EASy Avionics: Everything You Ever Wanted to Know About Vertical Navigation
EASy Avionics: Everything You Ever Wanted to Know About Vertical Navigation
Part 4 – Early and Late VNAV Descents
The purpose of this multi-part series is to de-mystify vertical navigation (VNAV) on Honeywell’s Primus Epic® for Dassault EASy (Enhanced Avionics System). Each article provides simple, easy-to-follow explanations in “pilot-speak.”
Operational differences exist between Honeywell Primus Epic® for Dassault EASy platforms (EASy II, EASy III, etc.), so it’s difficult to write a one-size-fits-all article. Therefore, this series references EASy II avionics. However, the final article in this series will be dedicated to discussing the differences between EASy II and EASy III with NG FMS.
This series covers the principles of VNAV and its implementation in Honeywell’s Primus Epic® for Dassault EASy avionics. However, since EASy avionics can be found in different Dassault Falcon aircraft types, it’s important to clarify that the AFM and CODDE manuals for your specific aircraft take precedence over any information presented in this article.
Entering the Descent Phase of Flight (POF)
This article, Part Four of this series on EASy VNAV, explores VNAV descents that begin either before or after the FMS-calculated top-of-descent (TOD) point and what to expect from the automation in these situations.
The ideal scenario to begin a VNAV descent and enter the descent phase of flight (POF) is to have the altitude preselector dialed down to a lower altitude with VALT as the active vertical mode. Then, when the FMS-calculated TOD is reached, the vertical mode changes to VPTH and the pilot-defined descent angle is flown.
As we all know, however, this ideal scenario is not always feasible due to ATC issuing an early descent prior to the TOD or a late descent past the TOD.
The typical ATC clearance that requires a VNAV descent prior to the TOD usually goes something like: “Falcon123, descend now to cross HAUPO at one-six-thousand, O’Hare altimeter 29.92.”
The crew then adds the 16,000-foot constraint to the HAUPO waypoint (refer to the first and third articles in this series on how to add altitude constraints).
A quick glance at the INAV, Vertical Situation Display (VSD), and waypoint list (Figure 1) shows that we are still about 13 miles away from the TOD.
|Figure 1. INAV, VSD, and Waypoint List – TOD Ahead of Aircraft|
If the flight plan is selected for display on the Primary Display Unit (PDU), the TOD point can be referenced here as well (Figure 2).
|Figure 2. TOD depicted on the PDU|
Using PATH Mode
Like any descent, ensure the altitude preselector is set to the new altitude – 16,000 feet in this example. Then, use the path wheel on the Guidance Panel (GP) to make PATH the new active vertical mode and set a descent path that will intercept the constructed vertical path. In this example (Figure 3), a descent PATH of -1.0 degree will be used. The commanded path, shown by the magenta line on the VSD, indicates the current descent path will intersect the FMS-constructed path just past SPANN.
|Figure 3. Using PATH to Intercept the Constructed Vertical Path|
However, as the Flight Mode Annunciator (FMA) is currently set, the aircraft will not capture the FMS-constructed path. The aircraft will continue descending in PATH mode at -1.0 degree until reaching 16,000 feet – not exactly what ATC cleared us to do. To intercept and capture the FMS-constructed path, push the VNAV button on the GP to make VNAV the armed vertical mode (Figure 4).
|Figure 4. FMA Set Up to Capture FMS-Constructed Path in PATH Mode|
Once the FMS-constructed path is intercepted, the vertical mode changes from PATH to VPTH. The manually commanded descent path angle of -1.0 degree on the FMA is replaced by the descent angle used by the FMS in constructing the path to make the altitude restriction at HAUPO (Figure 5). In this example, -2.4 degrees was the default descent angle as set in the Auto Speeds tab of the Avionics window. From this point forward, the descent is managed like any other VPTH descent to a waypoint restriction.
|Figure 5. Transition From PATH to VPTH|
Using Vertical Speed Mode
Alternatively, Vertical Speed (VS) mode can be used to establish a descent to intercept the FMS-constructed path. Note that in VS mode, only the green actual flight path line extending from the nose of the aircraft is presented on the VSD (Figure 6). As such, it is more difficult to predict exactly where the FMS-constructed path will be intercepted versus using PATH mode to initiate the descent.
|Figure 6. Using VS to Intercept the Constructed Vertical Path|
Pushing the VNAV button on the GP arms VNAV and sets up the FMA to intercept and capture the FMS-constructed path once it is intercepted (Figure 7).
|Figure 7. FMA Set Up to Capture FMS-Constructed Path in VS Mode|
Once intercepted, the vertical mode changes from VS to VPTH. The manually commanded descent rate on the FMA is replaced by the descent angle used by the FMS to construct the path to make the altitude restriction at HAUPO (Figure 8). From this point forward, the descent is managed like any other VPTH descent to a waypoint restriction.
|Figure 8. Transition from VS to VPTH|
|Figure 9. Applying the Vertical Direct-To|
The FMS will only allow a vertical direct-to to be performed if the new FMS-constructed path angle is between 1 and 6 degrees. To see what this new path angle is after applying the vertical direct-to, look for the cyan-colored descent angle next to HAUPO in the waypoint list. This is the descent angle that will be commanded once the Activate softkey is selected. Here, the resultant angle is 1.83 degrees.
|Figure 10. Viewing the Resultant Vertical Direct-To Descent Angle|
With VNAV active and the altitude preselector dialed down to the crossing altitude, activating the vertical direct-to will result in the aircraft beginning the VPTH descent. However, if you activated the vertical direct-to and either VNAV wasn’t active or the preselector wasn’t dialed down to the lower altitude, the vertical mode will not change to VPTH. Once VNAV is made active and the preselector is dialed down, the aircraft will begin the VPTH descent without the need to activate the vertical direct-to a second time.
|Figure 11. Transition From VALT to VPTH and Resultant Vertical Direct-To|
If instructed to “cross HAUPO at one-six-thousand, O’Hare altimeter 29.92,” the crew will need to add the 16,000-foot constraint to the HAUPO waypoint.
Looking at the vertical deviation indicator (VDI) on the PDU (Figure 12) tells us that the aircraft is already flying well above the FMS-constructed vertical path.
|Figure 12. VDI Shows FMS-Constructed Path Below Aircraft|
In addition, the VSD depicts the aircraft flying above the FMS-constructed path (Figure 13). Either of these indications shows that a descent past the TOD will need to be performed to comply with the ATC crossing restriction.
|Figure 13. VSD Shows FMS-Constructed Path Below Aircraft|
Using the Automation
Since there is no VNAV descent flight-level change mode in EASy, some pilots are left wondering, “…How do I use the automation to descend and capture the FMS-constructed path from above?”
With VALT as the active vertical mode, the only required pilot action is to simply dial down the altitude preselector to the lower altitude – that’s it! Doing so automatically causes the vertical mode to change from VALT to VPTH (Figure 14). Or, if the preselector is set to a lower altitude with a non-VNAV mode active (such as ALT, PATH, or VS), pushing the VNAV button will cause VPTH to become the new active vertical mode.
Once VPTH is active, the flight director will command a descent angle equal to the FMS-constructed path angle plus an additional 2 degrees to intercept the path. Note that the FMA will display the FMS-constructed path angle (-2.4 degrees in this example), not the presently commanded angle to intercept. Here, the commanded descent path angle to intercept the constructed path is -4.4 degrees as indicated by the flight path symbol and flight director (Figure 14).
|Figure 14. Descending at -4.4 Degrees to Intercept the FMS-Constructed Path|
Once intercepted, the flight director commands the FMS-constructed path angle as displayed on the FMA (Figure 15). From this point forward, the descent is managed like any other VPTH descent to a waypoint restriction.
|Figure 15. Descending at -2.4 Degrees on the FMS-Constructed Path|
Using Vertical Direct-To
Sometimes, the resultant FMS-constructed path angle plus 2 degrees is a bit too steep for some passengers; or perhaps ATC assigned a descent speed that would be difficult to maintain without airbrakes during the steeper descent to intercept the path (Figure 16).
|Figure 16. Resultant Descent to Intercept Path Is Too Steep|
If you find yourself in one of these situations, consider performing a vertical direct-to to the crossing restriction to fly a path angle that is less than what the resultant intercept path would be (Figure 17).
|Figure 17. Shallower Path Using Vertical Direct-To|
Setting up and performing this vertical direct-to is done in the same manner as previously described in the Early Descent section of this article.
This article (Part Four) in the series has introduced VNAV descents that begin either before or after the FMS-calculated TOD point and what to expect from the automation in these situations. Future articles in this VNAV series will explore additional functions in easy-to-understand explanations. Be sure to “tune in” next time for more EASy VNAV information.
Program Pilot Ryan Milmoe supports Embraer E-Jets and Dassault EASy for Honeywell Flight Technical Services. He can be reached via email at Ryan.Milmoe@Honeywell.com