Flight Data Recorders and Cockpit Voice Recorders

Introduction

Flight Data Recorders (FDRs) and Cockpit Voice Recorders (CVRs) were introduced as a tool to aid accident investigation after a serious and often catastrophic complex event. The need for such devices arose because traditionally, aviation failure events did not provide many 'first-hand witnesses' to such failures, especially when it involved aircraft flying at high altitudes. An example of the need for recording devices stemmed from the 1950s de Havilland DH106 Comet, the world's first commercial aircraft (Figure 3.1), and their successive unexplained crashes.

This type of aircraft entered commercial service with the British Overseas Airways Corporation (BOAC) on 2 May 1952. While a small number of unforeseen events took place in 1952 and 1953, the losses stemming from the successive disasters of 1954 are what made this aircraft infamous. A series of aircraft suffered catastrophic losses in-flight, and at the time it was noted that the loss of the aircraft the absence of any radio communications. The accident investigations were inconclusive, and the crash investigation teams were unable to immediately determine the cause of the accidents. While some debris from these events was recovered, the principal causal factor was not identified at the time that the debris was recovered.

The manufacturer (de Havilland) conducted a static pressurisation water tank testing (Figure 3.2) on aircraft G-ALYU. After a number of simulated pressure cycles were reached, the test airframe suffered critical failure, bursting around riveted joints (Figure 3.3) which in turn failed the mechanical resilience of the fuselage. This mechanical test was the pivotal investigation technique that would shape the industry, identifying a knowledge gap on flying aircraft.

FIGURE 3.1

BOAC Comet 1 Aircraft.

FIGURE 3.2

de Havilland Comet G-ALYU in the water tank for pressure tests.

FIGURE 3.3

Square window failure of Comet G-ALYU post the water tank pressure tests.

It was only due to these static tests that were performed that the manufacturer and accident investigators were able to explain the previous accident's principal causal factor. Once the root cause of the failure was determined, the remaining flying aircraft were modified to remedy this in addition to future production aircraft.

Flight Instrument Recording

Early attempts at recording flight performance characteristics were performed in the 1930s by Francois Hussenot and Paul Beaudouin in France. The recorder used light-sensitive photographic film to capture images of the critical flight instruments (altimeter, airspeed indicator, etc.) at a pre-determined time interval. This is the earliest recorded instance of a device being used, instead of a flight crew member taking such readings manually.

After the losses of the Comet aircraft in 1954, David Warren, an Australian research scientist understood the difficulties in the accident investigation processes, especially for those that had no natural causation factor. His solution to this problem was to describe the theoretical concept of a data and voice recording device that would be installed on the aircraft, in a peer-reviewed paper: 'A Device for Assisting Investigation into Aircraft Accidents' (1954). While Hussenot's devices recording devices were further developed in the 1950s, the lack of information in the aircraft investigations was problematic. Warren had a personal interest in the field, having lost his father to an unexplained plane crash as a young child. Warren developed a working prototype in 1956, which later went into production utilising steel wire as a means to record the cockpit voice recordings. Warrens device was known in the industry as the 'Red Egg' due to the colour and shape of the device casing.

Further FDRs and cockpit sound recorders were developed in the 1960s by Prof. James Ryan and Edmund Boniface, respectively, in the USA. Both such devices were patented and introduction of these recorders was slow to gain popularity. One such obstacle was the flight crews themselves. They believed that the airlines would eavesdrop on their private conversations onboard the aircraft, compromising their ability to pilot the aircraft, effectively. This impasse was later resolved by flight crews and airlines, with the understanding that the recorded sound data would only be obtained in the event of a recorded event (e.g. a plane crash). The recorders typically operated by encoding the data within the unit onto a continuous magnetic tape/ wire loop (Figure 3.4), with a continuous operation specification.

FIGURE 3.4

Flight data recorder fitted to a pressurised aircraft, exposing the magnetic recording wires.

FIGURE 3.5

An example of an early Flight data recorder fitted to Boeing 707.

The data recorders would continuously record up to 2 hours of flight performance data, whereas the CVRs record the last 30 minutes of audio. Early examples of such flight data recording (Figure 3.5) included just five parameters, e.g. altitude, magnetic heading, indicated airspeed, vertical speed indicator and the microphone - all recorded temporally.

The location of such FDR and CVR recording devices was situated in the tail of the aircraft, i.e. aft of the rear pressure bulkhead. The purpose of this location choice was based on the expectation that the tail would be most likely to remain intact post an event. The absence of jet fuel and minimised risk of fire was an important factor, as the recorders were both fragile and could be damaged in a post-crash fire event.

Certification and Flight Instrument Recording

Certification of commercial aircraft has traditionally been the legal responsibility of each sovereign nation that builds new aircraft. For example, for new aircraft certified in the United Kingdom, the Civil Aviation Authority has specified the minimum performance standards deemed necessary. The use of flight recording devices became mandatory in the early to mid-1960s by the respective regulators around the world. Such requirements specified which aircraft were to be included (judged by take-off mass, number of seats, etc.)

FIGURE 3.6

Orange painted Flight Data Recorder with distinct white stripes for recognition.

with remaining operational aircraft later included, with the need to retrofit these machines.

After a crash event, the recovery of the FDRs and CVRs was prioritised. To assist in this recovery, the cases of these devices were painted in bright, distinct colours to aid recognition (Figure 3.6).

Forced landing on water proved to be problematic for recovery by the search and rescue teams. The solution to this problem was the mandatory introduction of Underwater Locator Beacons (ULB) (Figure 3.7), which are battery-powered devices that emit an ultrasonic signature of a 10ms pulse once per second at 37.5 kHz ± 1 kHz for a minimum of 90 days. This frequency has been selected as there are no known natural sources that emit this output, and once the signal is detected, search teams are confident that the source is a ULB, allowing for a more effective recovery.

As recording technologies have matured and improved, new methods of storing data have become possible. Early data recording used coded information recorded as magnetic data onto a continuous metallic tape, and the limitations of the time meant that only a small number of parameters could be recorded. However, with the advancement of signal processing and storage technologies, the ongoing trend from 1985 onwards was to move away from magnetic tape storage (Figure 3.8), and to utilise solid-state recording.

FIGURE 3.7

Underwater Location Beacon fitted to FDRs and CVRs.

FIGURE 3.8

Magnetic tape-based FDR from a recovered aircraft post a crash event.

FIGURE 3.9

Solid-state-based data recording memory cards used in FDR.

The solid-state memory cards (Figure 3.9) permitted much more useful information to be recorded, thus the certifying manufacturing regulators (e.g. FAA, CAA, etc.) increased the number of monitored channels, frequency of monitoring and the total duration according to the technology 'of the day'. In later years, the agreed standards of recording have been published by the International Civil Aviation Organization ICAO, Annex 6 in the respective revision. These revisions state the minimum numbers of channels to be monitored and define the frequency of data logging necessary.

Decoding Flight Instrument Data from Data Recorders

Magnetic tape-based storage recording devices required a very specialist technical apparatus to download and interrogate the recovered information. These old FDRs were robust and designed to withstand the forces anticipated from a catastrophic event; removing the magnetic tapes was not a simple or straightforward task. The introduction of the solid-state recording memory cards (Figure 3.9) has allowed for more crash resilient FDRs and CVRs that are more likely to retain the integrity of the recorded data. Modern recorders have granted all the data to be recorded as discrete digital data values, rather than the older analogue data formats, which have been made possible by the improvements in data storage technology. As a direct result of this digital innovation, all data is stored in a digital format, namely Digital Flight Data Recorder (DFDR) and Digital Cockpit Voice Recorder (DCVR). The last recorded event of a crashed passenger aircraft using an FDR and CVR was on 24 November 1992 in China. A China Southern Airlines, B737-300 series aircraft crashed and while the CVR was not found, the FDR was recovered. Unfortunately, the FDR was found to be severely damaged in the post-crash

FIGURE 3.10

Solid-state-based Flight Data Recorders and Cockpit Voice Recorders including the underwater beacon.

fire exposing the tape to the environment. This resulted in the loss of flight data information.

DFDRs and DCVRs (Figure 3.10) are designed to withstand much more significant events than the earlier metallic recording based units, with the digital variants capable of retaining the recorded data without the loss or corruption of the information contained within the memory modules. This technological advancement has been possible by the new design of the units, with minimal moving components, the memory boards encased in a solid fire-retardant material that is in turn encased in a stainless steel shroud.

Conclusions

The introduction of FDR and CVR in large commercial aircraft has been driven by the need of the industry to identify the last moments of a flight before a crash. Early recorders used magnetic media that could only store a limited quantity of data, with any new data overwriting the previous content. The introduction of such devices was instrumental in helping accident investigators identify the technical problems and piece together the last minutes of a flight prior to a crash.

The improvement in the recording technologies has allowed for a much better understanding of accidents by investigation teams. Additionally, the transition from analogue to digital technologies has also permitted for a much more rapid means of downloading the data contained within the data modules to a ground-based computer. The flight performance data, for instance, can be uploaded into an approved flight simulator model, to recreate the same event in a flight simulator. This is particularly useful to allow for accident investigators to experience the same indications and characteristics that the respective flight crew observed.

The additional benefits of airlines utilising the DFDR data will be discussed in Chapter 5.

The next chapter will explain the primary flight controls of an aircraft, the automated controller (autopilot) that is used extensively in the industry. The air conditioning system, being a major component of the Environmental Control System, is explained in detail, because this system provides the sur- vivable conditions that are essential to human life when an aircraft is flying at high altitudes (i.e. greater than 20,000 ft).

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