Vertical Speed Indicator (VSI/IVSI): Principle, Lag, and Turning Error

022-02-05Pablo Asensio Martínez2026-04-011 min

The Vertical Speed Indicator (VSI) displays the rate of climb or descent in feet per minute (ft/min) or meters per second (m/s).

Principle of Operation

The VSI measures the rate of change of static pressure.

  • Capsule: Connected directly to the static vent (pressure changes immediately).
  • Case: Connected to the static vent through a restricted orifice called a metering unit (pressure changes slowly).
  • Operation: During a climb, the pressure inside the capsule drops instantly, while the pressure in the case remains higher for a moment due to the restriction. This differential pressure causes the capsule to compress, indicating a climb.

Approximate Conversion: $1 \text{ m/s} \approx 200 \text{ ft/min}$.

Instantaneous VSI (IVSI)

Conventional VSIs suffer from lag (a delay of several seconds). The IVSI overcomes this using an accelerometer system (dashpots/pistons).

  • Mechanism: When a climb/descent is initiated, the vertical acceleration moves a piston, creating an immediate pressure change in the capsule to drive the pointer before the static pressure differential builds up.
  • Turning Error: In steep turns (bank > 40°), the accelerometer may react to G-forces, causing erroneous indications.
  • Turbulence: IVSI can be over-sensitive in turbulence; pilots should scan trends rather than instantaneous fluctuations.

Blockage

Since the VSI relies solely on static pressure:

  • Static Blockage: The pressure in the capsule will eventually equalize with the pressure in the case (trapped pressure). The instrument will fall to zero regardless of the aircraft's actual vertical motion.
  • Pitot Blockage: No effect.