Product Focus: CVRs & FDRs

By Capt. Edward R. Hanson Jr. | April 1, 2001
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First, the disclaimer: It’s impossible for me to write about cockpit voice recorders (CVRs) and digital flight data recorders (FDRs) with less than a passing interest in the subject. I fly jets that have these devices installed. So read on, knowing that I have a point of view, a perspective, about them. After all, that could be my voice on the tape (CVR) and my actions captured in the black box (FDR).

The problem: Airplanes sometimes crash. And the people who buy them, fly them, regulate them and ride in them want to know why. They want to know if something was done to the aircraft or its components that made the event inescapable.

The airframe and component manufacturers need data to detect possible design flaws. The owners and operators need data to perhaps amend procedures and/or training. And the public needs assurance that such an accident won’t happen to them, and that the person or organization causing the accident will be held accountable.

More than needs, the public demands answers. Tasked to provide those answers in the United States are the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB).

The Solutions

During an accident investigation, wreckage is retrieved and the crash site is measured and charted. Witness marks (physical evidence that investigators use to determine the accident’s cause) are noted and photographed. A witness mark could be broken tree limbs along the flight path leading to the crash site or the initial impact scar where the aircraft hit the ground.

Before CVRs, that is what accident investigators did: sift through the wreckage, analyzing only what they could find and making educated guesses. Their guesses often were that an accident ultimately was caused by human error, i.e., the pilot. Usually, the pilot could not defend his or her actions because he or she died during the event. Hence, the CVR.

The original CVRs had a 30-minute tape loop that captured from area microphones what was said in the cockpit. More than conversation, they could discern the sounds of controls and switches being positioned. For example, to move a Boeing 727’s flap handle requires lifting it from its stow position and moving it through several "gates." Each handle movement produces a unique sound as it clicks against the gates. Thus, the accident investigator could make a reasonable deduction of whether the handle was moved and the flaps selected.

The CVR is a rich trove of information. Even the background noise of an "empty" channel can be processed to reveal data about the aircraft. One case involved a vibration hum on an otherwise empty, unused channel. Accident investigators matched the hum to another pattern, a physical abnormality within the aircraft, which eventually was blamed for the crash.

Area microphones, feeding directly to the CVR, can only do so much. The FAA and the NTSB needed better recordings, so the Federal Aviation Regulations (FARs) were amended to require that Part 121 air carriers use boom mikes when flying below 18,000 feet.

Discovering that potentially critical data is still unavailable, the FAA may soon also require that CVRs capture the last two hours of conversation. This extra 90 minutes of recording represents a major retrofit to airline fleets and a quantum leap in data availability for accident investigation and prevention.

The Digital FDR

Beyond the CVR, accident investigators need flight data recorders to determine what the aircraft did in response to control inputs. The original FDRs only recorded very basic information such as heading, altitude, airspeed and climb rate. Now, state-of-the-art digital FDRs can capture more than 100 aircraft parameters.

With all this information, an airline can go "data mining." A carrier can nip accidents in the bud by detecting trend information on equipment and on how it is operated. This information, in turn, can be employed to develop training emphasis items.

But issues exist regarding CVRs and FDRs, and here is where my point-of-view comes out of the cage. The combination of the CVR and FDR information constitutes an intrusion of the pilot’s work environment–by potentially everyone. Not that a pilot wants to hide anything from the accident investigators. But he may not want his last actions and words played on the evening news and subjected to inexpert analysis by the public. A harmless remark to the copilot to blow off a little tension could appear as a cavalier attitude by a plaintiff’s attorney seeking a settlement. A pilot’s grunting final words as the aircraft breaks apart should not be played for the prurient interest of the public.

Yet the information is vital.

The data used for trend analysis can be facilitated by industry/government agency agreements to de-identify individual records, while maintaining data integrity. The FAA, pilots unions and airlines can agree that information from CVRs and FDRs will be used to rectify fleet behavior–to correct, but not punish. Such agreements are in place at several air carriers. From my perspective, this is more to assure that privacy can and will be respected.


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Avionica Inc.


L3 Communications

Raymond Engineering Ops./Kaman Aerospace Corp., Memory Systems Division

RTX Systems Inc.

Flight Systems Engineering Inc.

Sfim Inc.

Smiths Industries

Spirent Systems

Teledyne Controls

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