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The Operation of Flight Recorders

Introduction, History and Development

The purpose of flight recorders was originally to aid in answering the question of why an air-accident had occurred, or as an American crash investigator would say find the ‘probable cause’ of an accident. Today flight recorders are also used to spot potential problems, before they can cause an accident.

Work was started on the design of crash-protected flight recorders in the 1940’s, however the first flight recorders did not enter service until the 1950’s, on some military aircraft. Non-military aircraft did not start to employ them until the latter years of the fifties. The main problem during these early years was that the technology of the day was not really capable of producing an accurate recording device that could withstand the massive forces and high temperatures it would experience in a catastrophic aircraft crash. The USA was the first country to make it a legal requirement that commercial aircraft carry flight recorders when on the 1st of August 1958 the Civil Aviation Authority (CAA later renamed the Federal Aviation Authority - FAA) issued a mandate requiring certain commercial aircraft classes to be fitted with a flight recorder. Two years later in 1960 the British government followed suit, requiring that all civilian passenger-carrying aircraft weighing over 20,000lbs should be fitted with a flight recorder. The most common method of data recording in early flight recorders was thick metal foil. Transducers sensed such parameters

as airspeed, altitude, engine speed, engine temperature, etc. and sent signals to the flight recorder which were then recorded on the metal foil by scribe arms connected moving coil meters and air pressure mechanisms scratching the readings onto the foil as traces, with the foil constantly moving. The foiled typically moved at six inches per hour this gave an accurate relative time reference. The foils were capable of recording for around 400 hours continuously before they had to be replaced. As the traces were etched into the metal of the foil it obviously meant that each length of foil could only be used once. The metal used for the foil was generally Incanol Steel (high Nickel content) which was believed to be virtually indestructible, however the problem of crash-survivability had still not been satisfactorily solved. The foil was read by overlaying it with a glass plate that had calibrated scales written on it. The first recorders produced in Britain ( those manufactured between 1957 and 1965) used a slightly different method of data capture and different recording media. These recorders used spools of stainless steel tape or wire as their recording medium. Instead of the continuous trace method used by the American recorders the British recorders noted the signals from the transducers in a repetitive sequence roughly once per second and recorded these readings on the tape/wire. The casings of these early recorders tended to be a titanium box lined with heat insulation material In 1958 the world authorities had defined the minimum requirements for crash-survivability of a flight recorder, for example they specified that a recorder must be able to survive at least a 100g impact. The standards they agreed on were more influence by the limitations of the technology of the day than the conditions that a flight recorder would have to cope with in an actual crash. Consequently these minimum standards were too low. In 1965 the standard for impact force was increased to make it more realistic, the original 100g impact requirement was to a 1000g impact. By this time of course aircraft had become faster so the forces involved in a crash had increased in magnitude. Most flight recorders of this period only actually tracked around five parameters, this was also a problem as this amount of data was almost totally insufficient to effectively aid air-crash investigations carried out by bodies such as the NTSB (National Transportation Safety Board) of the USA. Also in 1965 a law was passed that introduced a new type of recorder. All the recorders previously mentioned were of the type known as Flight Data Recorders (FDR’s), but in 1965 all commercial airlines were required to install Cockpit Voice Recorders (CVR’s) on all their aircraft. The purpose of the FDR is to track the physical behaviour and performance of the aircraft, the function of the CVR is to record the sounds (e.g. speech, engine noise)  that are audible in the cockpit.

By 1987 it was clear that the current legal requirements for flight recorders were completely insufficient. So the requirements were changed so that recorders were required to track and store more flight parameters. Most of the major airlines upgraded their flight recorders long before they were required to by law, it being in their best interests to do so as finding the cause of an accident with one of their aircraft helps to recapture the publics trust. However, even today there are still some old aircraft operating these early (foil) type recorders however, these recorders will become obsolete as the supply of Incanol steel foil is running out. As the technology developed magnetic tape became the preferred recording medium, it had been introduced with the CVR as the best way to record the audio data. Magnetic tape was not without its problems though as it required a much higher degree of protection than metal foil/tape/wire. Also as the aircraft developed they came to incorporate more and more electronics (avionics). Towards the end of the 1960’s and the beginning of the 1970’s considerably more sophisticated aircraft came into service (e.g. the Boeing 747 and the DC 10) these aircraft necessitated the introduction of more sophisticated flight recorders, capable of monitoring more parameters, these new parameters included engine data, flap position, etc.. These parameters had to be recorded by the FDR if it was to be of any serious use to those responsible for investigating air accidents. The only viable way of recording so many parameters at the time was magnetic tape. In 1970 a new avionics unit was introduced - the Flight Data Acquisition Unit (FDAU), this device collected all the signals from the aircraft systems and the sensors scattered all over the aircraft, sorted them converted them into a binary format then transmitted them to the FDR via a 2-wire serial data bus. This method of formatting the data was adopted as the industry standard and has been used ever since. Although these magnetic tape systems are still in operation now, almost thirty  years after they were first introduced, they have not been problem free. As well as magnetic tape being far more delicate than the media it replaced, magnetic tape mechanisms tend to be intricate and therefore expensive and time-consuming to maintain. The latest technology in this area is the ‘Solid-State’ flight recorder, so-called because it has no moving parts. These recorders use either large capacity computer memory chips (integrated circuits) or semi-conductor memories to store the data, hence they have no moving parts. The lack of moving parts means that solid-state recorders can survive harsher crash-survivability standards and therefore worse crashes than their magnetic tape predecessors.

Another advantage of not having moving parts is that they do not require maintenance or overhaul. It is also easier to retrieve the data that they record, this data can then be used to analyse the aircraft’s performance and identify any possible malfunctions in the systems that the recorder monitors. The first Solid-State Flight Data Recorder (SSFDR) to be certified for use was the Fairchild SSFDR model F1000. The data recorded by the CVR (i.e. audio data) requires far greater memory capacity than the FDR, consequently the Solid-State Cockpit Voice Recorder (SSCVR) took longer to develope. Wheras SSFDR’s became commercially viable in 1990, SSCVR’s did not until 1992 when the 30 minute capacity version came on the market. Then in 1995 the 2-hour capacity SSCVR was released. Both the SSFDR and the SSCVR consist of banks of the memory chips/ semi-conductor memory mounted in a crash-protected enclosure with a power supply, an Ultrasonic Locator Beacon and the decoding and controlling electronics. As these are very new systems they have been designed so that they are compatible with the systems that the older tape recorders receive their data through. One possible future advance in flght recorder technology is a recorder that records visual data from the cockpit, the technology to store video on solid-state memory is not too far away.

Types of Flight Recorder

Flight Data Recorder (FDR) The FDR’s basic function is to record as much data as possible about the aircraft in flight. Current regulations require FDR’s to monitor at least 28 parameters such as time, airspeed and heading. In general the FDR monitors parameters relating to the aircraft’s airframe, engines and avionics systems. Although they are only required to record 28 parameters by law some FDR’s monitor and record over 300. No matter how many parameter’s a particular FDR monitors they will generally include the following:

Time                                                 Vertical acceleration Altitude                                            Pitch attitude Airspeed                                           Roll attitude Heading                                            Time for each radio transmission Thrust (each engine)                       Trailing edge flap position Leading edge slat position              Thrust reverser position Speedbrake position                       Marker beacon passage (on approach slope) Autopilot engagement                     Longitudinal acceleration Control column position                 Lateral acceleration Control wheel position                    Pitch trim position Master warning                               Main gear squat switch status Angle of attack                                 Outside air temperature Hydraulic system low pressure      Groundspeed Glideslope deviation (above/below  Autoflight control system mode approach) Localiser deviation (left/right of Radio altitude approach)

[List taken from Penny and Giles Aerospace]

Some FDRs even record smoke alarms. After an accident the FDR is retrieved and the data is read from it. This data can then be used in a variety of ways to aid in the determining of the ‘probable cause’ of an accident. One of the methods used by the NTSB, the body charged with air-crash investigation in the USA is to use the FDR data to construct a computer animated video of the flight of the aircraft, this is of course is a great aid to visualising what happened. The FDR generally has enough storage space to record the incoming data for 25 hours continuously, after this time the new incoming data is recorded over the oldest data so that the FDR always holds the last 25 hours of data.

Cockpit Voice Recorder The CVR ‘s function is to record all the sounds that can be heard in or from the cockpit. As well as the speech in the cockpit itself this also includes radio transmissions to and from the aircraft, sounds from engines/other parts of the aircraft that can be heard in the cockpit and passenger announcements. The CVR itself is basically a four-track audio recorder. Generally the four tracks are: 1. The pilot’s microphone 2. The co-pilot’s microphone 3. Passenger announcements Cockpit area microphone ( which is usually installed in the overhead control panel between the pilot and co-pilot.

The CVR will also record communications with air traffic control (along with all other radio communications) and automated radio weather briefings. As with the FDR, the recording is made in a loop with the newest data overwriting the oldest. The time period for the CVR recording loop is generally 30 minutes, however, as previously mentioned, there is now a SSCVR that holds the last two hours of audio. The CVR recording is not treated in the same way as other data sources in a crash investigation, this is because the contents of the tape includes irrelevant conversations between the pilots and the other people whose voices might appear on the tape - there has even been a case where a pilot used the CVR to send a last message to his family. These conversations are private and are not normally relevant to the cause of the crash, therefore there is no need for these parts of the recordings to be widely circulated. In the USA congress has actually banned the NTSB from releasing any part of a CVR recording to the public. To get around this problem in the USA a CVR committee is formed. The committee usually consists of a few people from the NTSB, the FAA, the particular aircraft’s operator, it’s manufacturer, it’s engines’ manufacturer and the pilot’s union. The committee’s task is to listen to the recording and produce a transcript of it in writing, this will then be used in the investigation. This transcript will not of course contain any of the sensitive material mentioned above. To determine the local time at which incidents on the tape occurred, the committee uses the FAA’s ATC tapes which have associated time codes, a special computer program is used to match the air traffic control tapes with the CVR to obtain the timings of the events. If there are events on the tape that require more accurate timings then a digital spectrum analyser is used. As the transcript only details the portions of the CVR recording that are relevant to the accident this transcript can be released to the general public on conclusion of the investigation. Combined Voice and Flight Data Recorder (CVFDR) There are some aircraft where, although it is desirable to have a flight recorder, saving weight is a priority, two good examples of this are fighters and helicopters. For this reason helicopters and military aircraft are permitted to carry a single CVFDR instead usual two units. Advances in integrated electronic circuits and manufacturing techniques mean that it is now possible to produce a CVFDR which is considerably smaller and lighter than the two-unit alternative, but which has the full recording capacity of each of them.

The typical housing for crash-protected flight recorders is still a titanium box lined with heat insulation within which the mechanisms are held.

Quick Access Recorder (QAR) As useful as the above recorders are for working out what went wrong and therefore how to stop a repeat of the accident, prevention is obviously preferable. This is the purpose of the QAR, it is similar to a FDR but stores more parameters at higher sample rates for much greater durations. The QAR is not crash-protected, its purpose is to gather very detailed data on the operation of the aircraft. The QAR is located behind the flight deck for easy access to retrieve the data from it - hence the name. The QAR data is then analysed to try and spot any possible malfunctions in any of the aircraft’s systems, so that they can be stopped before they become dangerous. Thus the QAR is a monitoring tool, and one which has become an integral part of many airlines’ flight safety monitoring programs. Today’s computers can take the QAR data and use it to perform detailed analysis of the aircraft’s airframe, navigation systems, autopilot, engines ,etc. and can also check that the aircraft is being flown within its safety envelope.

Placement of Flight Recorders The basic criterion for deciding where to install a flight recorder is simple - put it where it is most likely to survive the crash. This means away from major structural components, fuel lines and tanks, pressure bulkheads, etc.. One place commonly chosen is above the ceiling at the back of the passenger cabin, underneath the tail fairing.

Making Sure the Recorders Survive Things have come a long way since 1958 when it was the state-of-the-art in crash-survivability that had the major influence on the level of the minimum requirements for crash-survivability. Today the minimum standards are set according to the conditions that a flight recorder is likely to meet in a crash. These days to become certified for use flight recorders must pass a collection of extremely arduous tests. These include:

Fire This is two-fold, the flight recorder casings must protect the recording medium from 1100degC (the temperature at which aviation fuel burns) for thirty minutes (simulating the initial fuel fire after a crash). It must then be able to protect the recording medium from a temperature of 260degC for ten hours (this simulates a long baggage-fuelled fire). Common heat-insulation materials used include polymers, wax and gelled which -  all offer good protection from high temperatures.

Water protection and Deep Sea Water Pressure Resistance The casing must protect the recording medium for thirty days underwater at a pressure equivalent to being submerged at 20,000ft below the ocean surface.

Static Crush Test The recorder must withstand a 5,000-pound pressure applied against all six axis points.

Penetration Resistance The recorder must withstand a 500lb force being dropped on it from 10 feet up, the point of contact being a one-quarter inch diameter hardened steel spike.

Impact Test In this test the recorder is fired from an air cannon into an aluminium wall. This simulates the required, 3400g deceleration in 6.5 milliseconds (which is equivalent to going from 350 mph to zero in 16 inches).

Fluid Immersion Test To check that the recorder will not be eroded by any of the fluids that are most commonly carried by planes, the recorder must survive emersion for 48 hours in each of the following: hydraulic fluid lubricating oil aviation fuel fire extinguishing agents and toilet flushing fluid

One other specification is the battery life of the ULB (see next section), the battery should have a shelf-life of 6 years and an operating life of 30 days.

The above are the internationally recognised standards.

Finding the Recorders A serious crash site on land is generally littered with wreckage, often spread out over a large area, finding two relatively small boxes in such carnage can be quite difficult. This is why the so-called ‘black boxes’ (a nickname never used by the professionals) are in fact bright orange and clearly marked ‘FLIGHT RECORDER DO NOT OPEN’ in English and French. In the event of an aircraft staying relatively intact there are ‘cut-out’ panels marked on the fuselage giving their location. In the event of an over-sea crash, the flight recorders are located by their Ultrasonic Locator Beacon (ULB) which is activated when the recorder is immersed in water, it broadcasts a signal for at least 30 days on 37.5khz, with a range of up to 2 miles. The signal can be detected using a variety of hydrophone-based equipment.