The IM procedure enables the flight crew to achieve and/or maintain an ATC assigned spacing interval behind a specific aircraft landing on the same runway immediately in front of the IM aircraft, i.e., Target aircraft. Depending on the operational need of the controller and the geometrical relationship between the IM and Target aircraft, the controller may issue the IM clearance in either time or distance, and may dictate whether the ASG is to be achieved immediately or as late as the FAF.
After the flight crew enters the clearance information into the IM user interface (procedurally designed to occur after the flight crew have already entered the Ownship route information and forecast wind) and once the software receives the position of the Target (from ADS-B transmitted messages), the algorithm calculates an airspeed for the flight crew to fly to meet the clearance.
Once an airspeed is displayed to the flight crew, they assess the speed and determine as a crew if it is operationally acceptable or not. If it is, they manually enter this airspeed into the autoflight system via the Mode Control Panel (MCP). This procedure was recommended by an airline partner in previous simulation experiments to limit the head down time of the pilots, especially when operating below 10,000 feet (United Parcel Service in ref. 36).
When the flight crew use this procedure to fly the IM commanded airspeed, shown both on the
Display (CGD), previous research has indicated that the aircraft can cross the specified waypoint after the Target aircraft within five seconds of the ASG (ref. 11).
2.2.3.2 IM Spacing Algorithm
The basic goal of an airborne spacing algorithm is to provide an airspeed to the flight crew, which if flown, nulls the spacing error. The NASA-developed Airborne Spacing for Terminal Arrival Routes (ASTAR) algorithm uses detailed route information for both aircraft to allow spacing procedures to begin any time after the Target aircraft’s route is communicated to the IM aircraft.
A more detailed description of the ASTAR algorithm is provided in Appendix A.
In 2015, the algorithm was updated to version ASTAR13 to support new IM operations described in the IM industry standards (ref. 3 and ref. 4). These standards define five different IM clearance types: Achieve-by Then Maintain (CROSS), Capture Then Maintain (CAPTURE), Maintain Current Spacing (MAINTAIN), Final Approach Spacing (SPACE), and IM Turn (TURN).
Prior versions of ASTAR consisted of only a trajectory-based speed control law, and therefore could only support the Achieve-by portion of the CROSS clearance. To enable the full implementation of the IM operations (except for the IM TURN), the previous ASTAR Trajectory-Based Operation (TBO) speed control law (ref. 12) was augmented with a state-based Constant Time Delay (CTD) speed control law (ref. 13 and ref. 14). In these supported clearance types, ASTAR ceases to provide speed commands for spacing at the PTP, which is a point on the Ownship’s route that is either designated by ATC, part of the planned procedure, or a default location close to the FAF.
The TBO speed control law in ASTAR13 is designed to support IM operations both when the IM and Target aircraft are in-trail and when they are on merging routes. The spacing error is calculated using the time-to-go of the IM and Target aircraft along their predicted 4D trajectories. A proportional control algorithm with an additional ground speed compensation term is then used to determine the amount of speed compensation that is required to achieve a precise spacing interval at a controller designated achieve-by point. The IM commanded speed is calculated by adding the speed compensation to predicted 4D trajectory airspeed. A discretized IM command speed is shown to the pilots, who are expected to close the control loop by entering the speed into their aircraft’s mode control panel speed window. An instantaneous commanded speed is used to drive a FAST/SLOW indicator to help pilots maintain better speed conformance when decelerating or accelerating toward the discrete commanded end speed. Additional details of the TBO speed control law are described in Appendix A.3.1.
The state-based CTD speed control law measures spacing error differently than the TBO speed control law, and can only be used when the IM and Target aircraft are in-trail. The measured spacing interval is measured as the difference in time between when the Target aircraft arrived at a particular along-path position and when the IM aircraft arrived at the same along path position.
The spacing error is the difference between the measured spacing interval and the assigned spacing goal. A proportional control law is used to determine the amount of speed compensation that is needed to capture or maintain the assigned spacing goal. The IM commanded speed is calculated by adding the speed compensation to the Target aircraft’s time-history speed, i.e., the speed that
speed window. Similarly to the TBO speed control law, an instantaneous commanded speed is used to drive a fast/slow indicator to help pilots maintain better speed conformance when decelerating or accelerating toward the discrete commanded end speed. Additional details of the TBO speed control law are described in Appendix A.3.2.
2.2.3.3 IM Cockpit Human-Machine Interfaces
The cockpit avionic devices used during this experiment to conduct IM operations included an EFB and a CGD for each pilot (Figure 6). A detailed description and illustrations of the EFB and CGD displays are given in Appendix B.
Figure 6. IM Cockpit Interfaces: EFB (left) and CGD (right).
A high-level description of the functionality of the IM displays to assist the flight crew in accomplishing the IM procedure and conduct the IM operation is given below.
• A design goal of the IM cockpit display was to allow for intuitive data entry of Ownship data, forecast wind data, and IM clearance data. When entered in the typical sequence, the flight crew enters data in the EFB from left to right, top to bottom. Active data fields are highlighted in green, and arrows within the data field indicate another page will be presented when selected (similar to aircraft flight management systems). This functionality exists only on the EFB.
• To minimize the time required for the flight crew to monitor the IM equipment, changes to the IM commanded speed were made salient (noticeable difference) by changing the background from black to green to highlight the new IM speed. For additional saliency, if the flight crew did not set the new IM speed within 10 seconds in the mode control panel speed window, the IM speed display cycles from black to green. Once the flight crew sets the new IM speed in the mode control panel, the background returns to black with the speed shown in green. This functionality exists on both the EFB and CGD.
• The state of IM spacing algorithm (ARMED, PAIRED, etc.) and alert messages (Target off path, Ownship off path, etc.) are displayed to the flight crew. Although this functionality exists on both the EFB and CGD, only a subset of the most critical messages needed to conduct the IM operation are shown on the CGD.
• A rate cue for decelerating the aircraft, called the FAST/SLOW indicator, allows pilots to quickly compare the relationship between the IM instantaneous speed to the aircraft’s current speed. This functionality exists on both the EFB and CGD.
• A cue for the along-path position of the aircraft, called the EARLY/LATE indicator, provides the flight crew an awareness of their ability to meet the assigned spacing goal within the expected tolerance. This functionality only exists on the EFB due to insufficient space on the CGD, and only at certain times (as specified in ref. 4).
The EFB is mounted on the outboard panel and the CGD just outboard of the aircraft’s navigation display (see Figures in Section 4). Since pilots frequently use the EFB for non-IM tasks (e.g., reviewing approach charts) and it is mounted outside of the pilot’s optimal primary field of view (FAA defined as within ± 15 degrees horizontally of a level line of sight, and vertically level to a 30 degree downward line of sight), the smaller CGD is designed to show, within the pilot’s optimal forward field of view, only the critical subset of information needed to conduct the IM operation.