Horizontal situation indicator

It combines a heading indicator with a VHF omnidirectional range-instrument landing system (VOR-ILS) display.

Among other advantages, the HSI offers freedom from the confusion of reverse sensing on an instrument landing system localizer back course approach.

As long as the needle is set to the localizer front course, the instrument will indicate whether to fly left or right, in either direction of travel.

The HSI illustrated here is a type designed for smaller airplanes and is the size of a standard 3 ¼-inch instrument.

Airline and jet aircraft HSIs are larger and may include more display elements.

Illustration of the face of a horizontal situation indicator showing the key elements of the display
An electronic HSI on a primary flight display
A magnetic slaving transmitter in a remote indicating compass detects the magnetic field of the Earth via a fluxgate magnetometer .
A slaving control and compensator unit evaluates the data from the slaving transmitter and corrects the heading in HSI when the "slave gyro" mode is enabled.