sept 11 and the black boxes

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    And even if we forget the steel, there's the curious incident of the planes' black boxes - not ONE of the EIGHT black boxes - two per plane - survived! Not just those of the planes that crashed into the Twin Towers, but also of the other two planes! Not even of the one that crashed into the ground in Pennsylvania.

    Do you know the kind of crash these black boxes are designed to withstand? The National Transportation Safety Board (NTSB) web site gives valuable information regarding these devices.

    Regarding the Cockpit Voice Recorder it says:

    "The CVR records the flight crew's voices, as well as other sounds inside the cockpit. The recorder's "cockpit area microphone" is usually located on the overhead instrument panel between the two pilots. 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." [NTSB web site]

    Regarding the Flight Data Recorder it says:

    "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." [NTSB web site]
    Thus as we see each plane is equipped with two separate "Black Box" devices which relay different yet complimentary information.
    It goes on to say:

    "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 to obtain by other means." [NTSB web site]
    They are meant to be the hardy and sure way to get information regarding the last minutes before the plane went down.

    Here are the specifications of these devices including what they can withstand:

    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

    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

    These specifications indicate equipment which can survive just about any conditions and any crash. Indeed, Black Boxes have been recovered from crashes in swamps, mountains, and oceans.
    ABC News reports:

    "Although investigators look for an entire black box, sometimes the only parts of the device that survive are the recorder's crash-survivable memory units (CSMU). the csmu is almost indestructible. It is housed within a stainless-steel shell that contains titanium or aluminum and a high-temperature insulation of dry silica material." "It is designed to withstand heat of up to 2,000 degrees fahrenheit for one hour, salt water for at least 30 days, immersion in a variety of liquids such as jet fuel and lubricants, and an impact of 3,400 G's. By comparison, astronauts are typically exposed to up to six Gs during a shuttle takeoff." [ABCNews]

    So the Black Box is designed to withstand an impact of 3,400 G's. So what about the Pennsylvania crash site?

    "The voice recorder was said to be heavily damaged, and the manufacturer was being asked to help with further analysis. The plane that crashed in Pennsylvania was reported to have hit the ground in excess of 500 miles per hour." [ABCNews]

    The plane hit the ground at 500 mph. Taking the weight of the plane, its speed of descent, the G's end up much, much lower than 3,400.

    Since each plane has two separate Black Boxes which are designed to be indestructible in the event of a tragedy, that makes a total of eight black boxes. We are to believe that all the Black Boxes were damaged beyond use, while a measly paper passport survived!
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