The heading indicator (HI), called the directional gyro, tells the pilot the aircraft's heading relative to magnetic north by sensing rotation about the aircraft's vertical axis and displaying the reading on a rotating compass card. Unlike the vertically mounted gyro of the attitude indicator above it, the HI gyro is mounted horizontally and spins around a horizontal axis, driven by a vacuum pump or electric power. Because magnetic dip and turning errors gradually nudge the indication off, the instrument must be periodically reset from the magnetic compass, a requirement that underlines the mounting and operational differences between the HI and the artificial horizon located just above it in the six-pack.
What is the difference between a heading indicator and an HSI?
The difference between a heading indicator and an HSI is that a traditional heading indicator is limited to displaying aircraft heading. The Horizontal Situation Indicator, or HSI, is a more advanced aircraft instrument that integrates a heading indicator with a course deviation indicator. For example, it continually presents aircraft heading and a course-deviation needle operating with VOR/LOC or GNSS navigation. Thus, the HSI provides both heading and course information, eliminating reverse sensing and giving left always means left and right always means right.
Both instruments appear in a six-pack steam-gauge panel. The HSI is normally mounted below the artificial horizon in place of the conventional heading indicator. In this same location, it replaces the directional gyro while adding the CDI that a standalone course deviation indicator occupies elsewhere. The pilot therefore sees a single straightforward display where heading, course selector arrow, and deviation are combined. The HSI reduces the need for frequent cross-checking of separate instruments and offers better situational awareness.
Because the HSI is an entirely separate instrument, it can be slaved to a remote compass or flux gate and features automatic synchronization that minimizes drift and precession errors. It also accepts ILS signals, displaying glide-slope deviation, and can be coupled to an autopilot capable of following the heading-select bug and executing an ILS approach by following localizer and glide slope. By consolidating information, the HSI reduces pilot workload and enhances overall navigation accuracy compared with the traditional heading indicator.
What is the difference between a heading indicator vs turn coordinator?
The difference between a heading indicator and a turn coordinator is that a heading indicator presents only heading: it displays the aircraft's magnetic heading through a vacuum-driven gyro that remains rigid in space, so the numeric card or lubber line stays fixed while the aircraft yaws beneath it. A turn coordinator is an electrically powered gyroscopic instrument that combines two functions in one face: it first reveals the rate of roll by means of a canted gyroscope mounted at a 30-45 degree angle, then continues to show the rate of turn after the roll stabilizes. The miniature airplane silhouette rolls left or right with bank angle and moves proportionally to the rate of heading change, while the embedded inclinometer ball indicates slip or skid, confirming whether the turn is coordinated. Thus, the heading indicator shows which way is the nose pointing, whereas the turn coordinator shows how fast is the aircraft rolling and turning and if the turn is coordinated.
What is an attitude and heading indicator?
The attitude indicator is a direct-reading gyroscopic flight instrument mounted in the cockpit. It displays aircraft position relative to the horizon using a gyroscope that spins on a vertical axis, thereby measuring the aircraft's pitch and bank by the principle of rigidity in space. By visualizing the airplane moving around the rigid gyro, the pilot receives an immediate indication of pitch angle and bank angle. Hashed lines at 10, 20, 30, and 60 degrees on the card mounted to the gyro arms provide a clear reference. Traditional round-dial attitude indicators are powered by a vacuum system that pulls air through a scooped rotor to spin the gyroscope, whereas modern glass-cockpit attitude indicators receive pitch, roll, and yaw data from the Attitude Heading Reference System (AHRS).
AHRS consists of sensors on three axes - solid-state accelerometers, electromechanical gyros, and a magnetometer or flux valve - that combine measurements through a Kalman filter to produce accurate attitude and heading readings. The magnetometer also furnishes the earth's magnetic field reference vector that corrects the heading reported by AHRS, eliminating the drift that appears until the system settles on a new heading. In transport-category aircraft a laser-ring gyro replaces spinning gyros for greater accuracy.
The heading indicator is driven by the same gyroscopic rigidity but spins its gyro on a horizontal axis so the card remains fixed in azimuth while the aircraft turns around it. The outside of the card depicts degrees of bank (heading) in 5- or 10-degree increments. Because the gyro is not north-seeking, the instrument must be periodically corrected with the magnetic compass, which itself swings wildly during turns and lags during acceleration. In glass cockpits the AHRS heading output replaces the traditional mechanical card, combining attitude and heading information into a single primary flight display so the pilot can cross-check both sources without moving the eyes from the center of the instrument panel.
What is the relationship between a heading indicator and VOR?
Heading indicators are not VOR related. The heading indicator and the VOR receiver perform separate tasks. A conventional VOR indicator presents lateral deviation information and has a course index that shows whether the aircraft is left or right of the selected VOR radial, while the TO/FROM indicator shows whether the aircraft is heading toward or away from the VOR station. On that conventional indicator left-right and to-from must be interpreted in the context of the selected course. Pilots have to mentally align the aircraft's heading with the course index of the VOR indicator.
The aircraft VOR receiver presentation is composed of four main parts: a dial to select the frequency, an OBS (omni bearing selector) calibrated 0-360 degrees that allows pilots to select and track a VOR radial, a course deviation indicator (CDI) that shows deviation from the chosen radial, and a TO/FROM indicator. When an HSI (horizontal situation indicator) is tuned to a VOR station the same information is shown in a more intuitive form: left and right always mean left and right, and the triangular arrowhead pointing to the VOR gives the TO/FROM indication.
Is the heading indicator mounted vertically?
No, the heading indicator is not mounted vertically. The gyro inside a heading indicator is mounted so its spin axis lies horizontal, parallel to the aircraft's longitudinal axis. This horizontal orientation lets the gyro sense any rotation the airplane makes about its vertical axis. The instrument case is fixed to the panel, but a double gimbal suspends the gyro, allowing the gimbal - and with it the compass card - to remain rigid in space while the airplane yaws around it. The vertical dial, which looks like the card of a vertical-card magnetic compass, rotates as the aircraft revolves about the vertical axis, keeping the lubber line aligned with the current heading.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
