Definition
The named or stated value of a component or quantity, used as a reference point. The actual measured value is normally close to the nominal value but may vary within an allowable tolerance.
Plain English
The labelled or 'on paper' value of something. Real-world measurements may be a little above or below it, but the nominal value is the figure used to identify and compare the part or quantity.
Context Anchor
Seen in aircraft maintenance, electrical system descriptions, instrument specifications, engine data, and performance information when a value is given as the normal reference number.
Derivation
From Latin 'nominalis', meaning 'in name'. A nominal value is the value 'in name' — what the part is called — rather than the exact value measured on the bench.
Why Pilots Care
Knowing a value is nominal reminds the pilot or technician that the actual reading may differ slightly. A 24-volt nominal battery may read 25 or 26 volts when fully charged, and that is normal — not a fault.
Analogy
A “12-volt” car battery is called 12 volts, but a meter may show a little more or less depending on its charge and use. Twelve volts is its nominal value.
Intuition Check
Do not read nominal value as exact value. It means the stated or named value used as the normal reference.
Example Sentence 1
The nominal value of the aircraft's electrical system is 28 volts, though the bus may read slightly higher when the alternator is online.
Example Sentence 2
Technicians compare actual voltage readings against the nominal battery voltage of 24 volts.