What is a vertical speed indicator in an aircraft?

A vertical speed indicator (VSI) is the aircraft instrument that shows the rate at which altitude is increasing or decreasing, usually in feet per minute. In aviation, a positive indication means climb, a negative indication means descent, and a settled zero means no detected change in static pressure—not necessarily proof of level flight.

In our Aviation & Real-World Flying coverage, VSI may also be called a rate-of-climb indicator or vertical-velocity indicator. It reports a rate rather than an altitude: the altimeter shows where the aircraft is vertically, while the VSI shows how quickly that reading is changing.

What does vertical speed mean in aviation?

Vertical speed is the signed rate of altitude change over time. An aircraft gaining 500 feet in one minute has an average vertical speed of +500 feet per minute (ft/min or fpm); one losing 300 feet in 30 seconds is descending at approximately −600 fpm.

A conventional aircraft VSI derives this rate from changes in atmospheric pressure. It does not show pitch attitude, climb angle, airspeed, height above the ground or terrain clearance. An aircraft can also have a nose-up attitude while maintaining zero vertical speed.

Most light aircraft use feet per minute, while some instruments use metres per second. Airliner displays may show rates of several thousand feet per minute. Always read the unit and multiplier printed on the instrument rather than assuming the scale.

How does a vertical speed indicator work?

A conventional VSI measures the pressure difference created when static pressure reaches one part of the instrument faster than another. It is connected to the aircraft's static-pressure system, not directly to the pitot tube.

Inside the instrument, a flexible diaphragm receives static pressure directly. The sealed instrument case receives the same pressure through a calibrated restriction:

• During a climb: outside pressure decreases. Pressure inside the diaphragm falls promptly, while pressure in the case falls more slowly, moving the pointer towards climb.
• During a descent: outside pressure increases. The diaphragm responds first and the reversed pressure difference moves the pointer towards descent.
• At a constant pressure altitude: pressure equalises through the restriction and the pointer returns towards zero.

This calibrated delay is what allows the instrument to calculate a rate, but it also causes the familiar lag. Our explanation of how static and pitot pressure feed the flight instruments covers shared-source blockages and the resulting indications in more detail.

What is an instantaneous VSI?

An instantaneous vertical speed indicator (IVSI) adds an acceleration-sensitive mechanism that produces an immediate trend indication while the normal pressure differential develops. It reacts faster than a conventional VSI, although “instantaneous” does not mean perfectly free of filtering, vibration or transient errors.

On a primary flight display, vertical speed normally appears beside the altitude tape. The exact symbology varies, so our flight-simulator PFD reading guide explains how to distinguish the live indication from bugs and selected targets.

How do you read a rate-of-climb indicator?

Read the VSI from its zero mark in the direction labelled climb, UP or positive, or towards descent, DN or negative. The pointer's position gives the estimated rate rather than the aircraft's altitude.

• +500 fpm: altitude is increasing at approximately 500 feet each minute.
• −700 fpm: altitude is decreasing at approximately 700 feet each minute.
• Zero: the instrument detects no continuing static-pressure change after it has settled.

Many analogue dials are marked in hundreds of feet per minute, so a pointer at “5” means 500 fpm only if the face indicates a hundreds multiplier. Other scales use thousands, metres per second or uneven spacing near the ends of the dial.

In a traditional six-pack, the VSI is usually the lower-right instrument. Our Cessna 172 panel walkthrough shows its position and how it is cross-checked with the altimeter, airspeed indicator and attitude indicator.

Does zero on the VSI always mean level flight?

No. A zero indication means that the VSI senses no continuing pressure change; it does not independently prove that the aircraft is maintaining altitude.

A conventional VSI may remain near zero for the first few seconds after a climb or descent begins. It can also return to zero after a static-system blockage, even while the aircraft changes height. Confirm level flight with a stable attitude, a stopped altimeter trend and, when available, independent altitude information.

The reverse can happen too: turbulence, disturbed airflow at the static port or an abrupt simulated weather-pressure change may produce a temporary non-zero indication without a sustained change in geometric height.

Why does a VSI lag or show the wrong rate?

Normal VSI lag comes from the calibrated pressure restriction, while persistent errors usually point to disturbed static pressure, a blockage, leakage, a display-source problem or unrealistic simulator conditions.

• Normal instrument lag: a conventional VSI takes several seconds to approach the established rate. Repeated pitch corrections made before it settles create needle chasing.
• Turbulence or manoeuvring: rapid pressure changes make the pointer fluctuate. Use the average trend rather than reacting to every movement.
• Blocked static source: a mechanical VSI normally settles towards zero and becomes unusable. The altimeter and airspeed indicator sharing that source may also become unreliable.
• Pitot-only blockage: this does not ordinarily disable a mechanical VSI because the VSI uses static pressure rather than pitot pressure.
• Alternate static selection: changing to cabin or another alternate source can cause a temporary jump as the pressures equalise.
• Static leaks or position error: leaks, slips, disturbed airflow or poor static-port modelling can create offsets or disagreement between instruments.
• Electronic-system failure: a failed air-data source may freeze, remove or flag the vertical-speed display. A red cross, warning flag or blank scale means invalid data, not zero vertical speed.

How do you diagnose a faulty VSI in a flight simulator?

A stable flight test separates normal lag from a panel, failure-setting or weather problem.

1. Return to normal simulation: leave slew, replay or active-pause modes and use the normal simulation rate.
2. Stabilise the aircraft: hold a steady attitude, power setting and airspeed long enough for a conventional needle to settle.
3. Check the measured rate: compare altitude change over a timed interval. A 250-foot increase in 30 seconds is approximately 500 fpm.
4. Cross-check other sources: compare the altimeter, standby instruments and any independent altitude data. If several static instruments misbehave together, suspect their common source.
5. Inspect simulator conditions: disable intentional instrument failures and check for abrupt live-weather pressure transitions or conflicting autopilot and controller inputs.
6. Compare another aircraft: if every aircraft has the problem, investigate global settings, weather or controls. If only one add-on is affected, its panel configuration or instrument modelling is the likelier cause.

A traditional pneumatic VSI does not need electrical power, but an electronic display and its air-data computer do. That distinction explains why a round mechanical instrument may continue working after an electrical failure while the PFD indication disappears.

How should pilots use the VSI?

Use the VSI to confirm and refine a climb, descent or level-off established with attitude and power, not as the sole pitch-control instrument.

1. Set attitude, power and configuration: establish the aircraft's expected climb or descent condition.
2. Confirm the trend: cross-check the VSI with the altimeter, attitude indicator and airspeed.
3. Allow for lag: hold each small correction long enough to assess its effect instead of chasing the pointer.
4. Anticipate the level-off: begin changing attitude and power before reaching the target altitude, then use the VSI to confirm that vertical movement is stopping.

A common planning approximation for a three-degree descent is groundspeed multiplied by five. At 120 knots, that suggests roughly 600 fpm. Wind, aircraft configuration, procedure constraints and path guidance still determine the rate actually required.

What vertical speed should you use?

There is no universal correct vertical speed because aircraft performance, weight, density altitude, airspeed, power and the required flight path all affect the answer. Light-aircraft climbs are commonly flown to a published target airspeed, with the resulting rate allowed to vary; descents may instead require a calculated profile.

Our guide to choosing a sensible climb or descent rate covers practical targets and the groundspeed-based descent calculation without treating one fpm value as suitable for every aircraft.

What does vertical-speed mode do on an autopilot?

In vertical-speed mode, the autopilot changes pitch to maintain a selected climb or descent rate. The selected value is a command; the separate VSI pointer or number shows what the aircraft is actually achieving.

A mistake we see often in simulators is concentrating on the selected rate while ignoring airspeed. In a climb, demanding too much vertical speed can make airspeed decay; in a descent, it can allow speed to increase. Some aircraft provide envelope protection or automatic thrust control, but basic systems do not guarantee it.

Choose VS mode when a particular moderate rate is required and airspeed can be monitored and controlled. When climb airspeed is the priority, an IAS or flight-level-change mode may be more suitable if the aircraft provides it and its procedures call for it.