T-6A INSTRUMENT NAVIGATION
The Horizontal Situation Indicator (HSI) or Electronic Horizontal Situation Indicator (EHSI)
displays navigation information to the aircrew as though they were above the aircraft looking
down. It is essentially a combination of a rotating compass card (actuated by the aircraft's
master compass system), a RMI, a course indicator, and a range indicator.
The aircraft heading is displayed on the rotating compass card under the top index or lubber line.
The bearing pointer indicates the magnetic bearing from the aircraft to the NAVAID or GPS
waypoint selected, while the radial position of the aircraft is indicated by the tail of the same
bearing pointer. When selecting a course with the course selector knob, a digital display of the
course selected will appear in the course selection window, while a graphic relationship between
the course selected and the present heading will be shown by the course arrow. The TO-FROM
indicator displays whether the course selected, when intercepted and flown, will direct the
aircraft to or from the navigation station/waypoint selected by the relationship of that station to
the fixed aircraft symbol. The purpose of the Course Deviation Indicator (CDI) bar shows the
relative position of the radial/course desired (selected in the course selector window) and the
fixed aircraft symbol. The series of four dots in the gauge center are scaled to represent distance
off course in degrees. The exact calibration varies between modes of navigation. The function
and use of the heading set and course set knobs are explained fully in applicable NATOPS flight
When range information is available through DME or GPS the distance to the VOR/DME,
TACAN station or GPS waypoint is displayed in NM. If the aircraft is not receiving a usable
DME signal, the range indicator will be obscured. A red X fail annunciation on the EHSI will
indicate loss of bearing information.
The aircrew may select the type of magnetic bearing information desired for display on the
The bearing pointer will not function in relation to ILS signals.
VOR navigation receivers, as well as GPS equipment, electronically measure the magnetic
course/bearing for display by the bearing pointers. Therefore, if there is a malfunction in the
compass system or compass card, the VOR or GPS bearing pointers do not point to the
station/waypoint; however, they may still indicate proper magnetic bearings. When a compass
malfunction is known or suspected to exist; the VOR and GPS radial displays must be
considered unreliable until verified by other means.
When the course indicator is used to display ILS/LOC signals, the course indicator provides
precise ILS LOC course information for a specific approach. The following information pertains
to course indicator functions and display when used on an ILS/LOC approach:
The TO-FROM indicator is blank.
INTRODUCTION TO AIRCRAFT FLIGHT INSTRUMENTS 2-5