3 Short and Long-term Intent
Target State (TS) reports are implemented in DO-242A in order to provide information about the aircraft’s active flight segment. The active flight segment refers to the current path and automation states being used for aircraft guidance and control. The primary elements of the TS report include the target altitude and target heading or track angle for the active flight segment. This information is called short-term intent. TS reports provide these intent elements even in cases where no TCP exists or TCP information is only partially available. Long-term intent includes information about TCPs and connecting flight segments, and is provided in a series of Trajectory Change (TC) reports. Both short and long-term intent are considered necessary for certain free flight operating environments.2
Figure 1 shows the relationship between information provided in TS reports and TC reports for an aircraft flying a simple trajectory between FMS/RNAV waypoints. The target track to waypoint ABC and the target altitude for the active flight segment are provided in the TS report. Three TC reports give information on waypoints ABC, DEF, and GHI. Note that this figure only represents one type of trajectory. Other trajectory types and the information used to fill the TS report and TC reports (if available) are described in the following sections.
Figure 1. TS and TC Report Information
A 2000 FAA-Eurocontrol sponsored Technical Interchange Meeting (TIM) on intent information included a recommendation in its outbriefing to, “Study the relationship between aircraft control loops and intent parameters.”10 This recommendation is important, in part, because the amount of intent information available for data exchange depends strongly on the transmitting aircraft’s current control state and equipment. These relationships were evaluated in several Boeing 777 simulator sessions and through a review of Airbus vertical flight modes.11 The TS and TC reports are designed to take advantage of intent information available when aircraft are operated in either simple or complex control states.
The three primary control states, referred to here as manual (no flight director), target state, and trajectory, are shown in Figure 2. With each additional outer loop, it is possible for an aircraft to communicate more information about future states and flight segments. While operating with target state control, one commanded state is available for the horizontal and vertical axes. The TS report provides these states in the form of target altitude and target heading or track angle. In the outermost loop corresponding to trajectory control, the aircraft has knowledge of multiple TCPs and connecting flight segments. TC reports provide this information. In the trajectory control state, the TS report provides target state information corresponding to the active flight segment.
Most commercial aircraft have several flight modes corresponding to the target state and trajectory control states shown in Figure 2. Flight modes are normally selected through the Mode Control Panel (MCP) or Flight Control Unit (FCU). They include choices such as hold current heading, hold current altitude, and maintain track between FMS/RNAV waypoints. The pilot can concurrently choose lateral and vertical flight modes that correspond to different control states, leading to different intent availability in the horizontal and vertical axes. Horizontal and vertical flight commands may be generated for manual flight using a flight director display mode, rather than through direct autopilot commands. No distinction is made between flight director and autopilot operation, since this information cannot be differentiated from ADS-B output reports.
FD: Flight Director
Figure 2. Aircraft Control States
Figure 2 shows typical equipment available on transport category aircraft that is capable of providing the associated information. Other flight hardware may also be able to generate this information. More sophisticated equipment is needed to transmit outer loop intent, although inner loop information on current target states may be difficult to transmit for older analog aircraft. An MCP or FCU is the primary interface between the pilot and autopilot when not operating in FMS automated modes. These interfaces allow the pilot to select target states such as altitude, heading, vertical speed, and airspeed. Since only the next target state is allowed in each axis, pilots often use the MCP or FCU for short-term tactical flying. Conversely, the Flight Management System (FMS) allows the pilot to specify a series of target states or flight segments through a keypad-based Control Display Unit (CDU). A pilot may program an entire route complete with multiple waypoints, speed, altitude, and time restrictions, and specify desired speed and altitude appropriate to the current flight segment. Because the FMS allows definition of consecutive flight segments, it is frequently used for long-term strategic flying.
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