Letters to the Editor

Subject: Flight Data Recorders Need Battery Back-Ups
From: Michael Slack mslack@slackdavis.com

Recently, an electrical storm rumbled through my section of Central Texas and knocked out the power in my neighborhood. As I walked past my child's bedroom, the only visible light came from my daughter's alarm clock that was running on its backup battery.

That is when it occurred to me: if something as simple and relatively unimportant as a child's alarm clock has a backup, why don't aircraft black boxes?

The last six minutes on Swissair Flight 111 are not known and will probably never be known, despite the efforts of air accident reconstructionists. Both the cockpit voice recorder [CVR] and the flight data recorder [FDR] on Swissair 111 stopped operating six minutes before the crash, leaving investigators without vital clues.

The CVR tapes cockpit noise including pilots' voices, engine noise and radio conversations. The FDR records key information such as altitude, airspeed and flap position.

Some investigators think the "black boxes" stopped recording due to electrical problems; others think the pilot cut the power while following an emergency checklist, to find why they had smoke in the cockpit, but such vital equipment should never be without power before impact. They should be powered by a backup power system independent of the plane's primary power system, much like office computer systems.

The lack of backup batteries, that switch on during an electrical failure to provide emergency power, is a critical flaw that should be immediately corrected. The recorders are the chief forensic tools in any air crash investigation, and must operate as long as possible to save all possible information.

Obviously, the recorders have never been given the design scrutiny they need. They need to be looked at as a separate, critical aircraft system.

Currently, recorders are plugged into the aircraft's emergency power supply. If the aircraft suffers a massive electrical shutdown, as is theorized about Swissair Flight 111, there is no backup battery power, and the information is lost.

Most airlines have been reluctant to discuss whether the change is a good idea, but Canadian and U.S. regulators might consider tapping alternative power sources for the two data recorders.

The solution is not beyond state of the art. We cannot continue to cripple along and modify these devices based on a reactive approach. These recorders need to be able to run in the event of a power loss. Power loss is a design question that should be on the engineer's checklist. A self-contained, backup power system is the only way to ensure that all the vital data is contained.

EDITOR’S REPLY:

I cannot agree with the following statements:

The battery backup idea is not new and has been looked at before. The problem that such a proposal presents is: how do you backup the data SOURCES that feed into the recorder? A recorder, which continues to run on its own battery, is worthless unless the data sources are also powered. Picture it as a regular tape recorder that has 200 microphones plugged into it to record 200 different persons, who are widely separated from the tape recorder and from each other.

The two recorders are usually powered from different electrical buses, so that a single bus failure will not cause both to fail. They are not powered from "emergency" buses; for good reason: The ultimate emergency bus is reserved for the absolutely essential items the pilot needs for survival, if all other power sources fail. Such items as the captain's flight data instruments, essential radios and lights are on the battery-powered emergency bus. Additional items, such as CVR and FDR, are not on that bus because it would drain the battery power even faster. With just the essential items powered, the pilot can only expect the battery to last 20 to 30 minutes. If the battery is drained before he is able to land or achieve flight in visual conditions, the plane will crash because the pilot will not know the attitude of the plane (spatial disorientation).

Swissair's FDR recorded over 200 parameters (think of each one as a microphone). Those data sources are powered by a variety of different buses, but many would be knocked out by the failure of a single bus. Each data source starts with a sensor that transmits data to the air data computer (ADC) which feeds it into the flight management computer (FMC). The FMC then transmits the combined data to the FDR, which is located in the tail.

Position data from the outboard aileron could only be recorded in the FDR if the aileron sensor, the ADC, the FMC and all connecting wires are powered. Much of the FMC is deliberately shut down when the only power source is the emergency battery bus.

When the pilot's information is reduced to that which comes only from the emergency battery bus, most of the sensors are no longer powered, nor is the ADC, nor is most of the FMC, nor is most of the wiring in the entire plane. Such a design priority is obviously necessary because landing the plane safely is more important than preserving data for accident investigation.

If you have a detailed design proposal, on battery backup for the recorders, that would also preserve the power for the data sources that feeds into those recorders -- without jeopardizing the essential items needed for a safe landing, I would love to read it.

For more information on flight recorders, go to:  Black Boxes.

November, 1998, revised August, 2002

Robert J. Boser    
Editor-in-Chief 
AirlineSafety.Com

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