Definition
The short delay between an actual change in the airplane's flight condition and the moment a cockpit instrument displays that change. The lag is caused by the mechanical, gyroscopic, or pressure-sensing characteristics of the instrument itself, meaning the indication always trails reality by a small but noticeable amount.
Plain English
Instruments don't update instantly. When the airplane changes attitude, speed, or altitude, the gauges take a moment to catch up.
Context Anchor
Seen during integrated flight instruction when a pilot compares outside visual cues with cockpit instrument readings.
Derivation
Lag' comes from older English meaning 'to fall behind' or 'move slowly.' Here it describes how the instrument's reading falls behind what the airplane is actually doing.
Why Pilots Care
Anticipating instrument lag prevents over-controlling the aircraft when making attitude or power corrections.
Analogy
It is like stepping onto a bathroom scale and waiting a moment for the number to settle. The weight is already there, but the display takes a moment to show it clearly.
Intuition Check
Instrument lag does not mean the instrument is broken. It means the instrument may naturally show a change slightly after the airplane actually makes that change.
Example Sentence 1
The instructor reminded the student to anticipate instrument lag during the climb, since the altimeter and vertical speed indicator would not show the level-off the instant the pitch was reduced.
Example Sentence 2
Recognizing instrument lag in the altimeter during a climb allowed smoother pitch adjustments without chasing the needle.