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Pilots are expected to have thorough knowledge of all the instruments and indicators within the cockpit to safely fly, steer, and land their aircraft with minimal fuel expenditure. Furthermore, pilots should also be equipped with the awareness to recognize any sudden failures or malfunctions reflected by indicators to minimize risk and avert a crash to the best of their abilities.
Airspeed indicators and vertical speed indicators are two commonly used aircraft indicators for a pilot to reference, and they are designed to indicate different flight performance functions. This blog aims to unravel the differences in the working mechanisms of these critical flight indicators so that you better understand their role and importance.
Vertical speed indicators are one device among a pack of six instruments within the cockpit designed to indicate an airborne aircraft's rate of ascent or descent in feet per minute (fpm), as per the US convention of naming flight units. VSIs are also known as Rate of Climb and Descent Indicators (RCDIs) or Vertical Velocity Indicators (VVIs). They use the static pressure system to determine the speed of an aircraft at a particular altitude.
Although VSIs function only on the measurement of static pressure, they still use an internal pressure difference to determine how rapid the ascent or descent of the aircraft is. Conventionally, VSIs use a circular scale instrument and a needle attached to a component of the Electronic Flight Instrument System known as an Electronic Altitude and Direction Indicator (EADI). More advanced VSIs use a digital display to indicate the rate of rising or falling of an aircraft.
A VSI's static system contains a diaphragm enclosed within an airtight barometric capsule. The diaphragm and the case connected to it intercept air at prevailing atmospheric pressure conditions. While the diaphragm receives air at full speed, the case receives air under static pressure. The static line of the static pressure system connects directly to the inside of the diaphragm via a small aperture known as a "calibrated leak." When an aircraft ascends, there is a concomitant decrease in pressure within the capsule, and a specialized mechanical system converts the capsule’s movement into that of the display needle.
Airspeed Indicators are used to measure the speed of an aircraft during motion in a forward direction. Although different units are used for measuring airspeed throughout the aviation industry, most pilots prefer measurement in knots (nautical miles per hour) as the standard. In older aircraft models, airspeed was usually displayed by a pointer that moved across a graduated scale. However, with recent improvements in modern airplanes, airspeed is indicated as a digital reading provided by the more advanced Electronic Flight Instrument System.
An airspeed indicator uses static and dynamic pressures, collectively called "ram air," while measuring the air flow from static and pitot ports. The difference between static and pitot (dynamic/impact) pressure is registered as differential pressure, and the net result is ultimately indicated by the airspeed pointer.
Both the pitot tube and the static port are located perpendicular to each other, where the static port collects ambient air from its location at the side of the fuselage or under it. In contrast, the pitot tube collects air from the aircraft wings where it is placed in the flow of air. The airspeed indicator directs the pitot pressure into the diaphragm and the static pressure into the airspeed case.
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