VNAV vs. FMS altitude
X-Plane knows two different types of VNAV, which can be described as “GA” and “Airliner”-type VNAV. The main difference is the availability of auto-throttle. “GA” without auto throttle can only do geometric descents – the vertical path is controlled by the autopilot, while the pilot controls the speed by the throttle (or speedbrakes). “Airliner” with auto-throttle uses the auto-throttle to set climb power in climb, hold the selected cruise speed in cruise, and comply with speed restrictions (if physically achievable) in descend.
VNAV climb and cruise
For Airliner-VNAV to be selected (dataref sim/cockpit2/autopilot/fms_vnav or command sim/autopilot/FMS) in the climb or cruise phase requires the plane to have auto-throttle. Upon selection in the climb phase, the autopilot vertical axis will be speed-by-pitch, and the autothrottle will go to REF mode. The target speed is provided by the FMS, and the target altitude will be the lower of sim/cockpit2/autopilot/altitude_dial_ft and sim/cockpit2/autopilot/altitude_vnav_ft. Upon level-off, the next climb will be automatic if the level-off was due to a VNAV restriction (ALTV) or must be initiated manually by altitude intervention if the level-off was due to pilot pre-selected altitude (ALTS).
If the selected cruise-altitude has been reached, the FMS goes into cruise phase. VNAV will be altitude hold on the vertical axis, and speed or mach hold on the auto throttle axis.
VNAV descent: G1000
VNAV descent works by arming the VNV PTH function on the G1000 by pressing the VNV button (sim/autopilot/vnav). The dataref sim/cockpit2/autopilot/vnav_status will indicate 1 for armed. The autopilot will monitor the vertical track error (sim/cockpit2/radios/indicators/fms_vtk_pilot) which must be intercepted from below. Also, the vertical track must be intercepted within 5 minutes (300 seconds) from arming VNAV. VNAV will not intercept a vertical track after that time, it needs to be re-armed in this case.
If the vertical track is intercepted within the time limit, VNAV will engage and follow the vertical path angle (sim/cockpit2/radios/indicators/fms_vpa_pilot) down to the flight plan target altitude (im/cockpit2/radios/indicators/fms_fpta_pilot). The descent rate will be modulated according to the forward speed and the vertical track error.
The autopilot will level off at the higher of pre-selected altitude (sim/cockpit2/autopilot/altitude_dial_ft) and VNAV target altitude(sim/cockpit2/autopilot/altitude_vnav_ft):
- If the level off is caused by a pilot-selected altitude, the indicated level-off will be “ALTS” (selected) and VNAV will be disarmed. For a subsequent descent segment, VNAV needs to be re-armed.
- If the lebel off is caused by the VNAV target altitude, the indicated level-off will be “ALTV” and VNAV will revert to armed. Thus, a new descent segment will be captured automatically.
VNAV descent: Airliner FMS
The behavior of the airliner FMS depends on the per-airplane setting “auto-arm VNAV descent” which is found on the autopilot settings page of Plane Maker. If Airliner-VNAV is selected (dataref sim/cockpit2/autopilot/fms_vnav or command sim/autopilot/FMS) and the plane comes close to the top-of-descent:
- With “auto-arm VNAV descent” selected, the VNAV descent is armed automatically when the plane comes close to the calculated top-of-descent and VNAV will intercept the vertical track automatically, provided that a lower altitude is selected in the autopilot altitude window (sim/cockpit2/autopilot/altitude_dial_ft). This is the behavior found in Boeing airliners.
- Without “auto-arm VNAV descent”, the autopilot must be allowed to initiate VNAV descent by either selecting “DES NOW” from the FMS or by pressing altitude intervention (sim/autopilot/alt_intv) following a lower altitude in the autopilot altitude window (sim/cockpit2/autopilot/altitude_dial_ft). Only then will the vertical track be intercepted. This behavior is found on Airbus airliners.
Note that the above mentioned behavior applies both to the initial descent from cruise altitude, and for all descents from intermediate level-offs, provided that those level-offs came from a VNAV altitude (ALTV) and not from a pilot pre-selected altitude (ALTS). In case of initiating descent from an ALTS level-off, the selected altitude must be lowered and altitude intervention must be pressed in order to re-engage VNAV descent.
Note that FMS VNAV uses geometric VNAV for the descent phase, so you will see both “sim/cockpit2/autopilot/fms_vnav” and “sim/cockpit2/autopilot/vnav_status” be used in that phase of flight.
Enroute vs Approach VNAV
Enroute VNAV is used up to the waypoint designated as the final approach fix. It will observe the pre-selected altitude (sim/cockpit2/autopilot/altitude_dial_ft) and never descend below it.
Approach VNAV is used after the waypoint designated as the final approach fix. It can be used to descend away from and under the pre-selected altitude, just like an ILS glide slope. This allows to set the pre-selected altitude to the missed-approach altitude while the airplane is descending to minimums.
LNAV/VNAV approach in general aviation aircraft
In small aircraft, approach guidance for RNAV (GPS) approaches is provided from the GPS unit or integrated glass cockpit (G1000) to the HSI source selector. It is imperative to set the source selector to GPS1 or GPS2 for an RNAV approach, and NAV1 or NAV2 for a VOR/LOC approach. In any case, the autopilot approach mode (LOC+GS) is used to track lateral and vertical paths. This is regardless whether the approach sensitivity is LNAV+V (advisory glidepath derived from WAAS altitude), LNAV/VNAV (WAAS-altitude glide path) or LP+V (localizer performance with advisory glide path, where the final approach course is derived from the FAS block and the glide path is derived from GPS altitude) or LPV (localizer performance with vertical guidance, both final approach course and glide path are provided by the FAS-block). In all cases, the lateral and vertical offset displayed on the CDI or HSI selected by the source selector will mimic and ILS and be intercepted by the autopilot as such. Height information in all cases is derived from WAAS altitude, so the barometric altimeter setting has no influence on the vertical path.
LNAV/VNAV approach in airliners
How the airliner autopilot tracks an approach not using ground-based (VHF) navigation depends on the setting of “Integrated Approach Navigation” on the autopilot system page in Plane Maker. This setting will be 0 for older 737s, 747s up to -400, 757, 767 and 777. It will be 1 for newer 737s, 747-800s, 787s and most Airbus airliners.
In airliners, approach course guidance for RNAV and RNP approaches is provided from the FMS unit. Vertical guidance can be provided based on barometric altitude (baro-VNAV) or from WAAS depending on the type of approach. So whether the correct altimeter setting actually influences your vertical path, depends both on the type of approach selected and the preferred vertical source selected on the FMS arrival page.
Both lateral and vertical guidance are available through the GPS1 and GPS2 source, however, the source should never be selected with the HSI source selector. In most airliners, the HSI source defaults to the on-side VHF NAV receiver, and the autopilot VLOC/APP mode will follow only that source. To follow LNAV without touching the source selector, the autopilot should be in GPSS (LNAV) mode (dataref sim/cockpit2/autopilot/gpss_status and command sim/autopilot/gpss) which it will have been for most of the cruise flight. To follow approach VNAV, whether barometric or WAAS, the autopilot should be in FMS VNAV mode (dataref sim/cockpit2/autopilot/fms_vnav and command sim/autopilot/FMS).
Summary: To intercept and fly the final approach of an RNAV or RNP approach, the MCP will have LNAV (GPSS in X-Plane speak) and VNAV (FMS VNAV in X-Plane speak) selected, and the FMA will display LNAV and VNAV PTH. This type of approach navigation is found in older 737s, 747s up to and including the -400, 757, 767 and 777 airliners.
A different way to deal with FMS-provided lateral and vertical cues during the approach phase is to “hijack” the LOC/APP modes of the autopilot and provide guidance “as-if” an ILS signal was present. Then, the pilot selects the approach button and the behavior will be the same for all types of approaches.
The corresponding on-side NAV signal will be overridden with FMS-provided LNAV/VNAV whenever an approach is loaded in the FMS that does not use VHF navigation and the active waypoint sequences to the final approach fix.
In this case, the autopilot will be in LNAV (GPSS) mode or in heading mode while approach is armed – the autopilot will automatically intercept the final approach course, deactivate LNAV (GPSS) mode and fly in APP mode laterally. Vertically, the autopilot can be in FMS VNAV mode or in altitude hold mode and will automatically intercept the vertical track, deactivate FMS VNAV mode and fly in APP mode vertically.
Summary: To intercept and fly the final approach of an RNAV or RNP or LPV or GLS approach, the MCP will have APP selected, and the FMA will display FAC and GP. This type of approach navigation is found in newer 737s, 747-800s, 787s and new Airbus airliners.