Why is the HSI unique?
The HSI is unique because it is a composite instrument offering multiple functionality in a single gauge. Features of the HSI include:
* The VOR-1 instrument gauge
* Localizer display for ILS runway heading
* Glideslope indicators for ILS approaches
* Heading adjustment for Autopilot
* Course (runway/Localizer) adjustment for ILS approaches
* TO/FROM Indicators (discussed in VOR training)
Basics of Operation
It's important to note early-on that the HSI refers only to the NAV-1 radio and the Autopilot. Although a multi-functional instrument, the HIS gauge is actually quite simple to understand and to use. Generally, the HSI is used in conjunction with the Autopilot, or as a VOR-1 gauge, or to manage an ILS approach, but may also be used as a simple Heading gauge for manual flight. The secret to keeping the HSI uncomplicated, is to see it as only the gauge being used for the immediate purpose. Example: If I'm using it as a simple VOR-1 gauge in non-Autopilot mode, then I'm not concerned with the Heading bug, flags, and so on. Okay, let's define some of the functionality a little more:
HSI Flag: This flag notifies you that the HSI is not receiving a radio signal.
Autopilot Heading Adjust: This adjustment rotates the Heading Bug around the compass rose; when Autopilot is activated, the airplane will follow the Heading Bug.
Heading Needle: This is in a fixed position; the center compass rose rotates with the positioning of the aircraft
Localizer & VOR Indicators: This is actually a set of split indicators. The outer sections are rotated around the compass rose using the Localizer/Course Adjustment, which declares the path (runway heading or VOR-1 radial) you wish to follow. The center bar indicates your position with regard to the Course or VOR-1 radial setting, and shows whether you are right or left of your desired path.
About The Glideslope
An ILS (Instrument Landing System) approach requires not only controlled horizontal positioning of the aircraft (via the Localizer) throughout the entire approach, but also controlled vertical positioning; meaning, we need to control our vertical descent on a specific glide-path that will lead us right to the end of the runway. The ILS radio system at the airport emits a highly-accurate radio beam that extends outward (and upward at a 3-degree angle) from the approach-end of the runway; this is called the Glideslope. Whether we're letting Autopilot manage the ILS approach, or whether we're flying the aircraft manually, the purpose behind an ILS approach is to follow not only the Localizer to keep us horizontally on-track with the runway centerline, but also to follow the Glideslope downward to the approach-end of the runway. The HSI has Glideslope indicators as shown in the picture below, and ideally we want to keep them vertically centered in the HSI window throughout our approach. If I am below the Glideslope, the indicators will be above center...If I'm above Glideslope, the indicators are below center of the window...vertically centered indicators means I'm ON the Glideslope path. Glideslope radio signals are generally limited to 15-20 miles, depending on terrain in the approach pattern.
Summary
We'll use the HSI extensively throughout the rest of the training, but for now let's understand that whether we're using the HSI for an ILS approach or as a VOR-1, functionality is similar: We tune NAV-1 radio to (either) the desired ILS runway frequency or the desired VOR-1 station frequency, tune a selected Course (runway heading or VOR-1 radial), and use the center-bar to follow the Localizer or VOR radial.
