6 Command and Planned Trajectories
A distinction is necessary between intent information that is actively used for aircraft guidance and control and other programmed targets residing within the automation system that are currently inactive. These types of intent are classified as the aircraft’s command and planned trajectories, respectively. Command intent is considered most reliable for short-term trajectory predictions,13 whereas planned intent may give valuable insights into a pilot’s long-term strategic plan. NASA’s AATT program is currently investigating various conflict alerting strategies that leverage both command and planned trajectory information.
The command trajectory refers to the path the aircraft will fly if the pilot does not engage a new flight mode nor change the targets for the active or upcoming flight modes. The command trajectory may include multiple flight mode transitions. Changes to the command trajectory normally result from a pilot input. However, a non-programmed mode transition may also occur that causes the aircraft to leave the command trajectory, e.g. reversion to speed priority on descent if the intended vertical path results in an over-speed condition.
The planned trajectory includes intent information that is conditional upon the pilot engaging a new flight mode. Without pilot input, the aircraft will only fly toward the command trajectory targets.
Figure 5a illustrates the difference between the command and planned trajectories for a simple descent scenario. In this case, the aircraft is flying a lateral and vertical FMS path that includes a planned altitude level off at the End of Descent (E/D). The MCP/FCU selected altitude lies between the aircraft’s current altitude and the E/D. Assuming the pilot doesn’t change the aircraft’s flight mode or targets, the aircraft will fly on the FMS descent path until reaching the selected altitude and then level off. This path is the command trajectory. If the pilot resets the MCP target at or below the E/D altitude prior to reaching the selected altitude, the aircraft will continue to fly along the FMS descent path and will level out at the E/D. The programmed FMS path beyond the selected altitude represents a planned trajectory. In today’s operational environment, selected altitude typically indicates an ATC clearance altitude. In this case, the pilot may choose to fly directly to the end of descent as soon as a clearance to the planned altitude is received.
Constant 090 Track throughout Descent
Top of
FL350
Descent (1)
MCP/FCU Selected Altitude (15,000 ft) MCP/FCU Altitude Level-off (2)
Altitude Constraint (3,000 ft)
Command Trajectory
Planned Trajectory
Waypoint ABC (End of Descent)
(3)
Figure 5a. FMS Descent with Intermediate MCP/FCU Selected Altitude
The command trajectory represents the active guidance targets used by the aircraft. It is therefore considered to be the most reliable source of trajectory intent.13 Because the command trajectory normally conveys a higher level of path integrity, additional constraints are imposed on TCPs labeled as “command”. The following conditions must be met for command TCPs:
1.
The transmitting aircraft can determine that a TCP is part of the command trajectory, as defined above, and
2.
The transmitting aircraft can determine that it is broadcasting all TCPs between the aircraft’s current position and the corresponding TCP.
Unless the transmitting aircraft can meet these conditions, the command/planned flags in the TC report must be set to “planned”. The determination of “command” versus “planned” must consider flight mode logic and targets resident in all auto-flight systems that support aircraft guidance. Command/planned status for the horizontal and vertical trajectories is considered independently (see Section 7).
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