44
$\begingroup$

In common parlance, "nominal" means "in name only", "small (amount or quantity)" or "stated, but not necessarily reflective of reality".

So, the general sense is that "nominal" means something that should not be taken at face value.

How and why, then, did it come to mean "working correctly" in space travel? Is the implication here that it really means "working as far as we can tell"? Or is there a deeper meaning?

$\endgroup$
  • 13
    $\begingroup$ Answered well at english.stackexchange.com/a/189686 $\endgroup$ – Organic Marble Jul 17 at 16:47
  • 1
    $\begingroup$ It's not accurate to say it means "working correctly". Firstly: it mostly applies to readings/values. Velocity can be nominal, whereas to say velocity is "working correctly" is meaningless. Second: even when it could be tied to a system as an indicator of performance, it still is not directly a measure of correctness. For example if a system over-performs, it would be working correctly, by could still be off-nominal. $\endgroup$ – ANone Jul 18 at 16:06
  • $\begingroup$ it's even better to use the word norminal $\endgroup$ – Sdarb Jul 19 at 21:24
  • $\begingroup$ Of 12: "6. Of or relating to the presumed or approximate value, rather than the actual value." and "10. According to plan or design; normal." $\endgroup$ – Peter Mortensen Jul 20 at 12:01
  • $\begingroup$ I feel like this question could use an illustrative usage example. :) $\endgroup$ – Ilmari Karonen Jul 20 at 16:41
82
$\begingroup$

As explained in the answer that Organic Marble dug up, aerospatial "nominal" is really a shorthand for something like within the allowed tolerances around the nominal (i.e. specified) value. We can speculate about how that shorthand evolved.

Example: Assume the thrust of an engine according to its design and specification -- its nominal thrust -- is 45 kN, but our mission rules (or whatever rules apply) say we're allowed to proceed if it's within +/- 10% of that figure. An engineer sees it's running at 42kN. How's the engine doing? The engineer might want to say "it's within the allowed tolerances from nominal."

That's a mouthful to say, so it gets shortened to something like "close enough to nominal" to save time, and eventually the literal meaning of "nominal" becomes irrelevant, and the engineer just says "nominal" to mean close-enough-to-the-expected-value and "off-nominal" (or "high", or "low", or "abort now-now-now") to mean not-close-enough.

$\endgroup$
  • 53
    $\begingroup$ It's not just aerospace. 'Nominal' is used in all of engineering. $\endgroup$ – Hobbes Jul 18 at 8:01
  • 2
    $\begingroup$ I'm not sure this answers the question, and in fact it seems a bit circular. So it's shorthand for tolerances around the nominal (i.e. specified or expected), fine, but OP's question is why nominal is being used to mean "specified or expected" when it doesn't normally have that meaning. $\endgroup$ – ArrowCase Jul 18 at 16:10
  • 1
    $\begingroup$ I'll remove "expected". "Specified" for "nominal" doesn't seem a stretch to me. $\endgroup$ – Russell Borogove Jul 18 at 16:17
  • 2
    $\begingroup$ @JaminGrey AFAIK / I've always understood that dressed ("pretty" smooth surface) timber is undersized because it was nominally [ :-) ] at full stated size when "rough sawn" and the act of dressing it has removed material on all major faces. I may be wrong :-) $\endgroup$ – Russell McMahon Jul 19 at 7:45
  • 4
    $\begingroup$ As an engineer, I'm naturally biased towards these words making sense, but to me, it seems very reasonable to use "nominal" in the "name" sense... its the named value. Everybody knows that in the real world you basically never achieve any named value, but it can be close enough that people may act as-if it is the named value. $\endgroup$ – Cort Ammon Jul 19 at 22:12
5
$\begingroup$

Terms like "correct" and "right" might imply the condition might be checked further beside the value being inside the acceptable parameters, so it’s good to stay away from that.

Terms like "normal" might imply not the specific condition or even worse a comparison with some historic averages.

For that reason having a less loaded term (implying you are only comparing the reading with the procedures/specification tolerances) seems to be a good choice.

The usual definition of the term „nominal“ coming from its origin („of name“ or „as it is named“) somehow reflects that in a double way. First, it references the fact that you are talking about the „numerical reading“, not the actual condition and secondly that somebody has called out for an expected value for the current condition.

Webster actually defines two meanings which both apply to those mentioned aspects:

: of, being, or relating to a designated or theoretical size that may vary from the actual : APPROXIMATE

the pipe's nominal size

And the actual definition used in aviation/engineering:

: being according to plan : SATISFACTORY

everything was nominal during the launch

$\endgroup$
  • $\begingroup$ More to the point, if the craft is supposed to be accelerating at 1.00g, but an accelerometer with specified accuracy of 0.02g reports that they are producing 0.11g, and the engines are actually producing 0.12g, the accelerometer reading would be "correct" within the accelerometer's specification, but represent a condition that is outside the specifications for the system as a whole. $\endgroup$ – supercat Jul 18 at 17:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.