Every measurement has some doubt in it: the instrument's own uncertainty, the reference standard's, temperature, the part's form, the operator. It matters most when a reading lands close to a limit.
- Accuracy ratio. A common rule of thumb is that the instrument's uncertainty should be no more than a quarter of the tolerance it checks (a 4 to 1 ratio), and many shops aim for a tenth. When the ratio is poor, a reading near the limit cannot tell a good part from a bad one.
- Decision rules. Decide in advance what happens to a reading close to a limit: accept it as read, apply a guard band (accept only inside a narrower limit), or re-measure with a better instrument. Write the rule into the inspection instruction, and follow your customer's rule where it sets one.
- Certificates. Read the lab's stated uncertainty against the tolerances the instrument checks. A lab uncertainty that is a large share of a tight tolerance is a reason to change the instrument or the lab.
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A management aid, not certification or a registrar audit. Using it does not by itself show conformity to AS9100D or any other standard, and your customers' requirements may add to it.
Clause numbers refer to AS9100D (SAE AS9100 Rev D) unless another document is named. The standards' text is not reproduced here: use your own licensed copy.
Aegis Insights are our own guidance, labeled wherever they appear: how we approach this work, not a requirement of any standard or customer.