Everything You Ever Wanted to Know About Vertical Navigation
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 impossible to write a one-size-fits-all article. Therefore, this series will reference 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 is important to mention that the AFM and CODDE manuals for your specific aircraft take precedence over any information presented in this article.
Part Two of this series on EASy VNAV takes a closer look at performance entries, discussing each input and where the entry impacts the performance in the flight plan. Optimum Altitude calculations are also discussed.
Pilots know the required inputs when entering a flight plan, but there may be confusion about when modes transition and which speeds are used.
The example flight profile below encompasses takeoff to landing with descriptions of where the FMS generates its performance information and what logic the pilot can expect during transitions. The flight begins with the airplane on ground and all performance initializations in the Avionics and Flight Management Window (FMW) complete.
On the runway with takeoff speeds computed and FMS speeds selected on the Guidance Panel (GP), the Primary Display Unit (PDU) displays V2 at the top of the airspeed tape (Figure 1).
|Figure 1. V2 Speed Target During Takeoff Roll on the PDU|
During liftoff, as the aircraft accelerates through V2, the speed target increases to V2+10 (Figure 2).
|Figure 2. V2+10 Speed Target Shortly After Liftoff|
Once above the takeoff safe altitude (TOSA), VFT becomes the new FMS speed target (Figure 3).
|Figure 3. VFT Speed Target Above TOSA|
A VNAV climb may be initiated by pushing the VNAV button on the GP. The vertical mode changes from T/O to VCLB and the FMS speed target is displayed to the right of VCLB in the Flight Mode Annunciator (FMA), as shown in Figure 4.
|Figure 4. Initiating a VNAV Climb With FMS Speed Target of 200 Knots|
During the initial climb, the FMS speed target is the lesser of:
– The current slat/flap setting limit speed, or
– A waypoint speed restriction on the departure procedure, or
– The speed defined in the Departure phase of flight (POF) in the Auto Speeds tab (Figure 5)
|Figure 5. Departure POF Auto Speeds|
Once the aircraft exits the Departure POF boundaries (Figure 5), the FMS speed target is the lesser of:
– The current slat/flap setting limit speed, or
– A waypoint speed restriction on the departure procedure (Figure 6), or
– The Auto Speeds “Speed Limit” (typically set to 250/10000), which limits the aircraft to 250 knots below 10,000 feet (Figure 7).
|Figure 7. Speed Limit in the Auto Speeds Tab|
Passing through the Speed Limit altitude (10,000 feet in this example), the FMS accelerates the aircraft to the Climb POF speed unless further restricted by a waypoint speed constraint on a departure procedure.
As mentioned in Part One of this series, the FMS remains in the Climb POF until the aircraft reaches the Init Crz Alt as entered by the pilot during Performance Initialization. So even during intermediate level-offs, the FMS will continue to command the selected climb speed (Figure 8).
|Figure 8. Climb Speed|
The Cruise POF starts at the completion of the Climb phase (top of climb) and ends at the beginning of the Descent phase (top of descent). As the aircraft levels off from the VNAV climb, the vertical mode changes from VCLB to VASL and then to VALT. However, if a non-VNAV vertical mode such as vertical speed (VS) was used during the final portion of the climb, the vertical mode changes from VS to ASEL and then to ALT. In this case, after leveling off and with ALT now the active vertical mode, pushing the VNAV button re-engages VNAV and changes the active mode from ALT to VALT. At this point, once in ALT or VALT, the FMS speed command transitions from climb to cruise speed (Figure 9).
|Figure 9. Cruise Speed|
If the crew desires to level off at a final cruise altitude lower than the Init Crz Alt, they must manually go back into the FMW and update the Init Crz Alt to the new lower value. Otherwise, the FMS will stay in climb mode and anticipate there is another climb coming, leading to incorrect performance predictions.
Now, if a higher than planned cruise altitude is assigned by ATC or requested by the crew, the FMS internally adjusts the Init Crz Alt up to the new higher altitude set in the preselector. However, the actual displayed Init Crz Alt value does not change.
Often while in cruise, a higher altitude is desired due to performance, ride quality, etc. When a cruise climb results in an altitude change of less than 5,000 feet, the FMS remains in the Cruise POF and the aircraft will climb at cruise speed (Figure 10).
|Figure 10. Cruise Climb Less Than 5,000 Feet|
However, if the altitude change is 5,000 feet or more, the FMS will change to the Climb POF and the aircraft will climb at climb speed (Figure 11). From a passenger comfort standpoint, this is something to consider before pushing the CLB button to initiate a climb of 5,000 feet or more using VCLB or CLB, due to the difference in climb rate. If your climb Mach number is less than your cruise Mach number (for example 0.74 and 0.80 Mach, respectively) when pushing CLB, the aircraft will begin to climb and pitch up relatively steeply to slow to the 0.74 Mach climb speed. Also, ATC speed assignments must be considered when performing this kind of cruise climb because the speed target will change as previously described.
|Figure 11. Enroute Climb of 5,000 Feet or More|
Note that in both climb instances discussed above, the Init Crz Alt internally adjusts up to the new altitude set in the preselector once the climb has been initiated. However, the actual displayed Init Crz Alt value does not change.
During cruise, sometimes a lower altitude is desired due to more favorable winds, ride quality, etc. Once a descent is initiated, the FMS transitions to the Descent POF, and the descent speed schedule is commanded, unless additional pilot action is taken. To transition back into the Cruise POF, the crew must manually adjust the FMW Init Crz Alt to match the new lower cruise altitude. Once the Init Crz Alt is adjusted, the cruise speed will once again be commanded by the FMS.
The Descent phase begins when a descent from cruise altitude is initiated. This typically begins at the Top of Descent (TOD) point as calculated by the FMS (Figure 12). TOD is the intersection of the cruise altitude and constructed descent path based on the FMS-defined descent angle (typically 3 degrees as set in the Auto Speeds tab) from the airport field elevation or highest altitude constraint.
|Figure 12. Image of TOD Point on VSD|
The Descent phase can be initiated from three different scenarios:
1. A descent initiated at the FMS-calculated Top of Descent (TOD)
2. Prior to TOD
3. A descent initiated after the TOD point
These three unique scenarios are explored in detail later in this series; however, the common requirement for each is that the altitude preselector must be set to a new altitude below the cruise altitude.
The Descent POF is flown based on descent speeds and default descent angle as set on the Auto Speeds tab in the Avionics window.
When flying level at cruise in LNAV and VALT, the crew will receive an aural alert one minute prior to reaching the TOD, and the vertical track alert (VTA) will appear just to the left of the altitude tape on the PDU along with the vertical deviation indicator (Figure 13).
|Figure 13. Vertical Track Alert (VTA)|
Once the vertical path is intercepted, the vertical mode changes to VPTH and the aircraft begins the VNAV descent (provided the altitude preselector has been dialed down).
The FMS is now in descent mode (Figure 14) and will descend initially at the pilot-entered angle of descent and descent airspeed/Mach number (as entered in the Auto Speeds tab). This descent angle is maintained until the aircraft passes the first constrained waypoint. After meeting the constraint, the angle of descent may be adjusted by the FMS to apply smoothing to adjust the descent path to meet any additional altitude constraints along the flight path. For more information about descent path smoothing, please refer to Part One in this series.
|Figure 14. Descent Speed and Angle of Descent|
While a three-degree pilot-entered descent angle is considered the default angle, some operators tend to use a shallower angle to help eliminate excessive energy build-up. Once the aircraft begins its descent, it remains in descent mode regardless of level-offs or smoothing adjustments until it reaches the speed/altitude limit.
As the aircraft continues its descent and approaches the speed/altitude limit (as displayed in the Auto Speeds tab, typically 250 knots/10,000 feet), the FMS will command a speed reduction in order to respect the speed limit, and the auto throttles will reduce thrust toward idle to begin slowing down.
In this example, the airplane remains at 250 knots below 10,000 feet (Figure 15) until it transitions to the Approach POF, as described in the next section.
|Figure 15. Speed Limit Flown Below 10,000 Feet|
A quick note about the Auto Speeds tab (as seen in the previous example figures): The speed being commanded by the FMS at any particular moment can be quickly determined by the active speed target being colored magenta in the Auto Speeds tab. If the question “where is it getting that speed from?” ever comes to mind during a flight, it can be quickly answered by simply selecting the Auto Speeds tab and looking for the magenta speed label.
Similarly, whenever MAN is selected on the GP, the Auto Speeds tab will display in cyan the speed the FMS would command if the switch on the GP was flipped back to FMS speeds (Figure 16). In other words, the cyan is used to indicate an armed speed; the speed target becomes the active speed (magenta) once FMS is re-selected on the GP. Again, this is very useful in answering the question, “what speed would be commanded if I now switch back to FMS speeds?”
|Figure 16. Cyan Color is Used to Indicate the Armed FMS Auto Speed|
With FMS speeds selected on the GP, once the airplane enters the Approach POF it will begin slowing to the clean configuration speed as set in the Auto Speeds tab. After that, each time the slat/flap configuration is changed, the FMS-commanded speed will change accordingly until the airplane is fully configured at the pilot-defined VAP speed.
The Approach POF boundary is defined by two possible conditions as set on the Auto Speeds tab. The first is commonly referred to as the “approach cylinder volume.” This “cylinder” is defined by an AGL height above the destination airport and radius distance from the airport. Once the aircraft enters this “cylinder,” the approach speed schedule becomes the active speed target (Figure 17).
|Figure 17. Entering the Approach Cylinder Activates Approach Speeds|
The second pilot input that controls the Approach POF boundary is the “1st App Wpt” checkbox. Selecting this checkbox instructs the FMS to command the approach speed schedule when the aircraft is 5 NM (flight plan distance, not radial distance) from the first waypoint on the approach (Figure 18).
|Figure 18. 1st App Wpt Checkbox Selection Activates Approach Speeds|
These two conditions (“cylinder” and “1st App Wpt”) are OR conditions, meaning that once either criteria is met, the FMS will enter the Approach POF.
Keep in mind that the “cylinder” values can cause the approach speeds to activate even when there is still significant flight plan distance remaining to the airport. For example, if flying the GLAVN1 STAR landing west with a “cylinder” of 15 NM/10,000 feet (Figure 19), the aircraft would begin slowing from the Speed Limit speed (i.e. 250 knots) to the Clean approach speed just past HOGGG (assuming the aircraft was below 10,000 feet).
|Figure 19. Undesired Early Transition to Approach POF|
To prevent this undesired early slowing, the crew must take action to either adjust the “cylinder” or switch to MAN on the GP.
An often less-understood feature of the EASy avionics is the Optimum Altitude calculation. The Optimum Altitude was designed to tailor performance predictions and provide different calculations based on the cruise speed mode selected on the Auto Speeds tab.
Most operators enter their desired (or flight planned speed), but remember, there are multiple selections that can be used for flight planning. The available selections are Long Range Cruise (LRC), Max Endurance (Max End), Max Speed (Max Spd), and Manual (Figure 20). Note that Manual in this context means a pilot-defined speed on the Auto Speeds tab, which is different than selecting MAN on the GP.
|Figure 20. Cruise Speed Selections Available|
Selecting Max Spd results in an optimum altitude where true airspeed is maximized. This altitude tends to be close to the Vmo/Mmo crossover altitude. The FMS-calculated Optimum Altitude is displayed in the FMW window when the Cruise POF is selected (Figure 21).
|Figure 21. Optimum Altitude (Opt) Displayed with Max Speed Selected|
If either LRC or Manual Crz Spd is selected, the optimum altitude is computed where the specific range (fuel economy) is optimized. This altitude is typically close to the ceiling altitude.
Finally, if Max End is selected, the optimum altitude is such that fuel flow is minimized.
This article (Part Two) in the series has introduced performance entries and when they are used by the FMS during various phases of flight. In addition, the relationship between optimum altitude and selected cruise speed was discussed. Future articles in this VNAV series will explore additional features and functions in easy-to-understand explanations. Be sure to “tune in” for more EASy VNAV information.
Program Pilot Ryan Milmoe supports Embraer and Dassault EASy for Honeywell Flight Technical Services. He can be reached via email at Ryan.Milmoe@Honeywell.com.