Vertical Speed Indicators

Vertical speed–how fast a helicopter is moving up or down–is a critical value pilots monitor in flight. A vertical speed indicator (VSI) or vertical velocity indicator displays this value to the pilot. This article will explore the operational principles of VSIs and their historical evolution.

In level flight a pilot will poll the VSI periodically to ensure it reads zero. In a climb, pilots often monitor the VSI to maintain a specific climb rate, like 500 feet per minute (FPM). Pilots may also use the VSI to help avoid dangerous conditions like vortex ring state in a low-speed descent.

Early VSIs were mechanical and included a gauge visible to the pilot. An example gauge is shown in Figure 1. This gauge would be in the cockpit in front of the pilot, often slightly to the right side. When the helicopter descends at 500 FPM the needle moves down, pointing to the lower .5 on this gauge (1 represents 1,000 FPM). Conversely, when the helicopter climbs at 2000 FPM, the needle would rise to point at the top 2 on the gauge.

In modern helicopters, glass cockpits display digital VSIs, usually on the right side of the primary flight display. An example is shown in Figure 2 below. The red rectangle on the right side encloses the VSI. Horizontal bars are typically shown every 500 or 1000 FPM. An arrow, black in this example, slides up and down and points to the current vertical speed. In this case, the arrow also encloses a text readout of the vertical speed, rounded to the nearest 100 FPM.

We now explore how the various generations of VSI worked, starting with the earliest.

Mechanical VSI

Early VSI systems included the following 4 key components.

• Static port. A small, flush-mounted opening on the side of the helicopter that collects the outside, ambient air pressure.
• Aneroid diaphragm. A flexible, hollow container connected directly to the static port via a tube. The diaphragm expands / contracts as the external air pressure changes.
• Sealed case. This case also receives air from the static port, but through a restricted passage known as a calibrated leak. The calibrated leak intentionally slows the rate at which air can enter or leave the case, creating a pressure difference associated with the climb/descent rate.
• VSI gauge. This is the gauge mounted in the cockpit, visible to the pilot. The needle is mechanically linked to the diaphragm / case so that it moves when the outside pressure changes.

How VSIs work

The VSI needle is driven by the pressure differential between the diaphragm and the sealed case. This is explained in the following scenarios.

• Level flight. The air pressure inside the diaphragm and the air pressure inside the case are equal, pushing the needle to zero.
• Climb. As the helicopter climbs, the outside air pressure decreases. This lower-pressure air immediately enters the diaphragm, causing it to contract. However, the pressure inside the case changes more slowly due to the calibrated leak. This pressure differential squeezes the diaphragm and drives the needle upward on the instrument panel, indicating a climb.
• Descent. As the helicopter descends, outside air pressure increases. This higher-pressure air immediately enters the diaphragm, causing it to expand. The pressure in the case increases more slowly, creating a differential that drives the needle downward, indicating descent.

Lag time

This VSI has an inherent time lag of many seconds before it provides a stable and accurate rate of vertical change. In fixed-wing aircraft, this lag is manageable, but for the dynamic maneuvers of a helicopter, it can be a significant drawback. A helicopter pilot is often making rapid, small changes in pitch and power. A lagging instrument can lead to "chasing the needle," where the pilot over-corrects for a climb or descent that is no longer happening. This is the reason for the next improvement.

Instantaneous Vertical Speed Indicator (IVSI)

The time lag in traditional VSIs motivated the Instantaneous Vertical Speed Indicator (IVSI). IVSIs remove lag by including accelerometers to boost the pressure differential and associated needle response. This enhances pilot control of vertical speed and is particularly useful in formation flight and IFR.

How IVSIs work

When a helicopter accelerates vertically, the accelerometer detects this and “front loads” a pressure differential in the instrument. This effectively moves the needle before the usual pressure difference builds up. After a few seconds, as the pressure differential catches up, the accelerometer's effect subsides, and the IVSI functions just like a normal VSI.

Inertial reference systems (IRS)

Another device that can estimate vertical speed on a helicopter is an Inertial Reference Systems (IRS). This is often integrated with an Inertial Navigation System (INS), and uses gyroscopes and accelerometers to continuously track position, attitude, and velocity.

Once an IRS is initialized with a known position and velocity, its accelerometers measure the aircraft's acceleration, and its gyroscopes measure angular velocity. A computer then integrates this data to determine an accurate and independent measure of the aircraft's vertical velocity.

Notice an IRS-based vertical speed indication is completely independent of the pitot-static system and barometric pressure. This is a critical safety feature, as it provides a redundant source of vertical speed, immune to errors from a blocked static port, which would cause a conventional VSI to fail.

GPS-based vertical speed indication

While basic GPS only provides position and ground speed, advanced GPS units and flight management systems can calculate an aircraft's vertical speed.

Using successive altitude measurements, GPS can effectively estimate the rate of altitude change which is the vertical speed. This is completely independent of the VSI/IVSI/IRS estimates and hence is another useful redundancy for error checking.

Digital VSI

In modern helicopters like the Bell 525, the VSI may be estimated by a combination of the above instruments. The air data system (ADS) synthesizes measurements from several sensors and provides a digital vertical speed to the glass cockpit displays. This system reacts quickly to vertical acceleration, like the IVSI, but also facilitates visual alerts or warnings for conditions like vortex ring state.