Lag in an instrument's readout is defined as which of the following?

• A. The delay before an instrument needle attains a stable indication.
• B. The rate of change of altitude
• C. The difference between indicated and actual value
• D. The maximum deflection of an instrument needle

Answer: (A)

Lag is the delay between a change in the actual value and the instrument’s indication reaching a new stable value. This delay happens because moving parts and damping fluids in instruments take time to settle after a change, so the needle or display doesn’t jump instantly.

That’s why the correct description is the delay before the needle attains a stable indication. It captures the time response of the instrument, which is why readings can appear to trail the true value, especially during rapid maneuvers.

A few related ideas to keep straight: the rate of change of altitude is how fast you’re actually climbing or descending, not how quickly the instrument responds. The difference between indicated and actual value is an error in the reading, not the time it takes to settle. The maximum deflection is simply the instrument’s scale limit, not its response time.