The HSI is the primary Command Direction Indicator for the Aircraft and can be operated in the Slaved Mode (with the compass card slaved to the Gyro system in the tail, which obviates the need to make regular adjustments to compensate for gyroscopic precession) or in the Free Mode, where the Compass Card is regularly aligned to the heading shown on the Magnetic Compass, as is the card in the RMI
The Synchronising Unit "Slave/Free" selector switch permits selection of the mode, and the synchronization process is a part of the Pre flight checks - see Checklist.
The HDG and NAV flags have their usual function, and the appearance of the HDG flag should be treated as an emergency. See again the Checklist.
Heading Bug Rotated using the Right Hand (orange) marked knurled knob to select chosen heading.
Course Pointer The Yellow arrowed course pointer can be rotated using the arrowed knurled knob
Links to Garmin 530
Under the regulations governing the installation of the Garmin and its linking to the HSI, the HSI controls have been augmented to include two annunciator lamps, coloured Green and White.
The Garmin 530 can provide data to the HSI either from the NAV1 box or from the GPS system
NAV Box Input
In the first case, the data is generated via an external transmitter such as a VOR or an ILS Localiser and received via NAV1. It is displayed in the conventional manner and the yellow CDI (Course Deviation Indicator) Bar serves in the usual way to indicate an angular displacement from a given radial - which is set in the normal way via the yellow course pointer indicator needle.
Where the NAV1 set is tuned to a Localiser frequency, the CDI bar indicates angular displacement from the Localiser beam centre line irrespective of the direction that the yellow arrow is pointing - though is is of course usual (having "selected" and "identified" the localiser frequency) to "display" the inbound "front" ILS radial under the arrowhead, so that the CDI bar is approximately vertical on the approach and the aircraft's displacement from the centreline is in the sense indicated.
Whereas in theory it is possible to obtain a reverse reading when operating the HSI in localiser Mode, it is worth noting that unlike the VOR instrument, the HSI never gives a reverse indication when operating in "VOR" mode - because the compass card always rotates as the aircraft's position changes.
GPS System Input
In the second case, data fed to the HSI course direction system is fed from the GPS receiver. The Garmin displays a leg of a route (or the "Go TO" leg) as a mauve line on the screen, and the CDI bar on the HSI will display a displacement from that line, irrespective of where, in relation to the start or finish of the leg, the aircraft is. Full scale deflection of the CDI bar is thought to represent a deviation from the course line of 10 Nm.
Depending on which input to the HSI is being used, either the White or the Green annunciator will light, and this choice is made from the Garmin 530 CDI Button, which will also show the choice in he Garmin Message screen.
NOTE The green annunciator referred to here is nothing to do with the Glideslope Capture Annunciator which is illustrated elsewhere.
Glide Slope Input
The HSI has a conventional Glideslope Indicator, and providing certain conditions are met (see Autopilot Operations) the Glideslope Capture Indicator will illuminate when the aircraft is established on the Glideslope.
Autopilot - HSI - Garmin operations
It will be clear - when you have read and studied the instructions on the Autopilot and have appreciated the facilities offered by the Garmin - that it is quite possible to (a) Set the Garmin to fly either in VOR mode or to set up a GPS Track and then (b) Have the autopilot fly the aircraft on that track (either by reference to the CDI (select Omni on the Coupler) or by reference to the Heading Bug (set Hdg on the Coupler).
Autopilot approaches are explained in the section on the Autopilot page as is the matter of Glideslope Capture.
However whilst all the above is possible, it is required that these procedures are not attempted unless the Pilot is accompanied by a safety pilot or (if solo) has been checked out on Autopilot Operations.
Furthermore, Instrument Approaches using Garmin are not (yet) permitted under any circumstances.