Learn what the aircraft HSI course selector controls for VOR, ILS and GPS guidance, how autopilot uses it, and why it does not steer alone.
The course selector sets the desired navigation course shown on an aircraft’s HSI—usually a VOR course, an ILS front course or, in some systems, a GPS course. It rotates the course pointer and establishes the reference for lateral deviation. It does not turn the aircraft unless an autopilot navigation mode is engaged.
In aviation and real-world flying, the key distinction is that course describes an intended path over the ground, while heading is where the aircraft’s nose points. A COURSE, CRS or OBS knob on an HSI, primary flight display or flight-guidance panel performs the same basic reference-setting task; it is not an autopilot mode button.
What changes when you turn the CRS or OBS knob?
Turning the knob rotates the selected-course arrow and changes the reference against which lateral deviation is displayed. The course is normally expressed in magnetic degrees unless the system explicitly uses a true-north reference.
With a VOR selected, the course-deviation bar shows whether the aircraft is left or right of the chosen course line through the station. The TO/FROM indication tells you which sense of that line is selected. A centred bar means the aircraft is on the line; it does not prove that the aircraft is travelling in the intended direction or pointing at the selected number.
A mistake we see constantly is confusing the course pointer with the heading bug. Our explanation of heading, course and bearing in FSX shows the same instrument geometry used in real aircraft.
How does course selection differ for VOR, ILS and GPS?
The course selector’s effect depends on the active navigation source and the avionics fitted to the aircraft.
| Navigation source | What the course selector does | Common mistake |
|---|---|---|
| VOR | Defines the desired inbound or outbound course through the station and affects CDI and TO/FROM presentation. | Selecting the VOR radial rather than its reciprocal when an inbound course is required. A radial is always defined FROM the station. |
| Localiser or ILS | Sets the published front-course reference for the HSI and, on some installations, the flight director or autopilot. | Expecting the knob to move the localiser beam. The radio beam is fixed, and raw localiser deviation generally does not depend on the selected course. |
| GPS or FMS | May define a course through a waypoint in OBS or manual-course mode. During normal leg sequencing, the active leg usually defines the required track. | Turning CRS and expecting an active GPS route to change when the installation ignores the knob in normal sequencing mode. |
| ADF or NDB | Normally has no effect on the ADF bearing pointer. | Treating an NDB bearing as though it were a VOR course with CDI guidance. |
For practical VOR instrument interpretation, see our VOR tracking and CDI guidance. On an ILS, set the published front course; our guide to finding the correct ILS frequency and course explains where that value comes from.
Some glass-cockpit systems automatically insert an ILS course from their navigation database, while others require manual selection. Do not assume that dialling the reciprocal is enough for a back-course approach; use the published procedure and the aircraft’s specified back-course setting or mode.
Does the course selector make the autopilot turn?
No; the course selector supplies a navigation reference, while the selected autopilot mode decides what the aircraft follows.
- HDG mode follows the heading bug and normally ignores the course pointer.
- NAV mode follows lateral guidance from the selected VOR, GPS or FMS source once that mode captures or engages.
- LOC mode captures and follows the localiser. APR mode can also capture compatible vertical guidance, but the course selector itself commands no climb or descent.
If only the flight director is active, its command bars may show the required turn, but the aircraft will not turn automatically. If the autopilot is coupled, verify the flight mode annunciator rather than relying only on an illuminated button. Our overview of how NAV and LOC autopilot modes use their selected source covers that relationship in flight simulators.
How do you set the course selector correctly?
Set the selector for the path you intend to fly, then verify the navigation source, indications and autopilot mode separately.
- Select the navigation source. Confirm whether the HSI or CDI is displaying NAV1, NAV2, GPS or FMS guidance.
- Establish valid guidance. Tune and identify the VOR or ILS where applicable, or activate the required GPS leg.
- Set the planned course. Use the desired inbound or outbound VOR course, the published ILS front course, or a GPS course only when the avionics support manual course selection.
- Intercept the course. Fly manually or use heading mode to approach the selected path at a sensible angle while watching CDI movement and TO/FROM information.
- Engage and verify. Arm NAV, LOC or APR as appropriate and confirm capture on the flight mode annunciator. If the aircraft turns unexpectedly, return to manual or heading control and recheck the source, course and mode.
Do not keep twisting the course knob merely to centre a VOR needle. That redefines the selected course beneath the aircraft instead of correcting the aircraft back to the planned course.
Why can the CDI be centred while the aircraft is not following the course?
A centred CDI only shows that the aircraft is on the selected course line or signal centreline at that moment; direction, wind correction and autopilot capture are separate matters.
- The aircraft may be crossing the course rather than tracking it.
- It may be travelling along the reciprocal direction, especially if the TO/FROM indication was overlooked.
- The heading may differ from the course because a wind-correction angle is required.
- The autopilot may still be in HDG mode, with NAV or LOC merely armed and not yet captured.
- The HSI may be displaying a different source from the one the autopilot or pilot expects.
- A GPS or FMS leg may be supplying its own desired track and ignoring the manually selected course.