How does an aircraft gauge how fast it is traveling? How do pilots know what altitude it is travelling at? These questions can be answered with a few pieces of technological equipment: the pitot-static system, airspeed indicator, and vertical speed indicator. These components are capable of providing the aircraft airspeed, altitude, and Mach number of a plane in flight, and relay this information to the pilots in the cockpit. Each one contributes to the overall safety and proper functioning of an aircraft.
A planes pitot-static system is comprised of a number of sensors which detect the ambient air pressure by the forward motion of the plane. This includes the air pressure that is affected (pitot pressure) and the air pressure that is unaffected (static pressure). The pressure is used on its own, or in combination with each other, to provide indications of various flight measurements including altitude, airspeed, Mach number, and vertical speed.
Pitot pressure is measured in a pitot tube; this open facing tube is positioned along the axis of the aircraft. The pressure measured in the tube is a combination of static pressure and pressure from the aircraft’s forward movement. Commercial aircraft are installed with at least two independent pitot systems to ensure redundancy in case of a malfunction. Pitot pressure differs from static pressure in that static pressure is measured through a number of vents as opposed to a tube. The vents that measure static pressure are situated aerodynamically at neutral points on the fuselage. Vents are positioned on either side and feed into a common tube; this cancels out any errors arising from the positioning of the vents. Most commercial aircraft have at least two independent static systems to provide redundancy, similar to pitot pressure. An airspeed indicator is what compares the pitot and static pressure systems to determine the aircraft’s travelling speed.
An airspeed indicator typically measures the rate of travel in knots, or nautical miles per hour. In a simple indicator design, pitot pressure is fed into a barometric capsule—which is located in a sealed container— that is fed with static pressure. One end of the capsule is fixed while the other end is connected to the instrument pointer by a suitable system. The speed that is displayed on the indicator is the indicated airspeed. This is the speed of the aircraft relative to the body of air which it is flying through. This device is a bit different from a vertical speed indicator.
A vertical speed indicator is an instrument which indicates the rate of climb or descent or an aircraft, or altitude. Similar to an airspeed indicator, a barometric capsule is contained in a sealed case. The capsule is then fed with static pressure from the pitot-static system, which is installed with a calibrated nozzle. This nozzle restricts the passage of air so that there is a time delay between a change in static pressure. If the aircraft climbs or descends, the pressure within the capsule will increase or decrease, adjusting the altitude displayed.
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