8.2.3 Top of Climb (TOC)
Top of Climb is the TCP endpoint of the climb phase of flight, i.e. Top-of-Climb designates the point where the aircraft levels off at a desired cruise altitude. Top-of-Climb is specified by latitude, longitude, and time-to-TCP estimates, as well as the selected cruise altitude. Note, after a TOC TCP, the next TC report contains a vertical TCP with either a target altitude (which can be the current cruise altitude or an intended step change altitude) or the Top-of-Descent (see below).
8.2.4 Top of Descent (TOD)
Top of Descent is the planned endpoint of the cruise phase of flight, i.e. Top-of-Descent designates the point where the aircraft is scheduled to begin descent from cruise altitude. Top-of-Descent is specified by latitude, longitude, and time-to-TCP estimates, as well as the selected cruise altitude. The next TC report after a TOD should contain a Target Altitude or Altitude Constraint vertical TCP with altitude value less than the cruise altitude at TOD. (Note: ideally all points where a vertical transition from level flight begins should be delimited as TCPs also, such as start-of-climb from an intermediate flight level. However, the pilot may simply use the autopilot interface with a new selected altitude and manual engagement to start such flight segments, or alternately may use an “At” constraint at a waypoint with FMS engagement of the next vertical transition segment to achieve the same purpose.)
8.2.5 Estimated Altitude
If the aircraft is in climb or descent mode transitioning towards the next level-off altitude when a lateral waypoint or TCP is sequenced, the altitude value is typically estimated by the FMS, i.e. if the aircraft is not maintaining a target altitude or subject to an altitude constraint at the waypoint, then the altitude value provided by the FMS is an estimated altitude.
8.2.6 Altitude Constraints (At, At and Above, At and Below)
Altitude constraints are often used in the climb and descent phase of flight to maintain separation of departure, arrival, and over-flight traffic patterns in congested airspace. Altitude constraints are provisioned in DO-242A, because current FMS buses may not provide such information to external data users. Representation of altitude constraints is considered essential for future versions of this MASPS (after Revision A), because vertical path intent is not complete until such intent data is available. Moreover, altitude constraints are the basis for implementing vertical RNP using altitude “window” constraints in future RNP systems.6 Altitude constraint TCPs will require specification of waypoint latitude and longitude, time-to TCP, the actual altitude constraint value, and the type of constraint, i.e. At, At and Above, or At and Below. The exact representation of such constraints is currently under consideration, i.e. how to accommodate window constraints consisting of a simultaneous At and Below and an At and Above constraint at the constraint fix. Three bits are provisioned in DO-242A to accommodate future expansion.
9 Equipage Class Requirements
In the original MASPS, Level A0 and Level A1 equipage provides basic state vector broadcast capability for VFR and IFR users, respectively. In addition, Level A2 equipage was defined to support extended range ADS-B applications to at least 40 NM range and provide at least a single TCP broadcast in order to assure the validity of trajectory predictions for several minutes look ahead. Level A3 equipage was similarly defined to support extended range applications such as flight path de-confliction out to 90 NM range and provide at least two TCP broadcasts to assure continuity of trajectory predictions near the first TCP, and to achieve at least five minutes trajectory look ahead time.1
DO-242A equipage classes retain the concept and overall capability of Level A2 and Level A3 equipage, but revise the definitions of these equipage classes to better reflect horizontal and vertical autopilot and RNAV capability. A minimum Level A2 ADS-B system will have the ability to broadcast TS reports for target altitude and target heading, and at least one TC report. The reason for requiring target altitude is to assure that a Level A2 system has some intent capability in both horizontal and vertical axes, i.e. to support extended range predictions in both horizontal and vertical dimensions. A minimum Level A3 ADS-B system will have Level A2 capability and the capability to broadcast multiple (up to four) TC reports. The reason for allowing up to four TC reports, as compared with two TCPs in DO-242, is that there are several conditions where two TCPs are insufficient to predict ahead five minutes or to 90 NM range. Specifically, routing changes are quite frequent in the terminal area transitioning towards final approach or on initial departure after take-off. Under these conditions, additional TCPs may be needed to achieve desired look-ahead time for terminal area planning applications. Other potential applications that could require more TCPs include air-ground planning applications for en route traffic flow management14-16 and transition between free flight air-air operations and ATC managed traffic.7
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