TOPIC 13.4.2.5: COCKPIT VOICE RECORDER (CVR) STUDY NOTES

Introduction to the Cockpit Voice Recorder (CVR)

The Cockpit Voice Recorder (CVR) is a fundamental flight safety device, commonly referred to as a "black box." It is designed to monitor and store an exhaustive record of cockpit activity, including all signals transmitted and received, crew conversations, aural warnings, and any tones or incidental noises occurring in the cockpit environment.

Key functional characteristics include:

• Recording Medium: Magnetic tape is used to record data continuously for a period of $30\,\text{minutes}$.
• Continuous Loop: The recorder operates on a continuous cycle where the newest information overwrites the oldest data as it is recorded.
• Objective: The CVR is intended to be downloaded and analyzed following an accident to determine the cause or provide critical information for investigative purposes.

CVR Physical Characteristics and Location

Despite its nickname as a "black box," the actual unit is painted a high-visibility bright orange, or orange with white stripes. This color choice assists in locating the device among debris or under water.

Installation Location:

• The recorder is installed in the aft fuselage area, specifically forward of the vertical fin.
• This location is selected because the rear of the aircraft is the most likely section to survive a crash impact.
• The CVR is situated in the same region as the emergency beacons.

Component Breakdown:

1. Cockpit Control Panel: Includes the cockpit area microphone, a monitor, and an audio jack.
2. Cockpit Voice Recorder Unit: Includes the magnetic recording mechanism and an Underwater Location Beacon (ULB).

CASA Maintenance and Testing Requirements

CASA AD/REC/1 Amdt 4 dictates the maintenance and functional testing requirements for CVR installations. Technicians must confirm the proper recording of the following sources:

• Radio Communications: All voice signals transmitted from or received by the aircraft radio.
• Cockpit Audio: All flight deck conversations, audio signals from alerting or warning devices, and general ambient sounds.
• Interphone System: Voice communications between flight crew members on the flight deck.
• Navigation/Approach Aids: Audio signals used to identify navigation aids introduced through speakers or headsets.
• Passenger Address (PA) System: Crew-to-cabin or crew-to-passenger communications, provided a recording channel is utilized for the PA.

Functional Test Circuitry and Indicators

The system includes internal test circuits to verify the integrity of all four audio channels.

Test Initiation:

• Testing can be triggered via the TEST switch on the control unit.
• It can also be initiated at the CVR unit itself via an ALL TEST switch or individual discrete switches for Channels 1, 2, 3, or 4 (if the unit is so equipped).

Test Signal Logic:

• When a discrete channel switch is pressed, a $600\,Hz$ tone is applied solely to that specific recorder amplifier circuit.
• When the ALL TEST switch is activated, the $600\,Hz$ test tone is applied to each of the four record amplifiers in an alternating sequence. Each channel receives the tone for approximately $0.8\,\text{seconds}$.

System Feedback:

• The record amplifier mixes the test tone with current audio inputs and sends the sum to the recording head.
• The monitor head detects this signal and sends it to the monitor amplifier.
• Aural Indication: The tone is routed to the HEADSET test jack.
• Visual Indication: The signal is sent to the TEST meter, providing a visual "GO" or "No-Go" indication. Two sharp needle deflections into the green "OK" zone indicate proper performance.

Bulk Erase Procedures and Safety Logic

A bulk erase circuit is integrated to allow for the complete clearing of the magnetic tape, a standard procedure often performed at the conclusion of a routine flight.

Activation Requirements (Safety Interlocks): To prevent accidental erasure of data during flight or from inadvertent contact, the following conditions must be met simultaneously:

• The aircraft must be on the ground (Weight on Wheels sensing).
• The landing gear must be down.
• The parking brake must be engaged.
• The ERASE switch must be held for a specific duration (cited as at least $2\,\text{seconds}$ in description, and $15\,\text{seconds}$ in the operation summary list).

Audio Signal Processing and Recording Channels

Input Handling:

• Inputs from the cockpit area microphone or remote microphones enter a preamplifier in the control unit. This boosts the signal to protect the signal-to-noise ratio during further processing.

Noise Suppression (Roll-off Circuits): Electronic suppression of engine and turbine noise is achieved via slope circuits that gradually increase attenuation over specific frequency ranges:

• Reciprocating Engine Aircraft: $1000$ to $5000\,Hz$.
• Turbine Aircraft: $150$ to $5000\,Hz$.

Channel Allocation:

• Channel 1: Third crew member or Passenger PA system.
• Channel 2: Co-pilot's station.
• Channel 3: Pilot's station.
• Channel 4: Cockpit area microphone.

Recording Specifications:

• Bias Generator: Outputs a $65\,kHz$ signal to the record amplifiers.
• Mixing: The audio signals and the $65\,kHz$ bias are mixed to create the combination bias voltage and record current applied to the 4-track recording head.
• Frequency Response: Approximately $150$ to $5000\,Hz$.

Mechanical Operation and Head Assembly

The recording mechanism uses a bridge assembly that houses three distinct heads:

1. Continuous Erase Head: Energized by the $65\,kHz$ bias current. It operates continuously while the recorder is running, ensuring a segment of tape is erased immediately before it reaches the recording head, or $30\,\text{minutes}$ after its last recording cycle.
2. Recording Head: A 4-track device that provides coincident recording of the four amplifier outputs.
3. Monitor Head: Detects recorded signals to verify quality; during system tests, it feeds the monitor amplifier for the test meter or headset jack.

System Installation and Reliability Standards

Specific regulatory and design standards govern CVR installation to ensure the highest probability of data retrieval:

• Power Supply: Must be connected to the bus offering maximum reliability without compromising essential or emergency loads. Power ($115\,V\,AC$) is typically supplied whenever aircraft generators are active.
• Automatic Termination: The system must contain an automatic mechanism to stop the recorder and disable any erasure functionality within $10\,\text{minutes}$ of a crash impact.
• Physical Protection: It must be installed as far to the rear as possible to minimize damage from impact, fire, baggage, cargo, or structural collapse. It does not strictly require being outside the pressurized compartment, provided other protective measures are met.
• In-flight Protections: The installation must be designed so that the recorder cannot be played back in the air, and erasure is physically impossible during flight.
• Operational Status: The system is operational immediately upon closing the CVR circuit breaker; there is no manual cockpit control switch to turn the system off.

Compliance and Maintenance Intervals

Following the requirements established after 1 July 1988, the following maintenance intervals apply:

• Analogue Technology (Tape-based): Intervals not exceeding $12\,\text{months}$ or $2000\,\text{hours}$ of time in service, whichever occurs first.
• Digital Technology (Solid-state): Intervals not exceeding $24\,\text{calendar months}$.
• Approved Programs: Maintenance may follow an approved aircraft-specific program under CASR 1998 regulation 42.140, provided it incorporates the requirements of the AD.

Terms and Definitions

• Wow and Flutter: Mechanical interruptions or airframe vibrations that cause the tape transport to vary in speed, thereby distorting the recorded audio signal. Electrical power interruptions can also cause loss of sync.
• Track: A band on the magnetic tape that contains one channel of audio, running parallel with the tape length.
• Crosstalk: Unwanted interference where signals from one communications channel leak into an adjacent channel.
• Erase Head: The lead-off head used to clear previous data by generating a strong, random magnetic field.
• Bias Oscillator: A generator providing the $65\,kHz$ signal necessary for the recording and erasing processes.