Definition
To remove unwanted air, moisture, contaminants, or old fluid from a system by flushing it out with a clean fluid, gas, or pressure source until only the desired substance remains.
Plain English
Clearing a system out so nothing unwanted is left inside before it is put back into service.
Context Anchor
Seen in aircraft maintenance instructions for fuel tanks, lines, brake systems, oxygen systems, and other systems that must be cleared before servicing or use.
Derivation
From the Latin purgare, meaning 'to cleanse' or 'to make clean.' The maintenance use keeps that core idea: cleaning a system by driving out what shouldn't be in it.
Why Pilots Care
Air trapped in hydraulic or brake lines can cause spongy, unreliable operation. Moisture in oxygen or fuel systems can lead to icing or contamination. A properly purged system is one that will behave predictably in flight.
Intuition Check
Do not read purge as simply “wipe clean.” In aircraft maintenance, it means clearing unwanted material from inside a part, tank, line, or system.
Example Sentence 1
After replacing the brake line, the technician purged the system to remove any air bubbles before signing off the work.
Example Sentence 2
After draining the fuel, the mechanic purged the lines to remove all vapors.