Editor’s Note: The title of this series highlights the inspiration for each article: every issue covered was reported to Honeywell by a pilot. PIREP will look at the things a system can’t do, either because of procedure design, database coding, the FMS, or physics. The hope is that pilots will begin to recognize and understand these limitations.
As SIDs and STARs become longer and more complex, there is a greater likelihood of having shorter routes consisting entirely of a SID and STAR with no en route waypoints. When the SID and STAR share a common waypoint, the FMS may keep the aircraft in an undesired vertical state, requiring pilot intervention.
Climb to Descent Mode Transition
The FMS remains in climb mode until the FMS a) reaches the Init Cruise Alt entered in Perf Init and b) sequences the last waypoint in the departure procedure. Once both of those are satisfied, it will sequence to the next vertical mode as appropriate.
This can cause an unexpected behavior in the FMS logic. If the arrival has a crossing restriction on the common waypoint between a SID and a STAR, or ATC assigns a restriction on that waypoint, the FMS will remain in climb mode and provide an “UNABLE NEXT ALTITUDE” message in the scratchpad when the crew expects the aircraft to descend to meet the constraint (See Figure 1.)
Figure 1. Altitude Constraint on Common Waypoint
A simple way to understand this problem is to look at the flight management system’s planning logic. The FMS logic is represented by the illustration below showing a profile view. Notice the FMS executes the climb profile as expected. The Problem Area is represented by the blue circle below. If the FMS hasn’t sequenced the last waypoint of the departure, it is anticipating another climb (based on climb speeds) but the crew is expecting (or has been assigned) a descent constraint. This is where the manual intervention technique (described below) becomes invaluable to the crew.
This is an opportunity to add a lesser known tool to the knowledge bank. The FMS can be forced manually from Climb to Descent mode or vice versa by entering the letter C (Climb) or D (Descent) into the scratchpad and placing it on the right side (vertical portion) of the flight plan adjacent to where the crew would like the switch to begin. The following example illustrates this concept.
An airplane is flying a SID as diagramed above, and one of the common waypoints on the arrival has a crossing constraint. The aircraft will remain in Climb mode until it reaches the initial climb altitude AND sequences the last waypoint in the departure. If that common waypoint has a descent constraint, the system will not honor it unless it has been switched to Descent mode by the method described above.
Finally, take a look at a quick operational example of switching from CLB to DES using the same technique. In the first graphic, notice the waypoint CYPRS in the CLB profile with an At or Above FL220 constraint. By entering the letter D in the scratchpad and line selecting it to the desired location (in this case R1), the mode changes from Climb to Descent as shown in the second and third graphics. This can also come in handy when the aircraft does not reach the altitude entered during PERF INIT and is still in climb mode as it begins approaching descent constraints in the descent path.
Questions or comments about anything covered in the PIREP series? Please contact Flight Technical Services at FTS@Honeywell.com.
Program Pilot David Rogers supports Honeywell EPIC and NG FMS for Flight Technical Services. He can be reached via email at David.Rogers@Honeywell.com