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2 Answers

Flight Computer Error? Does anyone know why?

Asked by: 3331 views ,
General Aviation

I just got a CX-2 electronic flight computer.

I know that from 29.93 to 29.92 is 10 feet.

But I was playing around on my CX-2.  I entered 29.93 - 29.92 and got 0.01.  THEN, I multiplied that by 1000 and got 10.00023.  Odd.  I'm wondering why it didn't just display "10."

Consequently, when I just enter 0.01 x 1000 it DOES give me 10.

I didn't know if the 10.00023 was an error or if anyone had any idea why it might display that?  And yes, I tried it multiple times.

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2 Answers

  1. Brian on Nov 06, 2010

    It is the degree of accuracy that computer accounts for. Nobody really cares about 23 ten thousandths of a foot (.07 millimeters), but someone will say the computer is better for it. 🙂

    The 0.01 you’re seeing in your first example is really 0.01000023 (9 digits). The computer just isn’t showing the remaining 6 digits, probably due to some limit in display, until you multiply and get 10.00023 (7 digits).

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  2. Koehn on Nov 06, 2010

    Computers work in binary, even when doing math. There’s no accurate way to represent 1/100 in binary (just as there’s no accurate way to represent 1/3 in ten decimal digits), so the computer has to round 1/100 to a number that’s close, but not quite the same value. When you multiply the not-quite-accurate number, you multiply the error too, and get a not-quite-accurate result.
    See this for a thorough explanation of floating point math.

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