Vertical Navigation Modes (VNAV)
If another vertical mode is touched while VNAV Mode is selected, VNAV Mode reverts to armed.
VNAV mode will not capture a descending profile unless the selected altitude is valid and at least 75 feet below the current aircraft altitude
Vertical Navigation (VNAV) Mode is available for enroute/terminal cruise and descent operations any time that VNAV flight planning is available.
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When a flight plan is active, VNAV data is valid, and the VNAV Key is selected on the GMC 507 or VNAV Button on the AFCS Control Window, VNAV mode is armed in preparation for descent path capture. ‘VNAV’ is annunciated in white in the AFCS Status Box. If applicable, the appropriate altitude capture mode is armed for capture of the VNAV Target Altitude (ALTV) or the Selected Altitude (ALTS), whichever is greater.
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When a descent leg is captured (i.e., vertical deviation becomes valid), VNAV Mode is activated and tracks the descent profile. An altitude capture mode (ALTS or ALTV) is armed as appropriate.
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When approaching the VNAV Target Altitude (or Selected Altitude) the system automatically transitions to ALTS or ALTV Mode with ALT Mode armed.
If the altimeter’s barometric setting is adjusted while VNAV mode is active, the autopilot increases/decreases the descent rate by up to 500 fpm to re-establish the aircraft on the descent path (without commanding a climb). For large changes, it may take several minutes for the aircraft to reestablish on the descent path. If the change is made while nearing the VNAV waypoint, the aircraft may not reestablish on the descent path in time to level off at the VNAV Target Altitude. In this case, the autopilot will revert to Pitch Hold mode upon passing the VNAV waypoint, and Selected Altitude Capture (ALTV) mode will be automatically armed.