Definition
A vertical line displayed alongside the altitude tape on a primary flight display (PFD) that predicts what the aircraft's altitude will be in six seconds if the current vertical speed is held. The length and direction of the line indicate the magnitude and sense (climbing or descending) of the predicted altitude change.
Plain English
A small line next to the altitude readout that shows where your altitude will be in six seconds if you keep flying the way you are right now. It points up if you're climbing and down if you're descending, and the longer it is, the faster you're moving.
Context Anchor
Seen on a glass-cockpit primary flight display, next to the altitude scale.
Derivation
"Trend" comes from the Old English trendan, meaning to turn or roll, and now means the direction something is heading. "Vector" comes from Latin vehere, to carry, and in aviation displays it refers to a line that carries information about both magnitude and direction. So a trend vector is a line that shows where a value is heading.
Why Pilots Care
Allows the pilot to anticipate and correct altitude deviations before they become significant.
Analogy
It is like the short prediction line on a moving map: it does not tell you where you must go; it shows where you are headed if nothing changes.
Intuition Check
Do not read it as a selected altitude or an instruction to climb or descend. It is only a short-term prediction based on what the aircraft is doing right now.
Example Sentence 1
As the altitude trend vector reached 10,000 feet on the tape, the pilot began reducing the rate of climb to level off smoothly.
Example Sentence 2
The pilot used the altitude trend vector to time the pitch adjustment for a smooth level-off.