Definition
A condition in which the distribution of weight along the aircraft's longitudinal (nose-to-tail) axis causes the airplane to be nose-heavy or tail-heavy, requiring the horizontal tail surfaces to produce a corrective force to maintain level flight.
Plain English
When the load in the airplane is shifted too far forward or too far back, the airplane tends to tip nose-down or tail-down, and the tail has to work harder to keep it flying level.
Context Anchor
Seen in weight-and-balance discussions, especially when checking whether the loaded airplane will stay within its approved forward and aft balance limits.
Derivation
Longitudinal comes from the Latin longitudo, meaning length. Here it refers to the lengthwise axis of the airplane — running from nose to tail. Unbalance simply means the weight is not evenly distributed along that line.
Why Pilots Care
It changes pitch stability, increases or decreases control forces, and can make the airplane difficult or unsafe to fly if left uncorrected.
Analogy
Think of a playground seesaw. If too much weight is on one end, it no longer balances correctly; an airplane can have the same kind of front-to-back problem.
Intuition Check
Do not read unbalance here as only a left-right problem. Longitudinal unbalance means the weight problem is along the airplane’s length, from nose to tail.
Example Sentence 1
After loading two heavy passengers in the back seat with no baggage forward, the pilot recognized a longitudinal unbalance and shifted some weight forward before takeoff.
Example Sentence 2
The weight-and-balance form showed longitudinal unbalance outside limits, so the crew moved two passengers forward before takeoff.