Understanding NG FMS Cruise Constraints

Figure 1. NG FMS VNAV Diagram

For example, one significant difference from legacy systems is the climb speed/power schedule. If a climb is entered that’s more than 5,000 feet (it’s actually 4,850 but 5,000 is for practical purposes), the system will climb based on the climb speed schedules. If it’s a smaller climb, the system will initiate a cruise climb and hold the cruise airspeed (Figure 2). 

Figure 2. Cruise Climb vs Max Climb

CRUISE OR DESCENT?

Another change in NG FMS revolves around the descent profile. Think about the following scenario: a crew is flying westbound against the winter winds. They are cruising lower than normal to lessen the headwind component. They paint an area of convective weather on the radar that they can easily top, so they elect to climb four thousand feet higher for a hundred miles to get over it. Once they’re on the other side, they elect to return to their original altitude. Think about this scenario for a minute - from a VNAV perspective, what should the FMS do? It clearly saw a climb to a higher altitude, saw a cruise segment of level flight over the weather, and now it sees a descent. Should the FMS VNAV enter the descent mode, assuming it will be a descent to the destination, or should it stay in cruise, waiting for another trigger to put it in to descent mode?

These are the types of scenarios engineers must evaluate and determine what course of action the crew would prefer. In the above scenario, the crew likely does not desire the airplane to remain in descent mode when the destination could still be hundreds (or thousands) of miles away. They want the system to be intuitive enough to realize they are going to be in a level segment again and remain in cruise. Once the aircraft is within the descent window for the destination, they want it to switch to the descent profile and remain there until landing.

ENTER STRATEGIC TOP OF DESCENT

The solution to the descent issue is resolved by implementing a Strategic Top of Descent. Most operators are familiar with the top of descent point calculated in the VNAV data page (Figure 3) and displayed on the MFD in the Vertical Situation Display (Figure 4). When there are no constraints during the descent, the TOD is calculated using the destination elevation and angle. When there are constraints in the descent profile, the TOD is calculated by working backwards from the most recent constraint. A Strategic Top of Descent is a fancy way of saying the FMS will command different logic, depending on how far it is from the Top of Descent, in an attempt to predict what the crew wants.

Figure 3. Top of Descent in VNAV Data page

Figure 4. Top of Descent icon on the VSD

This is where anticipating the crew’s desire comes in. The FMS logic attempts to differentiate whether a descent is a cruise descent or the actual descent for arrival. As illustrated on the VNAV diagram, The Strategic Top of Descent is the intersection of the cruise altitude (FMS Cruise Alt) and a path from the highest descent constraint, at the FMS defined descent angle (Figure 5).

Figure 5. Strategic Top of Descent description

Now, notice the ball note D1. It describes descents initiated 100 miles or more from the Strategic Top of Descent (the note reads, “Cruise Descent Initiated before 50nm/100nm from the Strategic TOD.” 100nm is what is currently implemented but will be changing to 50 with G-650 Block 3.) If the descent is initiated more than 100nm from the Strategic Top of Descent, the FMS considers it a Cruise Descent.

In a cruise descent (more than 100nm from TOD):

• The Altitude Preselector is dialed to the lower altitude and a vertical mode selected,
• The phase of flight and auto speeds remain as Cruise,
• The FMS cruise altitude is updated to the preselector value,
• If VS or FPA is active and VNAV is selected, VPATH will become the vertical mode and will limit the descent to 1000 feet per minute,
• If FLCH is the active mode and VNAV is pressed, VFLCH will become the active mode.
• The Vertical Deviation Indicator is not displayed in this case.

This follows the weather example given earlier. The FMS will make the descent at a comfortable rate, recompute times and fuel based on the new cruise altitude, and continue at cruise speeds entering the descent profile at the Top of Descent.

If an early descent (within 100nm of the Strategic Top of Descent) is initiated (see ball note D2).

• The Altitude Preselector is dialed to the lower altitude and a vertical mode selected,
• The phase of flight and auto speeds change to descent,
• The FMS cruise altitude does not change (it assumes we’re starting our descent early)
• If VS or FPA is active and VNAV is selected, VPATH will become the vertical mode and the descent logic limits the rate of descent to (-1000fpm or -500fpm) depending on aircraft altitude
• If FLCH is the active mode and VNAV is pressed, VIAS/VMACH will become the active mode, targeting the higher of the Altitude Preselector or Flight Plan Constraint Altitude.
• The Vertical Deviation Indicator is displayed and referenced to the constructed (descent) path.
• The VSD reconfigures to show the constructed path, removing the level segment, and corresponds to the Vertical Deviation Indicator.

This scenario attempts to predict that the crew is starting the descent early and will not be performing a prolonged level-off at a cruise altitude. Therefore, it limits the rate of descent until the constructed descent path is intercepted and computes time and fuel based on the descent profile in the FMS.

Last, we’ll look at a descent commencing at the Strategic Top of Descent. This is the ideal descent point and when most crews would prefer to descend. Ball note D3 describes what crews are used to when descending at the Top of Descent.

In a descent at the Top of Descent point:

• The Altitude Preselector is dialed to the lower altitude and a vertical descent (VS, FPA, FLCH) mode selected,
• The Vertical Track Alert and Vertical Deviation Indicator appears 1 minute prior to the Top of Descent,
• Approaching the Top of Descent, the aircraft begins a descent on the constructed (descent) path.
• If VNAV is selected, the vertical mode is VPATH.
• The Vertical Deviation Indicator is displayed and VPATH is controlling to the constructed (descent) path.
• Speed target is descent speeds.

Where operators can find themselves in trouble is not realizing the Cruise Descent region has a limited rate of descent built into the logic. If an altitude constraint exists in this flight regime, the VNAV system will limit the rate of descent to -1000 fpm and will NOT honor the constraint (Figure 6). A Vertical Direct-To will also not solve the issue. The pilot must use another vertical mode such as VS, FPA or FLCH and use the VSD to select a rate that will meet the constraint. 

Figure 6. A depiction of an altitude constraint not being honored due to the Cruise Descent or Early Descent logic

Another indication that the FMS may not honor the constraint (or is in the Cruise Descent logic) is the rate of descent in the vertical profile targeting 1000 feet per minute and the CRZ label (Figure 7). These are indications the system is using Cruise Descent logic and is limiting the rate of descent. If an altitude constraint is required, the crew must select VS, FPA, or FLCH, and select a rate that will comply with the constraint.

Figure 7. Rate of Descent and CRZ label

Summary

For anyone that may still be confused, the simple version is; if a descent is entered more than 100 miles from the top of descent, the system will enter a cruise descent mode. If an altitude constraint is involved, the system will not honor it. It will limit the descent rate in VNAV to -1000 feet per minute. This can be noted by referencing the VSD and seeing the descent path does not intersect the altitude constraint or by looking at the vertical profile and seeing the 1000 ↓/CRZ label in the profile. Selecting VS, FPA, or FLCH and an appropriate rate, is the only way to ensure compliance with the constraint.

 Look for a version for the Dassault EASy platform next month. As always, if you have any questions or comments, feel free to reach out to us at FTS@Honeywell.com.

Program Pilot David Rogers supports EPIC and NG FMS-equipped Cessna and Gulfstream aircraft for Honeywell Flight Technical Services. He can be reached via email at David.Rogers@honeywell.com.