Definition

An Electronic Horizontal Situation Indicator (EHSI) is part of the Primary Flight Display (PFD) in glass cockpit aircraft. The EHSI provides the same information provided by a traditional HSI, along with optional displays for weather radar, TCAS symbols, wind speed and direction, etc.

Description

In most Electronic Flight Instrument System (EFIS) installations, the EHSI appears on the bottom half of the PFD. Like a traditional HSI, the EHSI combines a heading indicator with navigational functions such as a course select pointer and a course deviation indicator. However, the EHSI adds other indications such as a drift angle bug, wind speed and direction, groundspeed, Distance Measuring Equipment (DME) readout, chronometer readout, and frequencies for communications and navigational radios.

In most EHSI designs, pilots can select additional information to display as well, such as weather radar, ground mapping, course waypoints, Traffic Collision and Avoidance (TCAS) indications, etc. (TCAS is also known as ACAS, for Airborne Collision Avoidance System.) Also, most EHSIs alert pilots to malfunctions such as miscompares between information on the captain's and first officer's sides. For example, if the two sides show different headings, an amber "HDG" symbol may appear. This tells the pilots that one side or the other is receiving bad data from an internal reference system or other component.