Summary
Highlights
The heading indicator, or directional gyro, is a basic flight instrument found in most aircraft. It operates on gyroscopic principles. In light aircraft, a vacuum pump drives the gyro for the attitude indicator and heading indicator, while the turn coordinator is electrically driven. The heading indicator shows the aircraft's current heading in degrees relative to Magnetic North. However, it lacks a mechanism to determine Magnetic North directly, requiring the pilot to align it using the magnetic compass. Unlike the magnetic compass, which suffers from errors during maneuvers, the heading indicator is more accurate and easier to read, making it the primary instrument for heading indication.
The heading indicator consists of a compass rose in the background, a miniature aircraft in the middle representing the actual aircraft's orientation, and a line extending from the aircraft indicating the current heading. In the lower-left corner, there's a cage and an adjustment knob. The compass rose displays 360 degrees of azimuth, with long lines for 10-degree increments and short lines for 5-degree increments. Cardinal directions (N, E, S, W) are represented, along with numerical headings every 30 degrees (omitting the last digit). Some indicators also feature relative heading markings for turns and a heading bug with a knob to select a particular heading, useful for manual flight and autopilot systems.
Reading the heading indicator is intuitive. The line from the miniature aircraft points to the current heading on the compass rose. For example, pointing to 'N' indicates 0 degrees, 'E' is 90 degrees, and so on. Relative heading markings are used when air traffic control requests a turn of a specific degree, rather than to a specific heading. For instance, a 45-degree right turn would involve aligning the current heading with the 45-degree relative marking indicated on the instrument.
The heading indicator utilizes the gyroscopic effect of rigidity in space. It incorporates a horizontal gyro with three degrees of freedom, meaning its plane of rotation remains rigid in space regardless of aircraft movements. When the aircraft turns, the instrument and aircraft turn, but the gyro stays fixed, allowing the instrument to measure the angle between the directional reference and the current heading.
Despite its advantages, the heading indicator has limitations. Mechanical stops prevent the gyro from rotating completely freely, causing it to 'topple' and become inoperative if excessive pitch or bank angles are reached. Pitch and bank limits are typically around 85 degrees for electric-driven indicators and 55 degrees for air-driven gyros. Additionally, the instrument suffers from gyroscopic apparent wander. Due to the Earth's rotation (15 degrees per hour), the gyro, which remains rigid in space, will appear to drift from its initial alignment with Magnetic North. This error is maximum at the poles and minimum at the equator. Real wander, caused by imperfections in the instrument's mechanisms, also contributes to drift but is usually small in modern, well-maintained gyros (around 1 degree per hour).
To correct for wander errors, the pilot must use the cage and adjustment knob. This knob disengages the gyro from the card, allowing it to act as a free gyro and preventing toppling during maneuvers. It also enables the pilot to realign the instrument with the magnetic compass. Pilots should realign the heading indicator every 10 to 15 minutes during flight, but only when flying straight and level at a constant speed, as these are the conditions under which the magnetic compass provides an accurate reading.
A variant of the heading indicator is the gyromagnetic compass, also known as the remote indicating compass or slave gyro compass. This instrument features a slave gyro that automatically and constantly realigns with Magnetic North by using a system of remote magnetometers, eliminating the need for manual alignment.