Cockpit Voice Recorder.

To fulfil the requirements of the Air Navigation Order (ANO) (Schedule 4 Scale S) aircraft 5700 kg issued with a type certificate after April 1971 must be fitted with either a Flight Data Recorder (FDR) or a Cockpit Voice Recorder (CVR) and aircraft greater than 11,400 kg must be fitted with both a FDR and a CVR.

The voice recorder system is installed to provide a continuous record of the last 30 minutes of flight crew communications and conversations. A permanent magnetic tape record is made of all conversations and other noises on the flight deck through the area microphone in the control panel, together with all transmitted and received flight crew communications, all navigation audio signals, and interphone audio that has been selected on the Captain's, First Officer's and Flight Engineer's audio selector panels.

VOICE RECORDER

Fig. 8.1. BASIC CVR SYSTEM

Audio from each of the four signal sources is recorded on individual channels on a 4 track, half hour closed-loop tape. At the end of the half hour period, when the tape has completed its closed-loop cycle, the recorder starts a new cycle, automatically erasing the previously recorded information and recording the new audio. Sensing of the aircraft on ground and parking brake set is used to permit bulk erasure of the recording.

Voice Recorder Unit

The voice recorder is a rack mounted unit housed in an internationally recognised orange painted case (for ease of identification at any possible crash site). The recorder consists of a drive unit, reel and tape, circuit board assemblies and an insulation assembly.

The drive unit contains a bridge assembly consisting of the recording, monitor and continuous-erase heads. A capstan assembly in the drive unit controls the rate of movement of the tape across the heads.

The reel and tape consists of a 300 foot endless loop of magnetic tape wound on a single sided reel. The tape is oxide coated on one side and graphite lubricated on the other.

The circuit board assemblies include four amplifier cards, a power supply card and a test circuit card.

One of the most important features of the unit is the insulation assembly, which encloses the drive unit assembly and protects it from shock and thermal damage (in the event of a crash). The insulator is made from an inert mineral fibrous mass that is shaped to surround the assembly. Around this material is a plastic cover which acts as a waterproof barrier. The insulating components are partially saturated with water. When the plastic cover is exposed to extreme heat, the cover melts and exposes the water saturated insulation. Low temperature melt plugs in the outer reel case allow generated steam to vent to atmosphere to provide an isothermal barrier between the external high temperature and the enclosed tape module.

Fig. 8.2. VOICE RECORDER UNIT

The voice recorder front panel has five TEST switches, an indicator and a headset jack which are used for test functions.

The TEST switches are placarded 1, 2, 3, 4, and ALL, are used in conjunction with the indicator and headset jack. The switches are used to test the recording channel by recording a test tone on the tape for playback through the monitor circuit. When a channel pushbutton is pressed, only the corresponding recorder channel operates. When the ALL channels pushbutton is pressed, all four channels operate in sequence.

The indicator has a divided dial to indicate a go/no-go circuit condition. The go condition is when the indicator pointer deflects into the green sector of the dial and the no-go condition when the pointer stays in the red. When the ALL channels test switch is pressed the indicator needle oscillates as the recorder test circuit sequences from track to track.

The Monitor-600 ohms headset jack provides headset monitoring of the test tone and all recorded information on all four tape channels through the test circuit.

An underwater locator beacon is mounted on the front face of the recorder. The locator beacon is a self contained battery powered acoustical beacon enclosed in a high impact casing. When the unit is submerged in water, the device emits a continuous series of ultrasonic pulses providing underwater search teams with a sonar type signal that indicates the location of a submerged voice recorder.

The unit makes a continuous four track recording from 4 audio sources. There are two record heads, one for tracks 1 and 3 and the other for tracks 2 and 4. The bi-directional tape has a capacity of 30 minutes. The power input is 115v 400Hz single phase, which is fed to the motor control circuitry to supply 130v (starting) and 70v (running) to the split phase motor.

Voice Recorder Control Panel

The voice recorder control panel allows remote monitoring and testing of the voice recorder unit, detects flight deck sounds and conversations, and controls bulk erasure of the recording tape.

The area microphone senses flight deck sounds and conversations and these audio signals are connected through a pre-amplifier to the voice recorder. The pre-amplifier is in the control panel and it raises the microphone output to a level suitable for recorder input.

Fig. 8.3. VOICE RECORDER CONTROL PANEL

The TEST switch tests all 4 recording channels in sequence, and therefore has the same function as the TEST-ALL switch on the recorder unit.

The ERASE switch operates the bulk erase circuit in the recorder and when pressed for a minimum of 2 seconds causes bulk erasure of the recording tape provided the aircraft is on the ground and the parking brake is set.

The go/no-go indicator is identical to the recorder unit indicator and it operates with the TEST switch to give an indication of the 600 Hz test tone level being recorded.

The HEADSET-600 ohms jack provides a point to monitor the audio recorded, on all the 4 channels of the tape.

CAA Configuration

Most CVR systems can be operated in either the CAA or FAA configurations. The difference between the two configurations is the audio being recorded on channel 1 of the recorder. In the CAA configuration the audio being recorded is the Hot Microphone audio from Captain's, First Officer's and Flight Engineer's audio selector panels. In the FAA configuration the audio recorded on channel 1 is all the selected audio and microphone signals from the Flight Engineer's audio selector panel.

In the CAA configuration the following is recorded:

Channel 1: Р1,Р2,Р3, Hot Microphone

Channel 2: P2, Audio Selector Panel

Channel 3: Р1, Audio Selector Panel

Channel 4: Area Microphone

CVR Operation

The voice recorder system is normally supplied from the Essential ac Bus.

Power is applied to the CVR when the aircraft is ready to taxy. This occurs when at least one Fuel and Ignition switch is ON, energising the associated fuel ON relays, and when ground power is removed. When ac power is applied it is fed to the internal dc power supply unit (which produces dc supplies for internal use) and also to the transport motor and the de-energised contact of the bulk erase relay.

With power applied to the unit the continuous loop tape is fed past the continuous erase head, and previous signals are erased using a 65 kHz bias signal. The tape moves past the record head and monitor head, and then back to the capstan drive to complete the loop. Audio signals from the audio selector panels and area microphone are fed through the record amplifiers to the record head. The recorded signals are picked up by the monitor head and amplified. The monitor amplifier output is connected to the 600 ohm headset jack on the recorder and control panel. All recorded signals are played back by the monitor head after approximately a 0.5 second top travel time.

At the completion of the flight, the crew can bulk erase the tape in about 10 seconds - providing that the aircraft is on the ground, with the parking brake set and the erase button is held for a minimum of 2 seconds. During the time of the erase function the 400 Hz tone is picked up by the monitor head providing a 400 Hz tone in the headset outputs.

When the test button is pressed, a 600 Hz tone is fed to the relevant channel or sequentially to each channel in turn where it is recorded and then monitored by the monitor head. The tone is amplified and filtered and if of sufficient magnitude will cause a dc voltage to be switched to the monitor meters giving a "go" indication, the tone being audible via the monitor jacks.

MAINTENANCE PRACTICES

The following is an example taken from an actual aircraft. Test at Control Panel

Plug headphones into the control panel and momentarily press the TEST switch. Two separate 400 Hz tones are heard in the headphones and the monitor LED illuminates twice.

Fig. 8.4. CVR SIMPLIFIED SCHEMATIC

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