NTSB seeks the Black Boxes!
Flight Data Recorders and Cockpit Voice Recorders
By Jim Sparks
This is a headline that accompanies most major aviation accidents and everyone who follows the wire services knows the black boxes are technically known as the Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR).
In fact "Black Box" does not justly describe these devices. These metal cases are required by regulations to be either a bright orange or yellow with reflective tape.
History of data recorders
During the dawn of the Jet Age, world aviation authorities decided that certain flight conditions should always be recorded. Five parameters were originally chosen and include:
This data was imbedded on a roll of steel foil, which was continually moving at a rate of about 4 inches per hour. Unfortunately once recorded, this data was impossible to erase or to record over, so every 400 hours, the foil roll had to be replaced. It soon became apparent that five parameters were insufficient to assist in accurately determining the cause of an aircraft accident. The list was later increased to include:
• Control Column or Pitch Control Surface Position
• Engine Power Output
• Pitch and Roll Attitude
• Time of each radio transmission
A new means or recording this information also had to be developed as the steel foil could not accommodate the additional data. Magnetic tapes were used. Although these are more delicate than the steel foil, they could hold significantly more information - plus they could be written over - thus decreasing maintenance costs. As the magnetic tape is not as durable as steel foil, data recorder manufacturers had to invent new ways of making the tape survivable in the worst conditions.
This is only a test
This was accomplished by developing crash resistant housings that are subject to severe testing requirements. These include crash impact testing, which requires the unit withstand an immediate deceleration from 360 miles per hour to zero. This involves firing the data recorder from a cannon at an aluminum wall. Static load testing is accomplished by applying significant compression loads against the six sides of the case for five minutes. Puncture testing is accomplished by dropping a pointed steel rod on the recorder from a height of about 10 feet.
The device also needs to prove resilience against the effects of fire and heat. A furnace is used to initially heat the Recorder to 1,100 degrees C for one hour then 260 degrees C for 10 hours. Water resistance is the final check and involves subjecting the device to pressures equal to those at 20,000 feet below the water for 30 days.
More recent Digital Flight Data Recorders (DFDR) have the ability to monitor and store information from up to 700 sensors. And by using electronic data storage there is no further need of magnetic tape, nor are there any moving parts contained within.
Different types of aircraft operations will abide by different criteria with regard to data recorders. United States Federal Air Regulation Part 91 appendix "E" provides a list of Flight Data Recorder Specifications. Even though this regulation does not mandate Flight Data Recorders for all, it does say that all turbine powered aircraft carrying 10 or more passengers with a crew of two as listed on the type certificate are candidates for an FDR installation. US Federal air regulations Part 135 and 121 governing aircraft for commercial use have somewhat different requirements.
Location is important
Location is an important factor when installing the Black Box. Many original installations placed the device in the aircraft avionics compartment. With earlier certification requiring the FDR to tolerate up to 100 Gs and the location frequently in the nose area most recorders did not survive sudden impact situations.
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