In astronaut lingo, why does "nominal" mean "okay"?

…considering that the other definition, if applied in that situation, would mean virtually the opposite?

“Only in name, you’re actually almost out.”

In aerospace engineering parlance, nominal means the designed (or named, hence the term) value for that particular parameter, which usually includes a tolerance–how much it can be off plus or minus. If the voltage on a bus is designed to be at 12 V +/- .5 V and it reads 11.89 V, then it’s nominal.

From the brief bit of study I did at university, I learned at least that aerospace engineering relies on far greater tolerances than other disciplines.

Or as my mate told me (I took civil engineering, he took aero), a civil engineering would build a plane so safe, it wouldn’t fly :wink:

Not just aerospace engineering, but anything at all that has a specification. You’re usually talking about nominals, limits, and hopefully all your values are within control limits and close to nominal. It doesn’t mean OK literally, rather, values for a given parameter are nominal (which is okay).

Heh, from what I understand, some types of airplanes, such as fighters, are intentionally built to be UNsafe, because of the added advantages in areas like maneuverability. If you are a Star Wars fan (well, a Star Wars Geek), you’ll know that the X-Wing can not be flown without an Astromech droid, because the droid performs many of the minute adjustments and tweaks that keep the fighter under the pilot’s control.

An F-16 Fighting Falcon, which came out a few years after Star Wars did, pretty much worked on the same principal, but the computer that made all the minute adjustments was build into the plane instead of being something that was plugged in every time the plane took off. So I’ve read, if you turned off the flight computer in an F-16, it would promptly go out of control before taking a wild unpredictable path that would eventually intersect with the ground. I’m not sure anyone would let you build a skyscraper that required constant adjustments by a sophisticated computer to keep it standing. :smiley:

That is basically correct although the term isn’t unsafe but unstable which is a very different idea in practice. Small planes are designed to be stable. Passenger airliners are to some degree as well. You can take your hands off the controls of a small airplane and most of them will just fly right where they are or close to it.

Stability in that case sounds good but it isn’t for something like a fighter jet. Instability gives fighters and aerobatic planes rapid maneuvability through every axis and that is what is needed for that application. Fighters have always been designed that way but fly-by-wire technology allowed us to build planes that are so unstable that they can’t be readily be controlled by humans but have a huge capability for maneuvarability.

Are you sure about that?

Hmm… well, those gadgets are ingenious little mechanical feedback devices, but I definitely wouldn’t call them sophisticated computers… and I think it’d be a stretch and a half to even call them ‘computers’ at this point.