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

> Learn how VSI and IVSI measure vertical speed from static-pressure changes, including lag reduction, blockage behavior, and acceleration-induced errors.

- **Subject:** 022 — Instrumentation
- **Section:** 022-02 — Measurement of Air Data Parameters
- **Topic code:** 022-02-05
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/instrumentation/measurement-of-air-data-parameters/vertical-speed-indicator-vsi/

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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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.