The horizontal situation indicator (HSI) was designed to eliminate some of the work of using very-high-frequency omnidirectional range (VOR) navigation and instrument landing systems (ILS). Like other cockpit instruments, HSIs can vary in appearance and operation by brand. Fundamentally, however, all HSIs indicate an aircraft’s horizontal position relative to a selected VOR or ILS.
The HSI works by combining a VOR/DME receiver and an electrically-slaved heading indicator. (Not all HSIs are slaved but the HSIs in the Flight Simulator aircraft are.)
Let's look at the various components of the HSI. The heading indicator shows the aircraft’s magnetic heading and its green pointer shows the course the pilot has selected. A course deviation indicator (CDI) shows the position of the selected radial relative to the aircraft’s current position. A dotted scale indicates the amount of deviation from course. (The number of degrees of deflection indicated depends on how far the aircraft is from the ground-based station.) The HSI also has an ADF for cross-reference, and includes a glide slope indicator and scale for ILS approaches.
Reading the HSI
The small airplane in the center of the HSI always points straight ahead, indicating the direction the aircraft is traveling. Using the HSI in the King Air 350 (shown above), the Seattle VOR is tuned on VOR1 (the instrument can display information for VOR1, VOR2, or the GPS). The To/From indicator shows that the aircraft is flying toward the station. The pilot has selected a course of 340 degrees (the green arrow) and, as a reminder, has set the heading bug on 340 degrees. The CDI indicates that the 340 degree radial of the Seattle VOR is to the left, or west, of the aircraft. The pilot has turned toward the CDI with an intercept angle of about 45 degrees. There's no need to calculate which way to turn, just turn toward the CDI.
The pilot holds that heading until the CDI begins to center and then intercepts the radial, turning to 340 degrees. As long as the pilot flies straight down the radial, the needle will stay centered. As a cross reference, the pilot can refer to the magenta ADF needle pointing at a non-directional beacon (NDB), 226 degrees from the present position.
When flying outbound from a VOR the same technique applies; fly toward the CDI. The selected course, however, is a radial pointing away from the station.
The example here shows an aircraft heading away from the Olympia VOR. The 060 degree outbound radial is selected on the course indicator. The To/From indicator points back, toward the station. The pilot has turned toward the CDI to intercept the radial.
The HSI receiver can also be tuned to an ILS and used on approach. The pilot selects the final approach heading on the course indicator and the CDI centers when the aircraft intercepts the localizer. Glide slope information is provided by the indicator and scale on one or both sides of the HSI. Once again the pilot flies towards the indicator. If the glide slope indicator is above the scale’s midline, the aircraft is below the glide slope and the pilot climbs to intercept. If the indicator is below the midline the aircraft is above the glide slope and the pilot descends to intercept.
HSI and GPS
Another advantage the HSI has is its ability to help the pilot track a GPS course. GPS, or global positioning system, is a network of satellites that continuously transmit coded data to GPS receivers. The receivers use the data to compute a position or track on Earth. A GPS receiver determines location by comparing the angular relationship between the receiver and the orbiting satellites. Being able to calculate a precise position without relation to a fixed ground station means that a course can be plotted direct to the destination with no radial or angle calculations. The pilot knows the aircraft’s current position by referring to the GPS latitude and longitude readouts or to the unit’s moving map display. By switching the HSI to GPS mode, the course needle and CDI indicate position relative to the GPS course. Once again, the pilot turns toward the CDI to intercept the course.
If you want to make this a hands-free task, use the Nav hold on the autopilot to have it do the work for you (see Using an Autopilot) and flip the VOR/GPS switch to GPS. Any way you look at it, if you’re navigating by GPS, VOR, or making an ILS approach, an HSI makes the job easier, and that should mean a safer flight.
Flight Simulator aircraft with HSI:
- Beechcraft Baron
- Beechcraft King Air
- Bell JetRanger 206B
- Boeing 737-800
- Boeing 747-400
- Bombardier Canadair CRJ-700
- Bombardier Lear 45
- Cessna Grand Caravan C208B
- Maule M-7-260C Orion
- Mooney Bravo
- Robinson R22