8.1.3 Direct-to Fix Lateral Transition
The Direct to Fix (DF) transition is defined implicitly as a path from the current horizontal position and velocity to the specified endpoint TCP. The transition typically consists of an initial turn transition to orient the velocity vector in the direction of the endpoint TCP, and a straight-line segment proceeding directly toward the specified endpoint (see Figure 11). The Direct-to Fix can be used as a means of specifying a fly-over turn toward the next waypoint, and is considered a non-precision trajectory type since DF segments are typically not repeatable or well defined in terms of turn behavior. Mandatory elements for the Direct-to-Fix TC report include the endpoint latitude, longitude and estimated time-to TCP, and a track-to TCP which can be computed from the latest reported position state vector as the direction from the aircraft position to the TCP (assuming that DF is the active flight segment). The track-to TCP will change dynamically in the turn transition phase until the aircraft velocity vector is aligned toward the endpoint TCP, and then remains relatively constant after the turn segment is completed. (Note: the DF transition is backwards compatible with the original DO-242 TCPs.)
ABC
Figure 11. Direct to ABC Lateral Transition Example
8.1.4 Direct to Fly-By Turn Transition
The Direct-to Fly-By is a combination of a Direct-to segment followed by a Fly-By turn. The information conveyed is very similar to the Fly-By turn transition, except for the meaning of the track-to Fly-By component, i.e. latitude, longitude, and TTG to the fly-by waypoint are required as well as track-to, track-from and turn radius components. If the DF to Fly-By is the active flight segment, then track-to may be computed as the inertial bearing angle from the current aircraft position to the fly-by waypoint. If the DF to Fly-By is preceded by an earlier TCP, then the track-to is computed as the bearing angle from the preceding TCP to the fly-by waypoint. However, the trajectory reconstruction process is inherently different for a DF to Fly-By compared to a TF to Fly-By transition, since the DF transition typically includes a turn segment to align the velocity vector toward the fly-by TCP, whereas the TF to Fly-By assumes a straight-line trajectory from the previous waypoint or TCP. Figure 12 shows a DF to Fly-By transition.
Figure 12. Direct to Fly-By Lateral Turn Transition
8.1.5 Radius to Fix Turn Transition
The Radius to Fix (RF) turn transition describes a constant radial turn over the earth, beginning at a turn start point that is the previous TCP and ending at the endpoint fix. Typically, RF turns are used to describe precision trajectories consisting of CF or TF to fix geodesic path segments and RF turn segments. Mandatory elements include the endpoint TCP latitude, longitude and time-to-TCP, the turn radius, and the track-from TCP. Turn direction can be transmitted also, but is not a required element. The turn center-point is constructed by first generating a line perpendicular to the track-from direction at the fix endpoint. The turn center-point is placed along this line segment at a distance equal to the turn radius from the endpoint fix. Care must be taken to achieve continuity of position and velocity when transitioning from the previous TCP to an RF turn segment. RF turns are considered a basic navigation leg type for implementing precision RNP routings. Figure 6 shows a geodesic path to fix entry and RF turn sequence.
8.2 Vertical TC Types
8.2.1 Unknown Altitude Type
This type is to preserve backwards compatibility with the original MASPS, i.e. a 3-D TCP is specified where the altitude value is an FMS estimate and may or may not represent one of the specified vertical TC types below.
8.2.2 Target Altitude
The Target Altitude TC type applies to level-off targets that end a vertical transition or denote the current maintaining altitude. This type contrasts with specific vertical transition types, such as Top-of-Descent and altitude constraints that specify defined 3-D endpoints. Some aircraft may be able to estimate the aircraft’s horizontal position at the Target Altitude trajectory change. Target altitude can be either an autopilot selected or an FMS target value such as selected cruise altitude. It is considered a TCP and separately reported and sequenced with other TCPs, if the command trajectory has a climb or descent transition that ends by leveling off at the target altitude. A target altitude TCP can be different than the target altitude in the TS report. For example, if the aircraft is maintaining cruise altitude prior to Top-of-Descent and the MCP selected altitude is set to an intermediate altitude, then the active target altitude is the selected cruise altitude, and the next two vertical TCPs are the Top-of-Descent point and the MCP selected altitude (see Figure 5a.). The only required TCP elements for this type are time to target altitude, target altitude, and altitude type, although latitude and longitude are desirable whenever available.
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