black Box' Flight Recorders were first thought of by Dr David Warren in the 1950's, in Australia, however the Australian Aviation Authorities did not approve the idea. In 1957 the first demonstrator unit was made and taken over to Britain for further development, where it was received very enthusiastically. However, surprisingly in 1961 Australia were the first country to make 'Black Boxes' mandatory in all aircraft after an unexplained crash. Nowadays every country in the world uses these recorders along with Cockpit Voice Recorders, which provide a vital part in explaining many accidents and can also help in preventing future accidents.

In the early recorders, which were manufactured in the UK between 1957 and 1965, the most popular method was to use spools of stainless steel tape or wire as the recording medium. This was then housed in a titanium crash-protected case along with heat insulation. Into this, signals from various positioned transducers, which sensed airspeed, altitude, engine speed and engine temperature, would be recorded in a sequence. This sequence would usually be repetitive of about one per second onto the stainless-steel media.

Nowadays, magnetic tape is the most popular medium for recording, along with more electrical systems being introduced as technology advances. Flight data recorders have now been in service for many years, but they still have many drawbacks, apart from being extremely expensive the tape mechanisms are very complicated and time consuming to keep maintained. The latest new technology recorders, which have been introduced, use large capacity computer memory chips. These chips also have no moving parts, meaning they are much more reliable, they require much less maintenance and can also survive greater fire and impact conditions. This chip type recorder is now available in both FDR's and CVR's consisting of banks of computer memory chips mounted in a crash-protected enclosure and with an ultrasonic locator beacon.

The Flight Data Recorders (FDR) that were introduced in the 1940's experienced many delays, because technology could not match the design requirements, for such a recorder, that could survive the forces of an aircraft crash and also the possible fire exposure until 1958. The initial requirement for FDR's was to record the actual flight conditions of the aircraft, for example the heading, altitude, speed, vertical acceleration and time. However the early devices had very limited recording capabilities and these parameters were embossed onto a metal foil, which could only be used once. This foil was thought to be almost indestructible, but the crash survival rate still seemed to be a problem.

Finally, in 1965 the recorders were upgraded from the original specification of 100g impact to a 1000g impact. However, the recorders only recorded five different parameters, which was not enough data for a meaningful accident investigation, hence in 1987 these recorders were deemed unacceptable, by the authorities, and additional parameters were required.

Investigators now decided that flight data alone could not provide all the accident information needed. Therefore technology covering the recording of sounds in the cockpit, such as the crew's conversation, airtraffic control communications and aircraft noises was now needed for accident investigators to be able to piece together accidents more accurately and meaningfully. A second generation of recorders that used a magnetic tape as the recording medium was created. The first product to use this new technology was the Cockpit Voice Recorder (CVR). In 1965, all commercial flights were required to install a CVR, which would record and retain the last thirty minutes of communications between the crew and any other noises within the cockpit.

All commercial, corporate and private aircraft are required by the Flight Aviation Authority to be equipped with two "black boxes" that record information about a flight. Both of the recorders are used to help reconstruct event leading up to an aircraft accident. The first "box" the CVR records radio transmissions and sounds in the cockpit, while the other, the FDR monitors many different parameters such as altitude and heading. Both recorders are installed in the most crash survival part of the aircraft; this is usually the tail section, which is located away from pressure bulkheads, fuel lines and major structural areas such as wing spars which may damage the recorder on impact.

Each recorder is also equipped with an Underwater Locator Beacon (UBL), this is used to assist in locating the recorders in the event of an overwater accident. This device is activated when the recorder is immersed in water. It then transmits an acoustical signal (37.5kHz) that can be detected with a special receiver. The beacon can transmit up to a depth of 14 000 feet.

Immediately after an accident, both recorders are removed from the scene as soon as they are located and transported away for processing. The information that is found stored on the recorders is then extracted using sophisticated computer and audio equipment. The investigators then use this information to help, along with other sources of information such as debris and eyewitnesses accounts, to try and determine the probable cause of the accident and also to try and stop similar accidents occurring again.

The CVR records the flight crew's voices, along with other sounds inside the cockpit. The recorders' microphone is usually found on the overhead instrument panel in between the two pilots. Sounds of interest that are captured on the recorder are:

• Engine noise
• Stall warnings
• Landing gear extension and retraction
• Crew conversation
• Communications with air traffic control
• Automated radio weather briefings
• Any other abnormal noise

From these sounds, parameters such as engine rpm, speed, system failures and the time at which these events occurred can then often be determined.

Specifications of CVR

• Time recorded...................30 minutes continuous, 2 hours for digital units
• Number of channels............4
• Impact tolerance.................3400Gs for 6.5ms
• Fire resistance....................1100 ^oC for 30 min
• Water pressure resistance......submerged 20 000 feet
• Underwater locator beacon.....37.5kHz
• Battery.............................6 year shelf life
• 30 day operation

The FDR onboard the aircraft records the different operating conditions of the flight. By law, all newly manufactured aircraft must have at least twenty-eight different important parameters monitored all of the time, such as altitude, time, airspeed and heading. Some FDR's can also record more than 300 other in-flight characteristics than can be of use to investigators. These characteristics monitored can include flap position, autopilot mode and also smoke alarms.

With the data retrieved from the FDR, the investigators can then generate a computer animated video reconstruction of the flight. This then helps the investigators to visualise the aeroplane's attitude, power settings and any other characteristics of the flight. All of this helps the investigating team to visualise the last moments of the flight right up until the accident.

Specifications of FDR

• Time recorded.................25 hours continuous
• Number of parameters........5-300+
• Impact Tolerance..............3400Gs for 6.5ms
• Fire resistance..................1100^oC for 30 minutes
• Water Pressure resistance.....submerged 20 000 feet
• Underwater locator beacon...37.5kHz
• Battery...........................6 year shelf life
• 30 day operation

Throughout the years recorders have had to survive the most horrific accidents. To ensure the survivability of an FDR or CVR it must pass a certain series of tests first.

The Crash Impact Test: This test is performed with a cannon and the recorder must survive an impact of 3400gs for 6.5ms. However recorders have survived crashes estimated to be more than 6000gs

Static Crush: A 5000 pound pressure is applied against all six axis points to the recorder

Pierce Test: A 500 pound weight, performed with a hardened steel pin, is dropped from a height of 10 feet .

Fire Test: The recorder is to be subject to 1100^oC for 60 minutes, then undergo 10 hours at 260^oC.

Both the FDR and the CVR have been proven to be two of the greatest safety inventions in the accident investigation process for the commercial airline industry. They can provide huge amounts of information that can be of great use to investigators that may be impossible to obtain by any other means. There are always improvements on to update equipment, and within the next few years, some form of cockpit video recorder that can store images in solid state memory and in conjunction with voice recording will be developed. This work is already in progress along with several other technological enhancements that may help to prevent other possible devastating aircraft accidents from happening.