Accident Investigation

To quote from the United Kingdom Air Accidents Investigation Branch, “The fundamental purpose of investigating accidents is to determine the circumstances and causes of the accident with a view to the preservation of life and the avoidance of accidents in the future; It is not to apportion blame or liability”. This has been the basic principle of accident investigation ever since the early days of commercial aviation.

As aircraft have become more and more complex it has become progressively more difficult to determine the cause of an accident, Initially, investigators relied upon analysing the wreckage. For example, cockpit instruments might well be stuck at the readings at the moment of impact, however this did not give clues about the flight leading up to the impact. For this reason, crash protected data recorders (and their close relatives, the cockpit voice recorders) were introduced.

Flight Data Recorders

Interest in crash-protected flight recorders as we know them today dates back to the 1940’s, but there was little progress until 1958 when the world authorities approved a minimum operating requirement for a FDR. The initial requirement was to record aircraft heading, altitude, airspeed, vertical acceleration and time. The five parameters were embossed onto a metal foil, which was used only once.

Flight data alone could not provide all accident information needed by the investigators, and recording of sounds in the cockpit, crews’ conversation, air traffic control communications and aircraft noises was needed. This started a second generation of recorders that used magnetic tape as the recording medium. The first product to use this new technology was the Cockpit Voice Recorder (CVR). In 1965, all commercial operators were required to install a CVR, which would retain the last 30 minutes of crew voice communications and noises within the cockpit environment. This same tape technology was extended to record data in a digital format, and the new recorders could record many additional flight parameters while meeting higher crash and fire protection requirements.

The Solid State Flight Data Recorder (SSFDR) became commercially practical in the early 1990’s. “Solid State” refers to storage of data in semiconductor memories or integrated circuits. The Solid State CVR (SSCVR) evolved later than the SSFDR because of the need for more memory capacity, now provided by “Flash” memory chips. More recently still, combined voice and data recorders have become available, and legislation now permits the use of so-called “dual-combi” installations with more relaxed dispatch serviceability requirements.

Data Transfer Systems

The basic problem of all flight data analysis systems is that the data is generated on the aircraft and needs to be assessed by people on the ground. In extreme cases such as flight test work, the data has to be assessed in real time, so air-to-ground telemetry is used to transfer the data. Indeed, there is some consideration of using satellite data transfer for some flight data, however for cost reasons this is not a realistic probability for the near future. We therefore rely upon systems that record the data we need on the aircraft, and transfer that post flight.

Quick Access Recorders

In the 1970’s, some airlines found that the FDR data could be used to aid maintenance if it was downloaded on a routine basis. Unfortunately, routine replay of the crash recorders reduced their life and was very time consuming, so a “Quick Access” Recorder (QAR) was introduced to record data in parallel with the crash protected recorder. This used a tape cartridge that could be removed and replaced during an aircraft turn-round.

Later QAR’s used optical disk technology to provide greater capacity and more rapid replay. These are being superseded by the Card Quick Access Recorder, using PCMCIA format solid-state memory. The capacity of data cards now exceeds the capacity of an optical disk and their extremely high reliability and lower price makes them a very attractive proposition.

Automatic Download Systems

No matter how easy it is to change over media, and how reliable the system is, any removable media system will always cost more to operate than a fully automatic system. At present, there are two main contenders for this. One system utilises mobile telephone networks to transfer data. The recorder makes the call when the aircraft door is opened and transfers data to an Internet Service Provider (ISP), who will then forward the data to the operator’s analysis centre. The second contender is the aircraft file server system, where QAR data is just one of a number of data files held on the aircraft’s file server. At the gate, a network link is made to the airline’s computer system and the data files transferred in that way.

In general, there is a balance to be struck between the installation and capital cost of the system and the operating cost. Which system is “best” for a given operation also depends upon the frequency of data transfer, the acceptable data latency and the route structure and available communications infrastructure.