In the Boeing 737NG FCOM, I've read this statement:

VNAV mode is terminated by any one of the following:

  • selecting another pitch mode
  • glideslope capture
  • reaching end of LNAV route
  • transition of glideslope intercept waypoint if G/S is armed
  • crosstrack deviation exceeds twice the RNP value during PTH descent for an active leg with a database vertical angle and LNAV not engaged

In the event of glideslope intercept waypoint transition, VNAV can be re-engaged.

What does "transition of glideslope intercept waypoint if G/S is armed" mean here?

  • 4
    $\begingroup$ Some context would help, where did you read this? $\endgroup$ – Ron Beyer Dec 15 '20 at 20:46
  • $\begingroup$ ...and which mark (dash no.) of B737 is it? $\endgroup$ – skipper44 Dec 16 '20 at 4:19
  • $\begingroup$ I ve read this in the FCOM 737 NG. $\endgroup$ – Arcil Tralleis Dec 16 '20 at 8:34
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    $\begingroup$ @ArcilTralleis I added a longer quote from the 737 FCOM. I think this is what you mean to ask about. If not, please edit again. $\endgroup$ – Bianfable Dec 16 '20 at 9:03
  • $\begingroup$ Means if glideslope mode is armed in the autopilot on an approach, VNAV mode will terminate and the aircraft will switch to glideslope following for altitude control when the aircraft intercepts the glideslope. $\endgroup$ – Carlo Felicione Dec 16 '20 at 13:00

As a generic (non-737 specific) answer, it means that when the Flight Director is armed to capture an ILS glideslope, the VNAV flight director mode, being a vertical path based on baro altitude, is cancelled once the aircraft passes through a waypoint that the RNAV system calculates to be the geographical intercept point of the ILS glideslope beam (the intercept waypoint) because the Flight Director is expected to capture and track the ILS glideslope and RNAV/VNAV guidance is no longer required.

  • $\begingroup$ I edited for clarification to mention that the "intercept waypoint" is effectively an RNAV calculation of where the glideslope intercept should occur, and at that point the RNAV/VNAV mode is cancelled in the expectation that the FD will now track the radio beam. $\endgroup$ – John K Dec 16 '20 at 15:51

More or less in consonance with @John K, here, step by step, is what happens on the later jets:

This is describing the VNAV mode behavior when crossing the "FI" ie the FAF, Final Approach Fix, waypoint, ie the GS intercept point.

The scenario would be that an ILS approach is selected via DEP/ARR page (i.e. from database), and there is a VNAV PATH from the FAF descending to and terminating at ~50ft above the LDG threshold.

If you're shooting an ILS, you're tracking the LOCaliser and the vertical mode is VNAV ALT or VNAV PTH and you're approaching the FAF (depicted as FI).

(Case 1) LOC has been captured GS is armed, the VNAV will disconnect and GS will be captured and become the active mode.

If GS is not armed,

(Case 2a) If the altitude window on the MCP is set to MDA, the airplane will continue on VNAV PTH and commence the descent from the FAF to the MDA.

(Case 2b) If the altitude is set to the FAF altitude or higher in the MCP window, the mode after crossing the FAF will be VNAV ALT.

Thus, in either case of altitude setting, if GS is not armed the VNAV will not disconnect crossing the FI (FI = FAF = GS intercept point)

  • $\begingroup$ Holy crap I never knew consonance was a word lol. $\endgroup$ – John K Dec 16 '20 at 5:03

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