Plain English Meaning
All the working parts of an aircraft, grouped by what they do — engine, electrics, fuel, brakes, instruments, and so on.
Full Definition
The interconnected mechanical, electrical, hydraulic, pneumatic, fuel, and avionics components that allow an aircraft to operate. Each system performs a specific function — such as producing power, controlling flight surfaces, supplying electricity, or managing fuel — and together they enable safe operation of the aircraft.
Context Anchor
You will see this term in flight training, preflight checks, aircraft manuals, and instructor discussions about how the airplane works and what to do if something does not work normally.