As aircraft are responsible for the lives of both passengers and aircrew, they are equipped with many components, ensuring that they operate optimally. Some of these parts include a set of six basic aircraft instruments that provide critical information to pilots during flight. The six primary aircraft instruments include the attitude indicator (AI), heading indicator (HI), turn coordinator, airspeed indicator, altimeter, and vertical speed indicator (VSI).
While reading the instruments is fairly straightforward, the systems that help them operate are more complex. More than that, some instruments are referred to by multiple names and acronyms, making it all the more complicated. As such, we will begin by covering the attitude indicator (AI), heading indicator (HI), and turn coordinator first, all of which are defined as gyroscopic instruments.
Attitude Indicator (AI)
The Attitude Indicator (AI), sometimes referred to as the Artificial Horizon (AH), serves as the primary attitude instrument in aircraft. Providing the pilot with critical information about the aircraft’s bank angle and pitch angle in relation to the horizon, AIs allow pilots to interpret the attitude of the aircraft and adjust accordingly. More than that, AIs provide a good indication of what the other instruments will indicate. For instance, if the AI indicates a bank angle that is beyond or below the wings’ level, the heading indicator will indicate a turn in the direction of the bank. Furthermore, the AI displays specific markings that enable the pilot to determine the pitch and bank angle with increased accuracy.
Heading Indicator (HI)
Also referred to as the Direction Indicator (DI) or Directional Gyro (DG), the HI is the primary horizontal direction indicator in aircraft. However, it suffers from drift errors and needs to be adjusted periodically with a magnetic compass. Some heading indicators consist of a heading bug feature that enables the pilot to move a colored marking to any heading on the HI for reference. It is important to note that the HI is not the same as the Horizontal Situation Indicator (HSI), which is an improved version that includes VHF Omnidirectional Range (VOR) and Instrument Landing System (ILS) indications.
Often called the turn and bank indicator, the turn coordinator provides information about the bank angle and coordination of the aircraft. Aircraft are in coordinated flight when the rudder input prevents the aircraft from slipping or skidding during a turn or when its tailplane is aligned with its flight path. This information is displayed by a “ball” in the instrument itself. The ball is positioned in a fluid-filled tube and is free to move in any direction. Meanwhile, the bank angle information is provided by a miniature aircraft or a vertical line that moves in the direction of the bank. Moreover, the turn coordinator has four white demarcations, each of which indicate a wings-level position and the bank angle needed for a standard rate turn.
The next set of aircraft instruments belong to the pitot-static system, those of which include the airspeed indicator, altimeter, and vertical speed indicator (VSI). To begin, the airspeed indicator measures aircraft airspeed. It is the only pitot-static instrument that uses both the static ports and pitot tubes to determine dynamic pressure, which is a measure of airspeed. It consists of colored rings within a dial that indicate different speed ranges. For example, the green ring denotes a normal operating speed while a red arc marks the speed beyond the aircraft’s capabilities.
The altimeter provides information about an aircraft’s altitude above Mean Sea Level. To do so, it must be adjusted to the correct barometric pressure setting. It utilizes information from the static ports to determine static pressure. The altimeter contains aneroid capsules that expand or contract based on the present static pressure. As they contract or expand, a series of gears and linkages display altitude information. It is quite simple to read an altimeter. The altimeter has two needles that can be read in a similar fashion to a clock. The long needle indicates hundreds of feet, while the short needle indicates thousands of feet.
Vertical Speed Indicator (VSI)
Sometimes referred to as a variometer or vertical velocity indicator, the last critical instrument is the Vertical Speed Indicator (VSI). Utilizing the aircraft’s static ports, the VSI can determine the Rate of Climb (ROC) or Rate of Descent (ROD) in feet per minute (fpm). Like the altimeter, an aneroid capsule inside the VSI expands and contracts based on changes in altitude. This change in pressure is measured and displayed on the VSI as the ROC/D. Unfortunately, this instrument lags so this should be considered when making adjustments based on the VSI alone.
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