EASy Avionics

Everything You Ever Wanted to Know About Vertical Navigation

However, most aircraft operate in a crowded and complex system, so this ideal vertical profile is rarely, if ever, possible.

The following terms appear in this series:

Vertical Profile: According to the FAA, vertical profile is “a line or series of lines in a vertical plane that define the ascending and descending portion of the flight plan.” When the vertical profile is referenced in this series, it refers to the vertical portion of the flight that takes it from a 2-D to a 3-D profile. EASy avionics have a Vertical Situation Display (VSD), an electronic display system that depicts the flight’s vertical profile. The VSD greatly enhances the crew’s situational awareness.

VNAV: A generic term for vertical navigation. The VNAV button on the Guidance Panel (GP) activates a VNAV sub-mode. When VNAV is engaged, the vertical path is controlled by the FMS and the altitude preselector. The auto-throttle will manage the power and performance of the aircraft along the vertical path.

Baro-VNAV: Barometric altitude information from the aircraft's air data computer (ADC) is used to compute vertical guidance. This vertical guidance along a path is either computed between two waypoints or as a descent angle to a single waypoint.

GPS Altitude: The only time GPS altitude can be accurate enough for use is when it has a correction applied, such as Satellite-Based Augmentation System (SBAS). This is why SBAS is required to fly an LPV approach. When a GPS approach to LPV minimums is conducted, the system uses satellite-based altimetry to construct the glide path for the approach. The vertical accuracy for GPS is typically 15 meters; SBAS improves this to 2-3 meters. The GPS status and altitude is displayed in the Navigation tab of the Sensors window on the Primary Display Unit (PDU).

Constraint: A condition that can be applied to any portion of the climb or descent profile of the flight plan. Instead of an unrestricted climb to cruise or a descent straight to the landing field elevation (LFE), waypoints can contain conditions on them such as cross at a certain speed, cross at or above (or below) a specified altitude, or even between two altitudes. A constraint may be entered manually by the pilot or may be coded as part of a procedure (SID/STAR or APPROACH). On the EASy avionics platform, constraints are easily identified by the altitude and/or speed being displayed in white in the waypoint list. There are several types of constraints that can be entered for climbs and descents.

A climb constraint can be cross “At,” “At or Above,” or “At or Below.” A speed can also be entered to keep the aircraft within specific confines during a procedure. Climb constraints are maintained until the waypoint containing the constraint is sequenced.

An “At” constraint is displayed with a white line above and below the constrained altitude.

An “At or Above” constraint is displayed with the altitude above a white line.

An “At or Below” constraint is displayed with the altitude below a white line.

A “Between” constraint is displayed as a combination of “At or Above” and “At or Below” constraints.

The following figure shows an “At or Below” climb constraint of 7,000 feet at ROPPR and an “At or Above” climb constraint of 8,000 feet at CEASR.

A descent constraint is similar but has more options. In addition to the “At,” “At or Above,” and “At or Below” options, a descent constraint can be to cross “At” a specific altitude and include an angle of descent. Additionally, the FMS can compute a vertical path through a “Between” constraint, which is a constraint that contains both an “At or Above” and “At or Below” altitude on the same fix. This gives the aircraft a window to pass through vertically and allows more flexibility for different aircraft types to operate more efficiently on arrivals. The following figure shows a “windowed” descent constraint between 10,000 feet and 7,000 at FBALL and an “At” descent constraint of 6,000 feet at PGSKN.

A speed constraint is displayed in white below a white line.

Despite looking like an “At or Below” type of constraint, all speed constraints are treated as “At” type constraints by the FMS. A more detailed discussion on FMS Auto Speed logic will be included later in this series.

The altitude constraint type at a waypoint can also be quickly determined by referencing the waypoint symbol type on the VSD. An “At or Above” constraint is shown as an upward-pointing triangle, an “At or Below” constraint is shown as a downward-pointing triangle, and an “At” constraint is shown as touching upward- and downward-pointing triangles. A “Between” constraint is displayed as a combination of the “At or Above” and “At or Below” triangles. The altitude associated with the constraint is displayed next to the waypoint symbol.

An altitude constraint can be inserted using the Cross dialog box.

If an altitude constraint is to be applied, the Altitude checkbox must be selected. If a descent angle to an “At” crossing restriction is to be specified, the Angle checkbox must be selected and the desired angle between 1 degree and 6 degrees must then be entered.

A speed constraint may also be entered by selecting the Speed checkbox and entering a speed.

An additional useful feature in the Cross dialog box is the Override checkbox. It can be used to manually override the FMS logic in determining whether the constraint is a climb constraint or descent constraint. The override is typically used when the planned cruise altitude was never attained (as set on the Alt/Spd tab in the Flight Management Window), or if a Standard Instrument Departure (SID) ends at the same waypoint where a Standard Terminal Arrival Route (STAR) begins. If this SID/STAR common waypoint had a climb constraint (as part of the SID) and a descent constraint (as part of the STAR), the Override checkbox would allow for specifying how the FMS should treat the waypoint constraint.

Smoothing: FMS smoothing logic provides a continuous descent path with multiple altitude constraints by modifying the descent angles required to meet the requirement of each constraint. However, if the resulting path angle between constraints is less than 1.5 degrees or longer than 20 miles, the angle is not modified. Instead, a level segment is inserted at the constraint altitude to connect the previous waypoint, as shown in the following two figures.

Note: EASy III calculates a continuous descent path in a different manner and will be addressed in a future article.

How the FMS Constructs a Vertical Profile

When constructing a vertical path, the FMS uses programmed data from the aircraft database, inputs from the crew, and environmental inputs. It then works from the landing runway backwards at the defined descent angle (typically 3 degrees, as set on the Auto Speeds tab in the Avionics window) until it reaches either the cruise altitude or a constraint.

If the system encounters a waypoint constraint, it calculates to the constraint and then calculates a path behind it at the defined descent angle (usually 3 degrees, as defined in “Descent” in the Auto Speeds tab). Once all the constraints are met and it has worked backwards to cruise altitude, the system has a complete descent profile computed.

The climb portion of the profile is similar to the descent in that the FMS will plan around speed or altitude constraints. The primary difference is that it doesn’t use angles in planning anything in the climb. Instead, it plans the climb based on speed and predicted aircraft performance from the aircraft database. This provides a reasonable expectation of the climb profile and Top of Climb (TOC) point, as shown below.

Once the climb is planned, we have a complete profile. The EASy avionics will then display the vertical profile in the VSD, as shown below.

The FMS will display the vertical profile in the waypoint list next to the waypoint name. As the following figure shows, an up arrow is displayed when the FMS plans to be in Climb Mode and a down arrow is displayed when the FMS plans to be in Descent Mode.

If there’s an altitude constraint at a descent waypoint, the descent angle and planned vertical speed are also displayed. This information will not be displayed if a vertical profile cannot be computed; usually a result of not completing the required performance initialization (Perf Init) information in the Flight Management Window (FMW).

In a perfect world, the aircraft would be allowed to fly the optimum path once it is constructed. In the real world, all bets are off as soon as the gear comes up. Aircraft are held down, notified of revised routes, given unplanned crossing restrictions, etc. The flight plan (both laterally and vertically) becomes dynamic. Despite its dynamic nature, once pilots have a vertical profile constructed, all they need is a basic understanding to use VNAV from takeoff through approach and landing.

VNAV Sub-Modes

As mentioned earlier, VNAV isn’t a mode but a series of sub-modes that apply to various phases of flight. The VNAV sub-modes for EASy are:

VALT/VASL: VNAV Altitude Hold and VNAV Altitude Select Capture

VALT and VASL behave just like ALT/ASL except the “V” prefix informs the crew they are operating with VNAV engaged. If the aircraft arrives at a climb or descent point and the altitude preselector is set to a different altitude, the aircraft will climb or descend automatically per the vertical profile.

VCLB: VNAV Climb

Similar to Climb (CLB), VCLB is an indicated speed mode used for climbs in VNAV. Unlike CLB, however, it will respect FMS altitude constraints. This is the only VNAV sub-mode that can be used during a climb.

VPTH: VNAV Path

VPTH is a descent mode. If the FMS-constructed vertical path is intercepted while in a VNAV sub-mode (or if VNAV is armed), the aircraft will descend in VPTH as long as the altitude preselector is set to a lower altitude. VPTH will respect altitude constraints during a descent.

VGP: Vertical Glide Path

VGP is an approach mode. It still tracks a defined vertical path, but will not honor (stop at) the altitude preselector. Once armed, VGP can only become the active vertical mode when the “To” waypoint is the final approach fix (FAF), or the aircraft is within 5 nautical miles (track distance) from the FAF, whichever occurs first.

VSBA: Vertical Satellite-Based Augmentation Guidance

VSBA is an approach mode. It is identical to VGP except that it is used to track an SBAS GPS-generated glidepath during an LPV approach.

A crucial piece of the VNAV system that isn’t a sub-mode is the altitude preselector. The preselector acts as a gate that limits the extent to which the FMS can control the vertical profile. For example, if the SID has a final altitude of FL190 and waypoints with crossing constraints, the altitude preselector can be set to FL190. After takeoff, the aircraft will follow the FMS VNAV guidance to meet the constraints, continue climbing when the constrained waypoint is sequenced, and then level off when reaching FL190. Except for VGP/VSBA, VNAV will never take the aircraft through the altitude on the preselector. Think of it like allowing the FMS to control the vertical profile, but only within the parameters defined using the altitude preselector.

The following rules also apply to VNAV:

• FMS must be the selected navigation source and LNAV must be engaged for VPTH or VGP to engage.
• VNAV will not operate until it has a “closed” flight plan in the waypoint list (although it will accept discontinuities) and all required Perf Init data is entered into the FMW.
• VNAV will never pass through the altitude in the preselector (except on approach in VGP or VSBA).
• VNAV attempts to keep the aircraft as high as possible for as long as possible.
• Acceptable VPTH angles are 1 to 6 degrees. Attempting to enter a constraint that produces a higher or lower angle will result in an FMS message.
• VNAV climbs are always done in VCLB.
• Pushing the VNAV button on the guidance panel arms VNAV, but the actual VNAV sub-mode is displayed in the flight mode annunciator (FMA) on the PDU.

VNAV Modes for Each Phase of Flight

The rest of this article will discuss how the VNAV sub-modes are used in different phases of flight (except for the approach phase, which will be covered later in the series).

Climb: Rule number one for VNAV climbs; they are always done in VCLB. The FMS never looks at a path or angles on the departure phase. The FMS honors all constraints during the climb out by climbing as quickly as possible and leveling off only when necessary to meet “At” or “At or Below” waypoint restrictions. Therefore, if the aircraft is climbing and the VNAV button is pressed, the only mode the crew should ever see during climb is VCLB followed by VASL/VALT during the level off.

Three examples of departing using VCLB are described below:

1. ATC Managed Climb
Departure is done by ATC stepping the aircraft up to cruise altitude either with or without the use of a SID. For example, “N12345, climb and maintain 4,000,” followed by “climb and maintain 7,000,” etc. When level in VNAV, the vertical mode on the FMA will be VALT. When the clearance to “climb and maintain…” comes, pressing the CLB button on the guidance panel will keep VNAV engaged. The vertical mode changes to VCLB and the aircraft climbs to the next target altitude where it will again level off.

2. Climb Via
Departure is done by ATC assigning a SID that has crossing constraints. For this example, reference the PRFUM 4 departure from Las Vegas in the following figure.

Assume ATC has given the clearance, “...climb via the SID.” Notice the SID has a top altitude of FL190. That will be set in the altitude preselector to ensure the FMS vertical guidance is followed to that point until ATC provides further instructions. Also, note that there are vertical constraints for ROPPR, CEASR, HITME, and KADDY. The FMS depicts the “At or Below” constraint at ROPPR in the WPT LIST as 7,000 feet below a white horizontal line.

The next three points, CEASR, HITME, and KADDY have “At or Above” constraints. The FMS depicts these in the WPT LIST (KADDY is not shown) with the altitude above a white horizontal line. Once the altitude is met, the FMS continues climbing to meet the next restriction or the top altitude.

Once VNAV is selected after takeoff, there is no need for additional button pushing. The FMS will initiate a climb in accordance with the speed schedule set in the Auto Speeds tab, shown below, and eventually accelerate to the Speed Limit followed by acceleration to the Climb Speed.

Reaching 7,000 feet, it will automatically level off and go into VALT until ROPPR is sequenced, then back to VCLB and resume climbing until it reaches FL190. If the FMS determines the aircraft cannot reach CEASR or HITME by the “At or Above” requirements, the UNABLE NEXT ALT FMS message is displayed.

3. Climb Via Is Cancelled by Amended Climb Clearance
If flying a SID and cleared to “climb via” and then subsequently given an amended climb clearance that cancels the “climb via,” ATC is now expecting an unrestricted climb to the newly assigned altitude. The easiest way to ensure the aircraft does not stop climbing to respect any pre-existing altitude constraints on the SID is to select a vertical mode other than VCLB (most likely CLB). VNAV may be re-engaged once past all SID constraints.

Cruise: Once the aircraft reaches the cruise altitude and target cruise speed, it will leave the climb phase of flight (POF) and enter the cruise POF. The FMS knows it has reached cruise altitude when the altitude attained is the INIT CRZ ALT as set in the pre-flight POF tab in the FMW (see the figure below).

One scenario that causes an issue is when a final altitude is accepted that is lower than the entered INIT CRZ ALT. For example, assume FL350 was entered in the FMW but ATC assigns FL330 for a final altitude. The FMS is still expecting another climb from FL330 to FL350 and, as such, stays in climb mode. Doing so prevents the FMS from transitioning into cruise and descent mode and can cause issues if crossing restrictions are assigned.

The fix is simple; if leveling off for a final altitude that is lower than what was entered during the FMS initialization, reset the INIT CRZ ALT (see the figure below) to match the new lower altitude (FL330). The FMS will transition into cruise mode and everything subsequently will work normally. However, if a higher than planned cruise altitude is assigned by ATC, the INIT CRZ ALT will automatically adjust up by reference to the altitude preselector and no pilot action is necessary to enter the cruise POF.

Descent: VNAV descents can be performed in VPTH or VGP/VSBA. VPTH will begin flying the FMS-generated descent path once the top of descent (TOD) is reached. The altitude preselector must be set to an altitude below the aircraft for this to occur. VGP and VSBA are the VNAV sub-modes used during final approach for eligible approaches. Always refer to the AFM for any limitations regarding the use of VNAV during an approach.

For this VNAV descent example, let’s assume ATC says, “N12345 proceed direct MAIER, descend via the BRUSR1 arrival, landing runway 8” (refer to the following two figures). We’ll assume the crew has previously verified all crossing restrictions in the FMS and that they match the charted procedure.

Since the clearance was “descend via,” the crew can set the bottom (lowest) altitude on the arrival into the altitude preselector. In this case, the bottom altitude is JAMIL at or above 4,000 feet; so, 4,000 feet will be set in the altitude preselector (displays as FL040 while the baro setting is STD). Note that once the preselector is dialed down, our current captured altitude is displayed in magenta to the right of the active vertical mode (VALT).

Proceeding direct MAIER, the lateral mode will be LNAV; the crew will then verify an active VNAV sub-mode is shown in the FMA (VALT in this example). With LNAV active, VNAV active, and a lower altitude set in the preselector, all three elements for a VPTH descent are in place, as shown below.

All that’s left to do now is wait to reach TOD and watch the vertical mode change from VALT to VPTH. The EASy avionics system will follow the calculated vertical descent path to meet all altitude constraints on the arrival.

The FMS also accounts for the distance required to slow down to meet the speed restrictions at BRUSR, BDWIL, ENSEA, FBALL, and PGSKN. Assuming FMS speeds and auto-throttle are engaged, these restrictions will be met without additional pilot action.

One final function that hasn’t been covered yet is the vertical direct-to. A vertical direct-to is like a lateral direct-to, but instead of navigating directly to a waypoint, the aircraft descends directly to an altitude at that waypoint. This is performed by opening the Cross dialog box and selecting the Vertical Dir To checkbox, shown below. The only limitations are that the vertical direct-to must be performed to an altitude constraint, the resulting descent angle must be between 1 and 6 degrees, and the altitude preselector is set lower than the current altitude.

Similar to performing a direct-to, which centers the needle on a new course to a fix, a vertical direct-to calculates a glide path from the aircraft’s current position to the altitude selected in the Cross dialog box (reminder: the altitude preselector must be dialed down as well). This is an easy solution for when ATC says “descend now to cross XYZ at 11,000,” or if a descent clearance is given and the aircraft has already passed the TOD. These types of scenarios will be explored more later in the series.

This first article in the series has introduced the VNAV system that is part of the Honeywell EASy II avionics. Future articles in this VNAV series will explore additional features and functions in easy-to-understand explanations. Be sure to “tune in” next time for more EASy VNAV.

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