Horizontal Situation Indicator
HSI is the acronym for Horizontal Situation Indicator. Sometimes it is also called Pictorial Navigation Indicator. It combines a standard Directional Gyro (DG) and Course Deviation Indicator (CDI) into one single instrument and reduces the pilots workload by displaying the aircraft's heading and position relative to a desired course in a compact manner.
The well known drift of a standard Directional Gyro is usually compensated by the use of a slaved gyro system that constantly corrects the heading indication.
The purpose of this document is to briefly describe the indicators and the usage of a HSI instrument.
Compass Card / Compass Rose
The Compass Card (1) is driven by the internal gyro. It shows the magnetic heading of the aircraft. Drift errors like in standard Directional Gyro are automatically corrected when in slaved mode.
The number on the Compass Card (1) under the Lubber Line (2) shows the current aircraft magnetic heading.
NAV Warning Flag
The red NAV Warning Flag (3) indicates, that either CDI or the GS indicators are unreliable. It is driven by the signal quality of the received nav station. Do not use the CDI or GS indicators for navigation when the NAV Warning Flag is visible.
HDG Warning Flag
The red HDG Warning Flag (3) becomes visible when the speed of the directional gyro is to low. Beware that there may occur compass failures that are not indicated by the HDG Warning Flag, so compare the indicated heading with the magnetic compass on a regular basis.
Heading Select Bug
The orange Heading Select Bug (5) indicates the heading you (the pilot) wants to fly. It may be coupled to the autopilot so can provide the "Heading Select" function. The autopilot will try to maintain the heading indicated by the Heading Select Bug. The Heading Bug can be adjusted by the Heading Select Knob (11)