Implementing VNAV support in FlightGear: Difference between revisions

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Implementing VNAV support in FlightGear (view source)

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'''Related:''' {{Issue|640}}.
'''Related:''' {{Issue|640}}.
== VNAV ==
The Vertical Navigation (VNAV) function of the Flight Management System (FMS) serves as an intelligent agent during all flight phases by automatically selecting appropriate targets (e.g. altitude, speed, and vertical speed) and pitch/thrust control modes to satisfy the objectives of each leg of the flightplan.
The VNAV function provides three automated features:
* VNAV automatically selects altitude targets and speed targets according to pilot MCP entries and the altitude and speed constraints in the FMS flightplan.
* VNAV automatically selects pitch and thrust control modes to fly to the targets. For example during descent, VNAV chooses between a FLCH descent, a vertical speed (fixed rate-of descent), and an FMS path descent. In the case where VNAV selects vertical speed control mode, VNAV also selects the vertical speed target.
* for the descent and approach, VNAV automatically provides an optimum path that is used as the reference for all automated altitude/speed target and control mode selections.
In a study of the software of contemporary VNAV functions, Sherry & Polson (1999), found that the typical VNAV function automatically chooses the active altitude target from a possible list of sixteen, and chooses the active speed target from a possible list of twenty-six.
{{Appendix|all|
* {{cite web |url=http://human-factors.arc.nasa.gov/publications/20051025130903_VNAVCogEngJournal1TM.pdf |title=Why VNAV in Descent is Difficult to Use }}
}}
== The Performance Database ==
There are typically two loadable databases that support the core flight management functions. These are the navigation database which must be updated on a monthly cycle and the performance database that only gets updated if there’s been a change in the aircraft performance characteristics (i.e., engine variants or structural variants affecting the drag of the aircraft).
The performance database contains aircraft/engine model data consisting of drag, thrust, fuel flow, speed/altitude envelope, thrust limits, and a variety of optimized and tactical speed schedules that are unique
to the aircraft.
Typically, the performance database contains data sets for
* take-off and configuration speeds
* approach and landing speeds
In real aircraft, the purpose of the so called "performance database" is to reduce the need for the pilot to refer to a performance manual during flight and to provide the FMC (flight management computer) with data required to transmit pitch and thrust commands to the FCC (flight control computer. The performance database is
also used by the FMC to provide detailed predictions along the entire aircraft trajectory. The data stored in the database includes accurate aircraft drag and
engine model data, optimal speed data, maximum altitudes, and maximum and minimum speeds. The FMS onboard the aircraft also performs fuel consumption
predictions based on a reference model: the FMS performance database. The PERF FACTOR entered in the M/CDU helps to the FMS predictions.
Implementing aircraft performance monitoring aims to determine the monitored fuel factor.
The main FMS aircraft performance predictions deal with:
* fuel consumption, time,
* climb and descent path,
* recommended maximum altitude, and
* optimization of speeds and cruise altitude taking into account economic criteria
defined by the airline Cost Index.
The PDB is derived from the IFP aircraft databases. Slight simplifications were taken into account because of the limited size of the FMS memory. For example, only one air conditioning setting is available (LO/ECON as appropriate). Per design, the aircraft performance databases are stored in the FMS Perf Data
Base (PDB). There is only a single PDB per family of aircraft.
At the center of the FMS functionality is the flight plan construction and subsequent construction of the four-dimensional aircraft trajectory defined by the specified flight plan legs and constraints and the aircraft performance. Flight plan and trajectory prediction work together to produce the four-dimensional trajectory and consolidate all the relevant trajectory information into a flight plan/profile buffer.
{{Appendix|all|
* {{cite web |url=http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_15.pdf |title=The Avionics Handbook: Flight Management Systems | author=Randy Walter (Smith Industries) }}
}}
== Trajectory Prediction ==


== Problem ==
== Problem ==
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Otherwise, it is not currently feasible to come up with VNAV/LNAV modes that properly work for different aircraft.
Otherwise, it is not currently feasible to come up with VNAV/LNAV modes that properly work for different aircraft.
The 3rd party approach is being used for TPPs data, but in the case of FDMs, we would need 100% accurate FDMs, too - so it would be better to directly use the FDM for computing a performance DB.
The 3rd party approach is being used for TPPs data, but in the case of FDMs, we would need 100% accurate FDMs, too - so it would be better to directly use the FDM for computing a performance DB.
Supporting VNAV/LNAV still is tricky for other reasons, and currently not on anybody's agenda - it's not just something that can/should be tackled by a single aircraft developer, it requires lower-level changes in FlightGear's architecture and the way FDMs are used.
Supporting VNAV/LNAV still is tricky for other reasons, and currently not on anybody's agenda - it's not just something that can/should be tackled by a single aircraft developer, it requires lower-level changes in FlightGear's architecture and the way FDMs are  


== Background ==
{{cquote|Altitude constraints are a mess - in the short term, they are best avoided. Keep in mind the GPS code doesn't really do proper vertical  navigation - it computes some data like the altitude change and climb/ descent rate for a leg, but I'm not sure if any real aircraft is  driving the VNAV mode of an autopilot from it.
{{cquote|Altitude constraints are a mess - in the short term, they are best avoided. Keep in mind the GPS code doesn't really do proper vertical  navigation - it computes some data like the altitude change and climb/ descent rate for a leg, but I'm not sure if any real aircraft is  driving the VNAV mode of an autopilot from it.


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<references/>
<references/>
== VNAV ==
The Vertical Navigation (VNAV) function of the Flight Management System (FMS) serves as an intelligent agent during all flight phases by automatically selecting appropriate targets (e.g. altitude, speed, and vertical speed) and pitch/thrust control modes to satisfy the objectives of each leg of the flightplan.
The VNAV function provides three automated features:
* VNAV automatically selects altitude targets and speed targets according to pilot MCP entries and the altitude and speed constraints in the FMS flightplan.
* VNAV automatically selects pitch and thrust control modes to fly to the targets. For example during descent, VNAV chooses between a FLCH descent, a vertical speed (fixed rate-of descent), and an FMS path descent. In the case where VNAV selects vertical speed control mode, VNAV also selects the vertical speed target.
* for the descent and approach, VNAV automatically provides an optimum path that is used as the reference for all automated altitude/speed target and control mode selections.
In a study of the software of contemporary VNAV functions, Sherry & Polson (1999), found that the typical VNAV function automatically chooses the active altitude target from a possible list of sixteen, and chooses the active speed target from a possible list of twenty-six.
{{Appendix|all|
* {{cite web |url=http://human-factors.arc.nasa.gov/publications/20051025130903_VNAVCogEngJournal1TM.pdf |title=Why VNAV in Descent is Difficult to Use }}
}}
== The Performance Database ==
There are typically two loadable databases that support the core flight management functions. These are the navigation database which must be updated on a monthly cycle and the performance database that only gets updated if there’s been a change in the aircraft performance characteristics (i.e., engine variants or structural variants affecting the drag of the aircraft).
The performance database contains aircraft/engine model data consisting of drag, thrust, fuel flow, speed/altitude envelope, thrust limits, and a variety of optimized and tactical speed schedules that are unique
to the aircraft.
Typically, the performance database contains data sets for
* take-off and configuration speeds
* approach and landing speeds
In real aircraft, the purpose of the so called "performance database" is to reduce the need for the pilot to refer to a performance manual during flight and to provide the FMC (flight management computer) with data required to transmit pitch and thrust commands to the FCC (flight control computer. The performance database is
also used by the FMC to provide detailed predictions along the entire aircraft trajectory. The data stored in the database includes accurate aircraft drag and
engine model data, optimal speed data, maximum altitudes, and maximum and minimum speeds. The FMS onboard the aircraft also performs fuel consumption
predictions based on a reference model: the FMS performance database. The PERF FACTOR entered in the M/CDU helps to the FMS predictions.
Implementing aircraft performance monitoring aims to determine the monitored fuel factor.
The main FMS aircraft performance predictions deal with:
* fuel consumption, time,
* climb and descent path,
* recommended maximum altitude, and
* optimization of speeds and cruise altitude taking into account economic criteria
defined by the airline Cost Index.
The PDB is derived from the IFP aircraft databases. Slight simplifications were taken into account because of the limited size of the FMS memory. For example, only one air conditioning setting is available (LO/ECON as appropriate). Per design, the aircraft performance databases are stored in the FMS Perf Data
Base (PDB). There is only a single PDB per family of aircraft.
At the center of the FMS functionality is the flight plan construction and subsequent construction of the four-dimensional aircraft trajectory defined by the specified flight plan legs and constraints and the aircraft performance. Flight plan and trajectory prediction work together to produce the four-dimensional trajectory and consolidate all the relevant trajectory information into a flight plan/profile buffer.
{{Appendix|all|
* {{cite web |url=http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_15.pdf |title=The Avionics Handbook: Flight Management Systems | author=Randy Walter (Smith Industries) }}
}}
== Trajectory Prediction ==


== VNAV for the 777 ==
== VNAV for the 777 ==
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