Definition
A description of the resting state of an electrical switch, relay, or valve in which the contacts or passage are open (not making contact, or not allowing flow) when the device is not energized or actuated. The circuit is completed, or the fluid is allowed to pass, only when the device is activated.
Plain English
The switch or valve sits in the open position when nothing is acting on it. It only closes when power is applied or something pushes it shut.
Context Anchor
Seen in aircraft electrical diagrams, switch descriptions, relay operation, and maintenance troubleshooting.
Derivation
‘Normally’ here means ‘in its normal, undisturbed state’ — that is, when the device is at rest with no power or pressure applied. ‘Open’ refers to an open circuit (no electrical contact) or an open passage (fluid can flow, in valves, or — in switches — no current can flow). Knowing this resolves the confusion that ‘open’ can mean opposite things for switches and valves.
Why Pilots Care
Correct understanding prevents miswiring safety circuits, warning systems, and automatic controls that depend on the default state.
Intuition Check
Normally open does not mean a door, panel, or valve is physically open. In this context, open means the electrical path is broken in the device’s normal resting state.
Example Sentence 1
The landing gear warning horn uses a normally open switch that closes when the throttle is reduced below a set position.
Example Sentence 2
A normally open pressure switch in the oil system closes to turn on the warning light if pressure drops too low.