Definition
A cockpit instrument that displays the level of suction or positive pressure produced by the aircraft's vacuum or pressure system, which drives the gyroscopic flight instruments such as the attitude indicator and heading indicator. The gauge is normally calibrated in inches of mercury (in. Hg) and has a marked normal operating range; readings outside that range indicate the system is not delivering enough airflow to spin the gyros at their rated speed.
Plain English
A small dial in the cockpit that tells you whether the system spinning some of your flight instruments is working properly. If the needle sits inside the normal range, those instruments can be trusted. If it falls outside, the instruments they drive may be giving false readings.
Context Anchor
Seen on the instrument panel during preflight checks, instrument scans, and troubleshooting when an air-driven instrument may not be working correctly.
Derivation
Vacuum' comes from the Latin vacuus, meaning 'empty,' and refers here to a pressure lower than the surrounding air, which pulls air through the system. 'Pressure' refers to air being pushed through the system instead. The gauge measures whichever method the aircraft uses, which is why it is labeled with both words.
Why Pilots Care
A low or zero reading indicates the gyroscopic instruments may soon lose power, removing reliable attitude and heading information especially in instrument conditions.
Analogy
It is like checking a tire pressure gauge before driving, except this gauge checks whether the aircraft’s instrument system has enough air force to work properly.
Intuition Check
Do not assume “vacuum” means empty space here. In this context, it means suction: air pressure lower than the air around the airplane.
Example Sentence 1
During the run-up, the pilot checked the vacuum/pressure gauge and confirmed the needle was within the green arc before taxiing for takeoff.
Example Sentence 2
When the vacuum/pressure gauge dropped below the required range in flight, the pilot cross-checked with the turn coordinator and prepared for possible gyro failure.