Learn what the backcourse button does, when to select BC mode, how it corrects localiser sensing, and why glideslope guidance must be ignored.
In real-world aviation, the backcourse button selects an autopilot or flight-director mode that reverses localiser steering for a published localiser back-course approach. Pilots use it only when the aircraft’s approved avionics procedure calls for BC, B/C or REV mode—not for a normal ILS, an ordinary outbound leg or an unpublished reciprocal approach.
What does BC mode actually change?
BC mode reverses the roll-steering commands used to capture and track a localiser from its back-course side. Without that correction, a conventional system may command a turn away from the required track because left-right localiser sensing is reversed behind the antenna.
The button does not retune the receiver, create a usable back course or turn the approach into an ILS. It changes how compatible flight-director or autopilot equipment interprets the lateral signal. On some installations it affects only the automation, so the raw CDI may still appear to reverse-sense.
The selected course also matters, but the required setting varies. Many legacy installations specify the published front-course heading even though the aeroplane is flying the reciprocal direction; other systems expect the published back-course inbound course or derive it from the loaded procedure. Our explanation of how course selection affects HSI and localiser guidance covers that distinction.
When should pilots press the backcourse button?
Pilots should select backcourse mode only for an identified, published LOC BC procedure and only when the installed equipment is approved to couple that approach.
| Situation | Correct use of BC mode |
|---|---|
| Published LOC BC approach with compatible avionics | Select BC, B/C or REV as directed by the aircraft or avionics manual. |
| Normal front-course ILS or localiser approach | Use the normal LOC or approach mode, not BC. |
| Flying outbound during another instrument procedure | Use the mode specified for that procedure. Outbound flight alone is not a reason to select BC. |
| Reciprocal side of an ILS with no published LOC BC procedure | Do not improvise an approach from the rear signal; it may be unusable and lacks protected procedure design. |
| No approved back-course coupling | Hand-fly or use another permitted control mode while monitoring the approved navigation presentation. |
A mistake we see often in flight simulation is treating BC as a general-purpose reverse NAV switch. It is not. The charted procedure, receiver source, selected course and installed autopilot logic must all agree.
How should pilots verify the setup before capture?
A correct setup is confirmed from the chart, navigation source and flight-mode annunciation rather than from the button lamp alone.
- Confirm the procedure: The chart must identify a localiser back-course approach, usually as LOC BC.
- Tune and identify the localiser: Select the correct navigation receiver and verify the identifier. The basic process follows the normal localiser tuning and interception sequence.
- Set the required course: Use the value prescribed by the aircraft flight manual, POH or avionics supplement. Do not assume the reciprocal is always correct.
- Arm the specified mode: Check that BC is annunciated as armed and then captured. Monitor the initial steering command instead of assuming the automation will turn correctly.
- Cross-check raw data and altitude: Continue monitoring localiser deviation, heading, step-down fixes and the published minimum descent altitude.
For the full interception and tracking technique, see our detailed guide to flying a published back-course approach.
Does backcourse mode provide a glideslope?
A conventional LOC BC approach does not provide an authorised ILS glideslope. Any glideslope movement received from behind the installation may be false, reversed or outside its designed coverage and must not be followed.
Pilots use the published step-down altitudes and minimum descent altitude rather than treating the approach as a precision ILS with a decision altitude. Advisory vertical guidance from another approved system does not change the published minima unless the procedure and equipment specifically authorise it.
Why can BC mode still command the wrong turn?
Wrong-way guidance normally indicates a configuration or interpretation error rather than a need to press BC repeatedly.
- The aeroplane is on the front-course side while BC mode is selected.
- The wrong localiser frequency, navigation source or course has been selected.
- The flight director is in another mode even though the BC button is illuminated.
- The autopilot reverses its commands, but the particular CDI presentation does not.
- The simulated aircraft models a different back-course procedure from the generic autopilot being expected.
If steering commands disagree with the chart, heading and raw navigation data, pilots discontinue reliance on the automation and follow the aircraft’s approved procedure. The practical rule is simple: use the backcourse button only when both the published approach and the installed avionics explicitly call for it.