Definition
The delay between an actual change in aircraft vertical movement and the corresponding indication on the vertical speed indicator (VSI). Because the VSI works by measuring the rate at which static pressure changes inside a calibrated leak, the needle takes a few seconds to settle on an accurate reading after a pitch change, climb, or descent is initiated or stopped.
Plain English
The VSI doesn't react instantly. When you start climbing, descending, or level off, the needle takes a few seconds to catch up and show the true rate.
Context Anchor
Seen when learning how a vertical speed indicator works and when interpreting climb or descent indications during instrument flying.
Derivation
Lag' comes from an older English word meaning to fall behind or trail. Here it describes the instrument trailing behind what the aircraft is actually doing.
Why Pilots Care
A pilot who waits for the VSI to catch up will overshoot altitudes; anticipating the lag allows smooth, accurate level-offs and rate changes.
Grounding Statement
After you start a climb, the airplane is already climbing before a standard vertical speed indicator fully shows the new climb rate.
Intuition Check
Do not assume the vertical speed indicator always shows what is happening right now. With lag characteristics, the indication may be a few seconds behind the airplane’s actual movement.
Example Sentence 1
Because of the VSI's lag characteristics, the instructor told the student to make a small pitch change and wait a few seconds before judging the result.
Example Sentence 2
During the descent check, the instructor reminded the student to hold the target pitch attitude and not chase the lagging VSI needle.