|Cockpit Voice Recorders (CVR) and Flight Data Recorders (FDR)|
Sounds of interest to an investigator could be engine noise, stall warnings, landing gear extension and retraction, and other clicks and pops. From these sounds, parameters such as engine rpm, system failures, speed, and the time at which certain events occur can often be determined. Communications with Air Traffic Control, automated radio weather briefings, and conversation between the pilots and ground or cabin crew are also recorded. A CVR committee usually consisting of members from the NTSB, FAA, operator of the aircraft, manufacturer of the airplane, manufacturer of the engines, and the pilots union, is formed to listen to the recording. This committee creates a written transcript of the tape to be used during the investigation.
FAA air traffic control tapes with their associated time codes are used to help determine the local standard time of one or more events during the accident sequence. These times are applied to the transcript using a computer process, which provides a local time for every event on the transcript. More precise timing for critical events can be obtained using a digital spectrum analyzer. This transcript contains all pertinent portions of the recording and can be released to the public at the time of the Safety Board's public hearing.
The CVR recordings are treated differently than the other factual information obtained in an accident investigation. Due to the highly sensitive nature of the verbal communications inside the cockpit, Congress has required that the Safety Board not release any part of a CVR tape recording. Because of this sensitivity, a high degree of security is provided for the CVR tape and its transcript. The content and timing of release of the written transcript are strictly regulated.
The FDR onboard the aircraft records many different operating conditions of the flight. By regulation, newly manufactured aircraft must monitor at least twenty eight important parameters such as time, altitude, airspeed, heading, and aircraft attitude. In addition, some FDRs can record the status of more than 300 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms.
With the data retrieved from the FDR, the Safety Board can generate a computer animated video reconstruction of the flight. The investigator can then visualize the airplane's attitude, instrument readings, power settings and other characteristics of the flight. This animation enables the investigating team to visualize the last moments of the flight before the accident. Both the Flight Data Recorder and the Cockpit Voice Recorder have proven to be valuable tools in the accident investigation process. They can provide information that may be difficult or impossible to obtain by other means. When used in conjunction with other information gained in the investigation, the recorders are playing an ever increasing role in determining the Probable Cause of an aircraft accident.
Flight Data Recorder
Time recorded ........ 25 hour continuous Number of parameters .............. 5 - 300+ Impact tolerance ....................... 3400Gs /6.5ms
Fire resistance ........................... 1100 degC/30 min Water pressure resistance ......... submerged 20,000 ft
Underwater locator beacon ...... 37.5 KHz Battery: 6yr shelf life 30 day operation
Cockpit Voice Recorder
Time recorded .......................... 30 min continuous, 2 hours for solid state digital units
Number of channels ................. 4 Impact tolerance ...................... 3400 Gs /6.5ms Fire resistance .......................... 1100 deg C /30 min
Water pressure resistance ........ submerged 20,000 ft Underwater locator beacon ...... 37.5 KHz Battery: 6yr shelf life 30 day operation
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