Definition
A short delay between a change in the aircraft's actual vertical movement and the corresponding indication on the vertical speed indicator (VSI). The VSI relies on a calibrated leak from a sealed case to measure pressure change rates, and that mechanical process takes a few seconds to settle, so the displayed rate of climb or descent trails the real motion of the aircraft.
Plain English
When you start climbing or descending, the VSI needle takes a few seconds to catch up and show the correct rate. That delay is called lag.
Context Anchor
Seen when reading the vertical speed indicator during climbs, descents, and level-offs.
Derivation
From Middle English 'laggen,' meaning to fall behind or move slowly. In aviation it keeps that everyday sense — the instrument is 'falling behind' the actual motion of the aircraft.
Why Pilots Care
Pilots must anticipate the delay and cross-check with the altimeter or attitude indicator to avoid overcorrecting pitch.
Intuition Check
Lag does not mean the instrument is useless or permanently wrong. It means the indication trails the airplane’s real movement for a short time.
Example Sentence 1
After leveling off at 5,000 feet, the pilot waited a moment for the VSI lag to settle before trimming for cruise.
Example Sentence 2
The lag in the VSI made the initial indication of descent appear smaller than the actual rate until the needle stabilized.