Lag is best 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 flight condition and the instrument displaying that change as a stable indication. This means after a bank, pitch, or altitude change, the needle doesn’t jump to the new value immediately but takes a moment to settle. The best description is focused on that response time—the time it takes for the instrument to show a steady reading after a change. The rate of change of altitude describes how quickly altitude is changing (vertical speed), which is a different concept. The difference between indicated and actual value refers to instrument error or bias, not how quickly the instrument responds. The maximum deflection is simply how far the needle can move, not how long it takes to respond.