Back Course Localizer

Gulfstream G450

Eddie sez:

The Back Course Localizer continues to offer opportunities for mistakes and nobody has come up with an idea better than the "BC" switch for reverser sensing of the localizer beam. Refer to Back Course Localizer for background information about:

  • Regulatory Requirements
  • Equipment Requirements
  • Advantages and Disadvantages
  • Limitations

Everything here is from the references shown below, with a few comments in an alternate color.

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Back Course Localizer

Approach Category

If you have ASC 007 you are a Category C airplane unless you have a maintenance logbook entry raising your maximum landing weight. If you do not have ASC 007, you are a Category D airplane, end of discussion. See G450 Approach Category for an explanation.

Approach Selection


Photo: MCDU arrival page (PHOG), from Eddie's aircraft.

Approach selection is made either through the MCDU (NAV > ARRIVAL) or the NAV display.


Photo: DU arrival page (PHOG), from Eddie's aircraft.

There is an advantage to making this selection on the NAV display: if the appropriate chart is selected in green from the CHARTS page, future changes to the approach on the NAV display will automatically change the selected chart.

Preview & Auto-Tune

[G450 Aircraft Operating Manual §2B-05-50 2.A. (2)]

  • The PREVIEW submenu is used to select the NAV preview mode. The NAV preview mode permits the flight crew to select a short-range navigation (SRN) source for an SRN-based approach (VOR, ILS, MLS, etc.) while FMS (Long Range Navigation) is selected for display and providing lateral/vertical guidance. A long-range navigation source (FMS) must be the active navigation source in order for the PREVIEW prompt to be displayed.
  • When the necessary conditions are satisfied, the auto NAV preview functionality performs the following:
    • Automatically selects the NAV preview mode
    • Selects the SRN source for the approach procedure entered in the FMS flight plan
    • Automatically sets the course on the display to the required in-bound course for the approach.
  • With auto NAV preview functionality, the flight crew is still required to arm the flight director for automatic transition from LRN (FMS) guidance to SRN guidance.
  • The EDS automatically selects the NAV preview mode when the following auto NAV preview enabling conditions are satisfied:
    • The FMS is the selected NAV source
    • The aircraft is within 75 NM flight plan distance and 30 NM direct distance from the destination airport
    • An SRN approach is selected in the FMS flight plan
    • Auto-tuning is enabled for the on-side NAV receiver
    • The on-side SRN source frequency specified by the FMS approach is tuned by the FMS.
  • When the auto NAV preview conditions are satisfied, the NAV preview information is displayed on the PFD. When the NAV preview information is displayed, the flight crew can modify the SRN course or deselect the NAV preview mode altogether.
  • Alternatively, you can preset the localizer frequency and the approach back course. (This would be advisable if you expect to be on a complicated STAR with short vectors to ILS final, precluding the auto-tune function.) You would enter the ILS identifier into appropriate LSK in the PROGRESS page and the ILS approach course in the Display Controller PREVIEW submenu.

Back Course Mode Selection


Figure: Back Course Mode Selection, from G450 Aircraft Operating Manual, §2B-08-00, Pg. 10.

[G450 Aircraft Operating Manual §2B-08-70 ¶;3.] The back course mode is selected by pushing the GP BC button. The flight director generates roll guidance commands for capture and tracking of the BC localizer indicated on the coupled PFD. Pushing the BC button on the guidance panel arms BC if the beam deviation is outside the capture range. After the BC mode has been armed, the flight director disarms the BC mode if the pilot pushes the BC button before the BC mode becomes active. The FD lateral mode transitions to BC CAP as a function of LOC DEV, course angle, distance-to-transmitter, and TAS.

Approach Armed


Figure: Back Course Arm, from G450 Aircraft Operating Manual §2B-08-00, Figure 21.

[G450 AOM, §2B-08-120 ¶14.] The BC mode automatically intercepts, captures, and tracks a BC localizer beam, using the selected navigation source displayed on the coupled PFD. The LOC navigation source is selected using the display controller. In the BC armed mode, the aircraft flies the heading intercept and is armed for automatic BC beam capture.


Figure: Back Course Mode, from G450 Aircraft Flight Manual §2B-08-00, figure 20.

Be careful about the intercept angle. If you are in LRN mode the FMS should do a fine job of leading the turn if your speed is fairly constant. If you are in SRN mode with a large intercept angle, the FMS doesn't seem to grasp the distance to course until it is too late.

Back Course Tracking


Figure: Back Course Tracking, from G450 Aircraft Operating Manual §2B-08-00, Figure 24.

[G450 AOM, §2B-08-120 ¶16.] The track submode is annunciated the same as BCc capture mode (i.e., no annunciator change occurs at the capture to track mode transition). The track mode follows the capture mode and occurs after the beam deviation is reduced to a minimal value. In the track mode, the flight guidance computer automatically compensates for crosswinds of up to 45° of course error.

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