747-4/8 "NG FMC" - VNAV ALT not changing to VNAV PATH automatically on approach
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Joined: Mar 2011
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747-4/8 "NG FMC" - VNAV ALT not changing to VNAV PATH automatically on approach
Aircraft in question is the B-747-400 and 747-8 NG FMC with blockpoint 4.0 software.
When when descending in VNAV SPD and below the VNAV path, the vertical flight mode annunciation changes to VNAV ALT once capturing the MCP altitude. When cleared for the approach, on a non-ILS or a published transition route to an ILS, typically the pilots will set either minimums or final approach fix altitude in the MCP altitude window depending on the type of approach. When on approach logic (flaps out of up, first approach waypoint sequenced, or close to the airoprt) and in VNAV ALT, most of the time, the FMA will automatically changet to VNAV PATH and begin a descent toward the next altitude constraint in the FMC or end of descent point, automatically as the VPI/VNAV path intersects the aircraft's altitude from above.
My question is: Why does VNAV ALT sometimes automatically change to VNAV PATH and resume descent when the the Vertical Path indicator crosses the airplanes altitude from above, but not always? Why does it sometimes remain in VNAV ALT and require a pilot to press the "altitude intervene" switch on the MCP to follow the path down? I realize you can press altitude intervene early and get VNAV path while level and below path (VPI), however I would just like to know why the FMC will automatically do this sometimes, and others not. I could not find this scenario mention in the FCOM Vol II; the scenario of VNAV ALT automatically changing to VNAV PATH on approach isn't specifically addressed that I could find.
When when descending in VNAV SPD and below the VNAV path, the vertical flight mode annunciation changes to VNAV ALT once capturing the MCP altitude. When cleared for the approach, on a non-ILS or a published transition route to an ILS, typically the pilots will set either minimums or final approach fix altitude in the MCP altitude window depending on the type of approach. When on approach logic (flaps out of up, first approach waypoint sequenced, or close to the airoprt) and in VNAV ALT, most of the time, the FMA will automatically changet to VNAV PATH and begin a descent toward the next altitude constraint in the FMC or end of descent point, automatically as the VPI/VNAV path intersects the aircraft's altitude from above.
My question is: Why does VNAV ALT sometimes automatically change to VNAV PATH and resume descent when the the Vertical Path indicator crosses the airplanes altitude from above, but not always? Why does it sometimes remain in VNAV ALT and require a pilot to press the "altitude intervene" switch on the MCP to follow the path down? I realize you can press altitude intervene early and get VNAV path while level and below path (VPI), however I would just like to know why the FMC will automatically do this sometimes, and others not. I could not find this scenario mention in the FCOM Vol II; the scenario of VNAV ALT automatically changing to VNAV PATH on approach isn't specifically addressed that I could find.
Joined: May 2006
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From: Here, there and everywhere
One situation could be reaching an altitude restriction before the waypoint associated with the restriction. Even if the altitude window has a lower altitude, VNAV logic will keep the restriction until getting to the waypoint. Pressing altitude intervent will get rid of the restriction on the active waypoint, and any press afterwards on the subsequent waypoints.
Thread Starter
Joined: Mar 2011
Posts: 195
Likes: 4
From: Earth
One situation could be reaching an altitude restriction before the waypoint associated with the restriction. Even if the altitude window has a lower altitude, VNAV logic will keep the restriction until getting to the waypoint. Pressing altitude intervent will get rid of the restriction on the active waypoint, and any press afterwards on the subsequent waypoints.
Joined: Sep 2016
Aviation Qualifications: ATPL
Posts: 52
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From: Australia
When on approach logic (flaps out of up, first approach waypoint sequenced, or close to the airoprt) and in VNAV ALT, most of the time, the FMA will automatically changet to VNAV PATH and begin a descent toward the next altitude constraint in the FMC or end of descent point, automatically as the VPI/VNAV path intersects the aircraft's altitude from above.
Joined: May 2005
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From: F370
Andmiz,
That would be true for the legacy FMC. With the -8, and the 747NG FMC the logic was revised so that an RNP/RNAV approach behaved more like an ILS.
In order to leave the VNAV ALT without pressing the MCP, the FMC must know that it is “on approach”. There are various triggers such as flaps being extended and within 20nm, etc but they don’t always work perfectly in the NG FMC. Hence the need to pay attention as you come onto the VNAV path and make sure the aircraft chooses PATH over VNAV ALT.
That would be true for the legacy FMC. With the -8, and the 747NG FMC the logic was revised so that an RNP/RNAV approach behaved more like an ILS.
In order to leave the VNAV ALT without pressing the MCP, the FMC must know that it is “on approach”. There are various triggers such as flaps being extended and within 20nm, etc but they don’t always work perfectly in the NG FMC. Hence the need to pay attention as you come onto the VNAV path and make sure the aircraft chooses PATH over VNAV ALT.
Joined: Sep 2016
Aviation Qualifications: ATPL
Posts: 52
Likes: 5
From: Australia
