Definition
A cockpit annunciator or warning device that shows the operating status of the electrical, vacuum, or pneumatic power source supplying the flight instruments, alerting the pilot when a power source has failed or is operating outside normal limits.
Plain English
A small light or gauge in the cockpit that tells you whether the power feeding your flight instruments is working properly. If that power fails, certain instruments stop giving correct readings, and this indicator is how you find out.
Context Anchor
Seen during preflight checks and instrument flight, especially in aircraft whose flight instruments depend on vacuum, pressure, or electrical power.
Why Pilots Care
Confirms reliable power to attitude, heading, and navigation instruments, which is essential for safe instrument flight.
Intuition Check
Do not read this as the flight instrument itself or as a backup power supply. It is the gauge, light, or display that shows whether the instrument’s power source is healthy.
Example Sentence 1
During the instrument scan, the pilot noticed the instrument power source indicator showing a vacuum failure and immediately shifted to the backup electrically-driven attitude indicator.
Example Sentence 2
When the alternator failed the instrument power source indicator immediately switched to the standby battery.