Definition
An imaginary straight line running lengthwise through the middle of the aircraft's fuselage, from nose to tail, used as a reference axis for measuring balance, alignment, and the location of components on the airplane.
Plain English
A make-believe line drawn down the exact middle of the airplane's body, from the front of the nose to the back of the tail. It's used as a reference point to describe where things are located on the airplane.
Context Anchor
Seen in weight and balance, stability, and center-of-gravity discussions when describing where weight is located across the width of the aircraft.
Derivation
Fuselage comes from the French fuseau, meaning spindle, because the body of an early airplane was shaped like a long spindle. Centerline simply means the line through the center. Together: the line through the middle of the airplane's spindle-shaped body.
Why Pilots Care
All horizontal arm measurements for center of gravity are taken from this line.
Grounding Statement
Picture a straight line drawn through the middle of the airplane’s body from nose to tail; weight on either side of that line affects left-right balance.
Intuition Check
Do not assume the fuselage centerline is a painted line you can always see. Here it means an imaginary reference line through the middle of the aircraft body.
Example Sentence 1
The baggage compartment sits directly on the fuselage centerline, so loading it does not affect lateral balance.
Example Sentence 2
Placing heavy cargo forward of the fuselage centerline moved the center of gravity ahead of the aft limit.