Horizontal Situation Indicator is a flight navigation instrument that combines the heading indicator with the course deviation indicator (CDI), combining VOR and ILS guidance to present a unified cockpit display. Normally mounted below the artificial horizon in place of a conventional heading indicator, HSI provides a pictorial presentation of the aircraft's position relative to the selected course or radial through a fixed aircraft symbol positioned on a circular compass card.
By simultaneously presenting heading information and VOR/ILS guidance, the instrument enables pilots to see at a glance how the aircraft is oriented and how closely it is tracking the intended flight path, reducing workload and refining situational awareness during en-route navigation and instrument approaches.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a horizontal situation indicator (HSI) in aviation?
The horizontal situation indicator is an aircraft flight instrument, commonly called the HSI, which combines a heading indicator with a VHF omnidirectional range instrument landing system (VOR-ILS) display and integrates information from the magnetic compass and a navigation instrument, thereby giving the pilot an indication of the location of the aircraft with relationship to the chosen course or radial.
The horizontal situation indicator (HSI) in aviation is a panel mounted cockpit indicator that integrates heading, course, and navigation data into a single instrument. At its core, the HSI is a rotating 360 degree compass card driven by a remote indicating compass system, which receives its information from a remote slaving transmitter connected to a magnetometer that senses the earth's lines of magnetic flux. This arrangement compensates for the errors and limitations found in older heading indicators, giving pilots an accurate, gyro-stabilized picture of aircraft current heading. A course deviation bar, centered on the display, indicates aircraft position relative to the selected navigation course, while a course select knob sets the desired navigation course and a heading bug lets you set desired heading. Because the HSI combines these functions, it is deemed a command instrument that simplifies navigation and instrument approach tracking.
The electronic horizontal situation indicator (EHSI) performs the same mission in glass-cockpit aircraft, where it is part of the Primary Flight Display. Instead of electromechanical cards, EHSI uses digital displays to show course, distance, wind direction, and optional symbols like TCAS traffic. Whether mechanical or electronic, the HSI tells the pilot at a glance where the airplane is, where it is pointed, and where it must go next.
What is the function of a horizontal situation indicator?
The function of an HSI is to indicate the selected course to the pilot. A Horizontal Situation Indicator provides a rotating display to show desired course and combines the functions of a CDI with a heading indicator. The pilot gets an indication of the location because the visual display gives an indication of the relationship of aircraft location to chosen course. By incorporating a course deviation bar, TO/FROM indicator, and rotating 360 degree compass card that indicates magnetic heading, the instrument presents navigation information so the pilot has better situational awareness.
The course arrow indicates selected course; the head of the course arrow points to the selected facility and the tail indicates reciprocal. Horizontal Situation Indicator can display localizer deviation and glideslope deviation when receiving navigation data from VOR/DME or from RNAV sources including VNAV-capable GPS. Processed information is displayed on PFD. Magnetometer senses earth's magnetic flux and ADC supplies heading data from magnetic sources like flux valves, altitude, and airspeed. In this way Horizontal Situation Indicator improves situational awareness by giving the pilot a comprehensive display that includes glide scope and visual representation of aircraft position relative to courseline.
How does the HSI work in aviation?
HSI works by combining VOR/DME receiver, GNSS, ILS, magnetic slaving transmitter and flux gate data into an electronic primary display. A fixed aircraft symbol at the center, a green pointer and a mode annunciation tell which course the pilot has selected. The slaving transmitter in the left wing sends magnetic north reference to flux gate. The flux gate drives the compass card so the card rotates to current heading while the heading bug interacts with autopilot. Course selector knob sets desired navigation course. Deviation dots glide left or right and the glide slope indicator moves up or down, so the pilot sees where the aircraft is relative to the waypoint. Modern HSIs overlay aircraft position and course onto digital maps and the same electronic display can be incorporated with the electronic flight instrument system. The autopilot can execute the ILS approach by following localizer and glide slope, so one instrument gives lateral, vertical and map guidance.
What are common horizontal situation indicator errors?
Common horizontal situation indicator errors fall into three groups: heading drift, compass-versus-HSI misalignment, and slaving-meter transients. After a few flight hours the gyro-driven card can drift about 5 degrees, so the cockpit indication creeps away from the outside magnetic bearing. Pilots notice this when they compare the HSI with the whiskey compass and see deviations of 5 degrees or more. The same comparison reveals that the HSI has become misaligned from the whiskey compass because magnetic slaving transmitter heading errors, caused by aircraft electrical fields or structure, add systematic offsets that do not change with aircraft heading but stay fixed along the local isogonic line.
During turns the slaving meter is a diagnostic aid. When the aircraft is in a turn and the card rotates, the slaving meter shows a full deflection to one side or the other. A left deflection indicates a counter-clockwise error of the compass card, while a right deflection indicates a clockwise error. These swings confirm that the flux valve, the direction-sensing device of the system, is being temporarily overwhelmed by the varying magnetic setting.
Because the HSI prevents reverse sensing errors on ILS localizer back-course approaches, crews sometimes forget that the basic gyro still needs an occasional reset. Reset must be done from straight and level, unaccelerated flight, where a push of the heading-drive button on the slaving control and compensator unit lets the pilot realign the card to the flux-valve reading.
What is the difference between an HSI and a heading indicator in aviation?
An HSI combines a heading indicator with a course deviation indicator to give a complete picture of the aircraft's position relative to a selected navigation course, whereas a heading indicator is a standalone instrument limited to providing heading information. The traditional heading indicator displays the aircraft's heading on a rotating compass card calibrated in degrees, uses a gyroscope to establish an inertial platform, and is not affected by forces that make the magnetic compass difficult to interpret. Because it drifts due to precession, it requires frequent calibration and cross-referencing with other instruments and must be reset from the magnetic compass periodically.
The HSI is an advancement that consolidates information by incorporating heading data with VOR and ILS signals, includes a course-indicating arrowhead and a reciprocal tail, a fixed aircraft symbol, and often a heading-select bug. It is normally mounted below the artificial horizon in place of a conventional heading indicator and is part of a remote-indicating compass system that compensates for errors of older heading indicators. While the heading indicator offers limited integration with other navigational systems, the HSI's automatic synchronization feature continuously corrects the displayed heading, refining accuracy and eliminating the repetitive manual adjustments required by its predecessor.
A Heading Indicator gave me magnetic bearing, but it provided a limited view of my position. The HSI showed my direction and my association to the path. Because the course variance bar was superposed immediately over the helming board, I comprehended my side placement comparative to the instruction without having to mentally combine readings from two distinct devices. I no longer had to cross-reference this with another VOR gauge, and the incorporation allowed me to concentrate further on cockpit command and decision-making.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the difference between HSI and CDI in aviation?
HSI is an improvement on the standalone CDI instrument, presenting a more advanced version of the same basic course deviation indicator. CDI is a simple instrument that shows aircraft lateral position relative to a course. Its needle deflects opposite to the aircraft's location, moving right when the airplane is left of course. Traditional CDI gives left/right deviations from the selected VOR radial chosen with the OBS knob, and it can be used with GPS or VOR navigation aids.
Because CDI is standalone and needs alignment with a separate compass, pilots must mentally combine heading and course data. HSI integrates CDI into a single display that pairs the course deviation bar with a slaved compass card, so heading and course appear together. Thus, HSI provides better situational awareness than CDI alone while eliminating the extra step of synchronising instruments.
What is the difference between an HSI and a VSI in aviation?
The Horizontal Situation Indicator (HSI) and the Vertical Speed Indicator (VSI) serve separate flight tasks. An HSI presents aircraft horizontal position relative to a selected course and heading, combining directional, track, and deviation cues on one face. A VSI is an instrument which indicates the rate of climb or descent of an aircraft. It shows, in feet per minute, whether the aircraft is climbing, descending, or in level flight.
The HSI relies on signals from a VOR, ILS, or GPS receiver and aircraft heading data to draw a top-down map of lateral situation. The VSI uses an internal pressure differential. It contains a diaphragm with connecting linkage and gearing to the indicator pointer inside an airtight case, and the aneroid capsule inside the VSI expands and contracts based on the change in altitude, translating the rate of change of pressure into pointer movement.
How to use a horizontal situation indicator?
To use an HSI, the pilot places the course arrowhead on the selected radial. Watching the glide-slope indicator, the pilot climbs if the dot sits above the scale midline, descends if it lies below, so the path re-centers smoothly. Deflection of the bar gives the same indication: one dot equals two degrees, so a half-dot calls for a gentle one-degree bank or roll out when the bar centers. Adjustment is continuous yet minimal. In trim air, correction does not exceed 3 degrees in 15 minutes, so the pilot nudges the heading only when the CDI drifts beyond half-scale. Each new waypoint demands only a twist of the course knob and no other re-calibration is required until the next navaid.
I learned to use the Horizontal Situation Indicator by first setting navigation rate. The device gave me a picture of my movement and allowed me to stay on course.I tracked my location relative to the VOR facility and managed to stay on track.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How to read HSI in aviation?
To read an HSI, start by confirming that on the compass card 330 degrees lies at the index . Next, spin the course-select knob until the course indicating arrowhead is set to 300, leaving a 30-degree intercept angle to the inbound track. The centre of the deviation bar is now zero, and each dot represents 0.5 deviation. One dot right, therefore, shows the aircraft is displaced 0.5 degrees to the right of the 300 degree course.
My instructor stressed the compass card is the base and the track pointer constitutes my planned route. I concentrate on the path pointer and on the path variance bar, because the path variance bar represents my side placement comparative to that route. I focused on maintaining the bar at the center. The To/From signal is pivotal: TO monitor verifies I am gliding toward the terminal, whereas a FROM screen represents I am proceeding off from it.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
