Aircraft Vertical Speed Indicator (VSI): How Does it Work?

The airspeed indicator tells a pilot their horizontal velocity, and the altimeter advises pilots of their current altitude, but how does a pilot know how fast that altitude is changing? For this you need to consult your aircraft’s vertical speed indicator (VSI).

What is an aircraft vertical speed indicator?

The vertical speed indicator (VSI) is one of the six-pack of instruments that is in the cockpit. It displays an aircraft’s rate of climb or descent in hundreds of feet per minute (in the United States). The VSI gathers its measurements via the pitot-static system. You may also hear the vertical speed indicator referred to as a variometer, Vertical Velocity Indicator (VVI) or a rate-of-climb indicator.

Main components of a vertical speed indicator

When you break it down, a vertical speed indicator is simply built and its workings easy to understand. There are just six main components that work together to generate a vertical speed reading.

Static Pressure Port

The static pressure port is a flush mounted opening placed on the outside of the aircraft in an area of relatively undisturbed airflow. It collects the ambient air from outside the aircraft and channels it into the static line.

Static Line

The static line is the hollow tube that connects the static pressure port to the case and diaphragm of the vertical speed indicator.

Case

The case is the housing that contains the other components of the vertical speed indicator. The case is connected to the static line via a connection with a calibrated leak.

Calibrated Leak

The calibrated leak is a special type of connection between the case and the static line that prevents the pressure inside the case from changing immediately when air flows into the case.

Diaphragm

The diaphragm is a flexible metal container located inside the case and attached directly to the static line. Rods and gears also connect the diaphragm to the needle on the face of the vertical speed indicator’s cockpit display gauge.

Face

The face is the front part of the vertical speed indicator which is visible in the cockpit and which the pilot looks at to get a vertical speed reading. The needle on the face of the VSI is connected to the diaphragm via gears and rods.

How does a vertical speed indicator work?
A vertical speed indicator works using the pitot-static system. External air is collected through the static pressure ports and flows through the static line into the case and diaphragm. The air flows into the case through the port with the calibrated leak while the diaphragm receives air directly from the static line.

When the aircraft climbs, the static pressure outside the aircraft decreases. This lower pressure air flows through the static line and the pressure in the diaphragm decreases. Since the calibrated leak slows the pressure change inside the case, the case will initially be at a higher pressure than the diaphragm. The diaphragm collapses down on itself like an accordion due to the pressure differential. This movement turns the rods and gears, moving the needle on the face of the VSI instrument.

The inverse happens when the plane descends. As the external static pressure increases and makes its way into the diaphragm and case, the diaphragm will be at a higher pressure than the case. It will expand and move the face needle in the opposite direction.

Difference between IVSI and VSI

An IVSI is an instantaneous vertical speed indicator. Like the standard vertical speed indicator, the IVSI also advises pilots of their rate of ascent or descent, however the IVSI is built to achieve a more lag-free reading.

On a VSI, there is a six to nine second delay from the time the aircraft changes altitude until the pressure differential between the VSI case and diagram causes the needle on the VSI instrument face to move. An IVSI decreases that time by using a pair of accelerometer actuated pumps along with the diaphragm for a near real-time reading which helps pilots to avoid “chasing the needle.”

To learn more about the pitot-static and other basic aircraft systems, read Dale De Remer’s Aircraft Systems for Pilots.

AircraftEducation

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