Flight Instruments: The Airspeed Indicator

Within the cockpit of an aircraft, there are many instruments and tools that help a pilot maintain safe and efficient flight. From knowing the current altitude to being aware of how much remaining fuel is left, understanding the complete state of the aircraft at all times is crucial. The airspeed indicator is an extremely useful instrument to measure speed. In this blog, we will discuss the airspeed indicator and how it is able to provide the pilot accurate speed readings during flight.

The airspeed indicator, as with many similar aviation instruments, relies on the pitot-static system for its basic functions. The pitot static system is composed of two main parts: the pitot tube and the static port. The pitot tube is most often installed in an area of a wing or the fuselage where it can be exposed to the wind and air surrounding the aircraft. Ram air pressure, referring to pressure of air that is forced into the tube due to placement, is measured to obtain total pressure. The static port, on the other hand, is placed in an area of the fuselage that is relatively undisturbed, such that it can measure average pressure. For redundancy and as a backup in case of blockage, multiple static ports may be placed on the aircraft fuselage.

As compared to other flight instruments, the airspeed indicator is the only instrument that relies on readings from both the pitot tube and static port. Within the airspeed indicator, there is a diaphragm that is surrounded with the static pressure from the static port. As the aircraft speeds up, greater amounts of ram air pressure enters the diaphragm, slowly overtaking the static pressure. When the ram air pressure surpasses that of the static pressure, the diaphragm begins to flex, which in turn moves a pointer on the instrument to let the pilot know how fast they are currently travelling. The airspeed indicator itself is not a true airspeed indicator, as it only displays indicated airspeed. Using calibrated airspeed and a computer to correct errors such as temperature and pressure variation, the true airspeed can be provided.

Depending on the size and weight of the flown aircraft, there are various markings made on the airspeed indicator to help establish limits. These are provided in colors such as green, yellow, and red, each to provide warnings and/or limitations. For example, if a pilot begins to take too quick of a turn, the dial will begin to quickly move into the yellow region towards red, instructing the pilot to slow down their airspeed to safer levels. These operating level markings are in place so that the pilot can maintain safe flight speeds during various operations.

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