There are six basic flight instruments in nearly every aircraft. These include the airspeed indicator, altimeter, attitude indicator, heading indicator, turn coordinator, and the vertical speed indicator. Each of them plays an important role in flight and instantaneously provides the pilot with key flight information. This blog will focus on the vertical speed indicator, including what it is and how it works.
The vertical speed indicator (VSI), sometimes known as a Rate of Climb and Descent Indicator (RCDI), signifies whether the aircraft is climbing, descending, or in level flight. In a VSI, the climb or descent is represented in feet per minute (fpm). It is an extremely helpful instrument for accuracy and stability of the aircraft’s movements, particularly in instrument flight. Instrument flight refers to conditions where visibility is so lacking that the pilot must rely solely on their instruments to guide the aircraft. The VSI, combined with the other five basic instruments, provides the pilot with a clear indication of the aircraft’s operating status.
The VSI is an example of a pitot-static system. A pitot-static system is a system of pressure-sensitive instruments used in aviation to determine figures such as airspeed, mach number, altitude, and altitude trend. The primary parts of a VSI are the diaphragm and an airtight instrument casing. The diaphragm is connected via linkage and gears to the needle on the face of the instrument. Static pressure lines are connected to both the inside of the diaphragm and the instrument casing. The casing around the diaphragm has a metered leak, which helps reflect the rate of climb or descent.
Pressure changes are measured in real time within the diaphragm as pressure causes it to expand or contract. The metered leak in the surrounding instrument casing also measures the pressure change, though the leak provides an intentional lag that allows the instrument to measure the pressure change more gradually than within the diaphragm. This delay is the result of consistent pressure leak and the corresponding rate of climb or descent as it is being measured on the instrument needle. After a few seconds in level flight, the two pressures will equalize and the vertical speed indicator will display 0 fpm.
While the VSI is a very reliable tool, it is not without limitations. The most prevalent limitations of a VSI are turbulence and static port blockage. Turbulence or other abrupt maneuvering can both cause a calibrated leak resulting in a delay of six to eight seconds, making the VSI effectively useless. If the aircraft is experiencing turbulence, the pilot should attempt to maintain a constant pitch attitude by monitoring either the attitude indicator or outside visual references. This can be more effective than relying on an imperfect instrument reading. Similar to an altimeter, which is also operated via a pitot-static system, a VSI will display ‘0’ when a static port becomes blocked. Additionally, no change will occur during either a climb or descent. Some airplanes are equipped with an alternate static source to provide a supplemental or backup source of static air to the flight instruments.
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