Which instrument is used to observe rate of climb or descent and is affected by lag characteristics?

• A. Altimeter
• B. Attitude Indicator
• C. Airspeed Indicator
• D. Vertical Speed Indicator

Answer: (D)

The instrument that shows how fast you’re climbing or descending is the vertical speed indicator, and it is the one that exhibits lag. The VSI measures the rate of change in altitude, not the altitude itself, by sensing the rate of pressure change in the static system and driving a diaphragm or capsule that moves the needle proportional to that rate. Because the mechanism responds through a restrictive flow path and a flexible diaphragm, it doesn’t react instantaneously to changes in vertical speed. That lag means the indicator can momentarily lag behind the actual rate during pitch or power changes, showing a delayed climb or descent as you begin and then level off, or as you adjust to a new rate. In practice, pilots cross-check the VSI with the altimeter and attitude indicator to manage climbs and descents accurately, using the VSI for the rate while the altimeter confirms the altitude change trend.