Rounding

There seems to be a theory around the bazaars that the only SAFE way to handle data from the Loadsheet etc is to round up. This is both odd and illogical. Consider:

At every stage of the process until it reaches the pilots, this data has been handled logically and safely using scientifically tried and tested rules for rounding.

Why would rounding up always be SAFER? If safer were the issue, we would weigh each an every pax. If the weight and balance is trending towards the right hand edge of the envelope (heavy), the safe path is to round up the weight and down the MACZFW as this is more limiting and places you nearer to the envelope edge. Equally, if you are at the left hand edge (light), you should round down the weight and down the MACZFW. If you are near Max ZFW, you should round up the weight. But that isn't the essence of the myths and legends: just round up. Fatally flawed thinking.

Fortunately for the more considered pilot, there is already a set of rules in place, called ISO 31-0. To summarise:

To replace a number having a given number of digits with a number (called the rounded number) having a smaller number of digits, one should follow these rules:

1. If the digits to be discarded begin with a digit less than 5, the digit preceding the 5 is not changed:
So 24.14 rounded to one decimal place becomes 24.1.

2. If the digits to be discarded begin with a 5 and at least one of the following digits is greater than 0, the digit preceding the 5 is increased by 1:
This doesn't apply to a Loadsheet MACZFW setting because we don't know the subsequent digits! But it applies to ZFW/TOW.

3. If the digits to be discarded begin with a 5 and all of the following digits are 0, the digit preceding the 5 is unchanged if it is even and increased by 1 if it is odd:
So 25.45 becomes 25.4 and 25.55 becomes 25.6. (Note that this means that the final digit is always even.)

4. If the digits to be discarded begin with a digit greater than 5, the digit preceding the 5 is increased by one.
So 26.77 becomes 26.8.

And on a vaguely similar topic, there is no such thing as a plural of an abbreviation. So kts does not exist, sorry. Nor are abbreviations written in capitals such as FT. It's lazy and it's wrong! Ah but....

Phil R has written to correct me! It seems that ICAO Doc 8400 'Abbreviations & Codes', agrees that there are no plurals but does allow for capitals. I still think it looks wrong and SI looks more consistent.

Tell me about it!

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Tuesday, 11 September 2007



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