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If some sub-system is not nominal, what do they say?

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3 Answers 3

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If something is nominal, then it is acting as expected, i.e., all measurements of the system are coming back in the normal/acceptable range. It's shorthand for all measurements of system X are operating as expected.

If something is not nominal, it does no good to say "not nominal", rather, one would say what is actually wrong. For instance, if fuel tank pressure was below its safe/acceptable value, they would report that the fuel tank pressure is low. That's why there appear to be a million dials or so in cockpits in addition to a computer interface. When something is off, exactly what is off is reported, and that is what the pilot / astronaut will relay.

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Mark has a fantastic insight - he is so right. If you're low on fuel, you'd scream Low on fuel! or just Emergency! If everything is just ticking along OK, they just keep mumbling "nominal." –  Joe Blow Jul 10 '11 at 11:28

I don’t know what they actually say (did a bit of Googling for Apollo records, but couldn't find what I wanted there). As “nominal” means “that has its nominal (aka normal) value”, the common word that comes to mind is abnormal.

In writing, however, it is perfectly fine to refer to such a situation as off-nominal. This term is found in plenty of technical reports and manuals of aircraft. See also, for example, this NASA webpage: Emergency, Abnormal, and Off-nominal Situations Study.

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I remember it being described as an anomaly being used to discuss deviations from the normal condition. I don't remember that in any radio calls. However in discussion, interview, and press release situations, it was used in reference to situations or results that were not normal or expected. One Wiki definition in under software anomaly is anything that differs from expectations, whether the expectations resulted from documentation, experiences, or otherwise. I think that works well for most other things.

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