How does a horizontal situation indicator (HSI) work?

Question

I am trying to understand the Horizontal Situation Indicator. To my understanding there is a slaving transmitter, usually in a wing (the left wing in the Piper Archer and Seminole), which sends information to a flux gate, which then sends information to the compass card in a slaved HSI. Is any of that correct? Are the slaving transmitter and the flux gate separate components?

Answer 1

Your understanding is partly right. Usually HSI indicators are associated with a slaving transmitter that includes the flux gate (the magnetic sensor), but also other elements such the directional gyro unit and a slaving control.

So a typical system will usually include the below components (there are different designs, so check the characteristics of the equipment in your aircraft):

• The Horizontal Situation Indicator (HSI) is the panel mounted cockpit indicator that provides a comprehensive display including usually at least heading, course, and glide scope. The heading indicator of the HSI is a remote indicating compass that gets its information from a remote slaving transmitter.
• The magnetic slaving transmitter includes the flux gate that is the magnetic sensor itself, and some electronics to send the heading information to the directional gyro and HSI. This transmitter and its associated flux gate is usually located in a wingtip to minimize as much as possible magnetic interference.
• a directional gyro unit that is usually remote mounted. It is using its internal gyroscope to keep and provide a stable heading reference during turns. This gyro unit is driven/slaved to the slaving transmitter.
• The slaving control is panel mounted and allows to selected either "slaved gyro" or "free gyro" mode. Usually, the system is in "slaved gyro" mode and will be slaved automatically. In "free gyro" mode, a slaving meter indicator displays the difference between indicator display and earth's magnetic field, and it is possible to align manually the switch using clockwise or counterclockwise buttons.

The manual of the Bendix-King KCS 55A provides a good introduction to HSI design and operation.

Answer 2

Garmin and others now make electronic HSIs. No more spinning mechanical gyroscopes. A magnetometer is installed in the wing or tail someplace not influenced by magnetic fields caused by current flow in wires.

Garmin indicates

GMU 11 is an affordable, microprocessor-based magnetometer. It can be used to accurately sense the Earth’s magnetic field alignment and provide this data to compatible ADAHRS processors for use in referencing aircraft magnetic heading. In essence, the magnetometer acts as an electronic compass, showing the ADAHRS which direction the aircraft is facing in relation to magnetic north.

Unlike conventional flux valve systems, which detect only the horizontal component of the Earth’s magnetic field, the GMU 11 magnetometer provides full 3-axis vector measurements for the most precise digital indication of magnetic field strength and direction.

ADAHRS components used on our popular G3X, G3X Touch and G5 electronic cockpit displays can use comparative inputs from GPS, magnetometer and air data computer information to achieve new levels of integrity, reliability and precision in digital flight reference.

What’s more, because it’s an all solid-state design, GMU 11 offers low power consumption as well as easy installation and calibration. On most aircraft, GMU 11 is mounted in a separate remote location from the ADAHRS and other electronics — typically outboard in the wing, tail section or vertical stabilizer — to minimize any external magnetic interference.

and

Rock-solid Attitude Reference When configured as a primary attitude indicator, G5 uses solid-state AHRS reference to provide smooth, steady and reliable horizon-based pitch and roll indications. In addition to aircraft attitude, G5 will also support display of airspeed, altitude, vertical speed, slip/skid, turn rate, configurable V-speed references, barometric setting and selected altitude — as well as visual alerts upon arriving at a preselected altitude. A built-in GPS receiver provides highly accurate groundspeed and ground track readouts. Plus, a dedicated rotary knob on the unit allows for easy adjustments to altitude bugs and barometric pressure settings.

Gain A Clearer Sense of Direction To provide even more situational awareness, G5 is also approved for installation as a replacement heading indicator/directional gyro (HI or DG) or horizontal situation indicator (HSI) in your panel. When paired with an affordable GMU 11 magnetometer, GAD™ 29 navigation data interface and select VHF Nav/Comms or GPS navigators, G5 can serve as your primary reference source for magnetic heading, VOR/LOC guidance and/or GPS course guidance — as well as providing distance and groundspeed indications. The unit displays both vertical and lateral GPS/VOR/LOC course deviation when available. And you can use the G5 instrument’s rotary knob to easily make and adjust course selections — or to control heading bug settings in DG installations. For added system integration, a single magnetometer can supply heading information to 2 G5 units simultaneously. Additionally, G5 can provide heading output to select third-party autopilots (with GAD 29B).