Harmon Technologies, Inc.

Horizontal Situation Indicator

The Horizontal Situation Indicator is an advanced instrument that combines several functions.  These are:

In most airplanes, this instrument is a simpler variation called a "directional gyro" that displays heading information only.  By combining many functions into one display, the pilot does not have to scan as many instruments and can more easily integrate the information and have a good picture of the situation.  Full featured HSIs are very expensive but can be extremely useful in high-workload situations.  N4907J is VERY fortunate to have an HSI.

The compass display shows the magnetic heading of the aircraft.  Because it is gyro stabilized, there is no swing such as is seen with magnetic compasses.  Usually, the pilot has to periodically reset the direction to the magnetic compass but N4907J has a neat feature called "remote magnetic slaving".  With this feature a remote magnetic compass sender is located in a wingtip so that it won't be disturbed by things in the cockpit and sends constantly updated magnetic information to the HSI display.  A small processor corrects for compass swing and automatically keeps the gyro set correctly.  Left on their own, gyros tend to slowly wander and must be reset every 15 minutes or so.  This compass slaving feature means there is one less thing for the pilot to remember.

The yellow line shows navigation information.  The little airplane on the face can fly toward the course-line, away from it, or along it.  This mimics the actual airplane's relation to the course set on the display.  If the little plane is centered on the yellow line, the real airplane is on the desired course.  There are times when the indicator is set in different ways so having the yellow line be off to one side does not necessarily mean anything bad.

The orange pointer (over the 12 in the figure above) is used to remind the pilot of the last clearence from air traffic control and to command the autopilot what course is desired.  When navigating electronically, the yellow course pointer may also be used to command the autopilot. 

The yellow pointers on either side of the HSI are not usually visible but pop into view when the airplane is executing an instrument approach.  They indicate if the airplane is too high or too low on the glideslope.  If the pointer is above the center, the plane is low, below the center, the plane is high.  The pointer points toward the desired direction.

At the beginning of an approach, the airplane usually approaches the glideslope from below so a "too low" indication is normal during the early stages of an aproach.

The yellow course and glideslope indicators are normally driven by the first navigation radio receiver (called "Nav-1").  On N4907J the yellow course line can also be driven by the Loran receiver if a selector switch on the instrument panel is properly set.  A light on the selector button shows which source is driving the yellow indicator.  If radio reception of the navigation signal is lost, the red "Nav" flag will appear.  Normally, the red flags are not visible.

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