Eddie Sez:

The PlaneView VNAV is smarter than you are except when it is dumber than you are. What determines which mode — dumb or smart — is up to you. What is alarming, however, is this: when the VNAV is smarter than you are, your license is at risk.

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue. There is an awful lot of blue here; the manuals are poorly written and a bit sparse with understandable technique. Of course all my technique is based on experience and I could have gotten some of this wrong. Please let me know.

For a non-aircraft specific discussion, turn the Procedures & Techniques / Vertical Navigation (VNAV) section.


General VNAV Rules

[G450 Aircraft Operating Manual §2B-28-40 ¶4.]


VNAV Climb

[G450 Aircraft Operating Manual §2B-28-40 ¶9.A.]

Figure: VNAV Climb Profile, from G450 Aircraft Operating Manual §2B-28-40 Figure 16.

A VNAV climb profile consists of the following:

  • VNAV is engaged in a VFLCH airspeed climb (1) after takeoff.

  • One thousand feet before reaching the constraint altitude, a VERT ALRT is given (2).

  • The flight guidance computer changes to VASEL to capture the altitude constraint (3).

  • The flight guidance computer switches to VALT at the constraint altitude (4).

  • One minute before the constrained lateral waypoint is reached, a VERT ALRT is given (5) indicating that an automatic climb begins at the waypoint.

  • The altitude preselector is set higher than the constraint altitude (5).

  • The flight guidance computers switches to VFLCH as the waypoint is passed (6).

  • The flight guidance computer performs a normal level off at the intermediate altitude preselector (7) switching from VFLCH to VASEL to VALT with the normal 1000 foot selector alert.

  • ATC clearance is received to the cruise altitude and the selector is dialed to the cleared altitude (8).

  • The FLCH button is pushed to resume the climb (8).

  • The flight guidance computer performs a normal level off at the cruise altitude (9) switching from VFLCH to VASEL to VALT with the normal 1000 foot selector alert and the speed command changes to the cruise value.

When you select VNAV in a climb, you are using a meat cleaver when what you might need is a scalpel. VNAV in a climb can make your life easier but sometimes the engines may be more power than VNAV can handle. Let's look at the Teterboro RUUDY FOUR departure as an example:

Figure: RUUDY Four RNAV Departure, Teterboro Jeppesen Terminal Chart, 10-3A, 1 Mar 13.

[G450 Aircraft Operating Manual §2B-28-40 ¶6.] All VNAV climbs are flown using VFLCH. Intermediate level offs are entered as waypoint constraints through the MCDU or they are set with the altitude preselector. VNAV never flies through the altitude preselector in any VNAV mode. When an intermediate level off is required due to an FMS waypoint altitude constraint, VNAV automatically resumes the climb after passing the waypoint when the selector is set above the current aircraft altitude. When the selector is not set above the current altitude, VNAV maintains the intermediate altitude when passing the waypoint. In this case, the climb is resumed by setting the altitude preselector higher and pushing VFLCH on the guidance panel.

Figure: From Eddie's notes.

[G450 Aircraft Operating Manual §2B-28-40 ¶5.] VNAV Flight Level Change (VFLCH) - In this mode, aircraft speed is controlled by the flight guidance computer (FGC) by the pitch of the aircraft. This mode is also referred to as speed on elevator. For most operations, the autothrottle is set to climb power rating for climbs and idle for descents. Exceptions are when climbing or descending only a short distance. In these cases, the throttle can be set to less than climb power or more than idle to avoid abrupt changes.

Figure: From Eddie's notes.

The AOM says VNAV will use climb power for climbs except when climbing "only a short distance." The problem is that right after takeoff the engines are at takeoff thrust and the VNAV will not pull the power levers aggressively enough in time for the pitch to react and you could overshoot the first altitude. (This is one of the most common causes of the many violations Teterboro issues every year.)

Figure: From Eddie's Notes.

[G450 Aircraft Operating Manual §2B-28-40 ¶3.] VNAV never passes through the altitude preselector.

I have found using FLCH works better after takeoff in this situation and using V/S-1000 fpm works better still. But you should plan on using the altitude preselector for each intermediate level off. I know using VNAV in the climb with the preselector at the final altitude should, by the book, work. But for these departures requiring immediate level offs quite often it doesn't. After hundreds of times flying this departure I have come to the conclusion VNAV should not be used for climbs immediately after takeoff.


VNAV En Route Descent - No Vertical Constraint Given

[G450 Aircraft Operating Manual §2B-28-40 ¶6.] Descents are flown as speed descents (VFLCH) or path descents (VPATH). The transition to descent is automatic, assuming the altitude preselector is set lower than the present altitude. One minute before TOD, the vertical track alert is given. Upon reaching the TOD, VNAV initiates either a VFLCH or VPATH descent. A speed descent (VFLCH) is flown when no altitude constraints exist in the flight plan during the descent. The TOD is calculated to place the aircraft at 1500 ft above the destination 10 miles prior to the destination. Also, the TOD is based on any speed constraints in the descent such as slowing to 250 kts below 10,000 ft. The pilot can initiate a VFLCH descent anytime by setting the altitude preselector to a lower altitude and pushing FLCH on the Guidance Panel. Following an intermediate level off at the altitude preselector value, the descent is resumed by dialing down the altitude preselector and pushing the FLCH button again on the Guidance Panel.

Figure: VNAV Flight Level Change Descent, from G450 Aircraft Operating Manual §2B-28-40 Figure 17.

[G450 Aircraft Operating Manual §2B-28-40 ¶9.B.]

  • From cruise altitude (1), dial down the altitude preselector to the ATC cleared altitude (2).

  • Push FLCH if already engaged in a VNAV mode or VNAV if not already in VNAV. The flight guidance computer switches to VFLCH and begins a descent (1).

  • One thousand feet before the altitude preselector (2) is reached, the normal altitude alert is given and the flight guidance computer does a normal level off switching from VFLCH to VASEL (3) to VALT (4).

  • An ATC clearance (5) is received to a lower altitude and the altitude preselector is lowered (6). FLCH is pushed (5) and the aircraft begins a descent.

  • One thousand feet before the altitude preselector (6), the normal altitude alert is given and the flight guidance computer does a normal level off switching from VFLCH to VASEL to VALT (7).

FLCH mode during descent brings the power levers to idle and controls airspeed by moving the elevator. It gives you a rough ride and in the G450 does a lousy job of holding speed. You will probably never see the so-called "speed descent" because you will almost always have a vertical constraint at the bottom of descent. But if you don't, say you are given clearance to land from a great distance out, you can select VNAV and it will remember to descend for you.


VNAV En Route Descent - Vertical Constraint Given

[G450 Aircraft Operating Manual §2B-28-40 ¶6.] A path descent (VPATH) is flown when there is an altitude constraint in the descent portion of the active flight plan. The path angle associated with the constraint is either a default angle computed by the FMS, procedure specified, or pilot-entered. During path descents, AT OR ABOVE or AT OR BELOW constraints are not treated as AT constraints. VNAV looks further in the flight plan to determine if other constraints exist. If so,VNAV flies a single path that meets all constraints rather than multiple paths that treats each constraint as an AT constraint. After passing the last waypoint with an altitude constraint, VNAV changes from path descent (VPATH) to a speed descent (VFLCH).

Figure: VNAV Path Descent Profile, from G450 Aircraft Operating Manual §2B-28-40 Figure 18.

[G450 Aircraft Operating Manual §2B-28-40 ¶9.C.]

A VNAV path descent can be engaged from VALT at any time if the altitude preselector has been dialed down before the TOD point is reached. The VNAV path descent mode is used to descend to a new flight level at a calculated or prescribed angle (between 1° and 6°). The following steps apply:

  • An altitude constraint is entered at a waypoint (1). The FMS calculates an angle and TOD (2) for the path descent.

  • At the TOD, the flight guidance computer switches from VALT to VPATH and begins a descent (2).

  • The altitude preselector is set to the ATC cleared altitude (3).

  • One minute before reaching the TOD, a VERT ALERT is given (4) and the vertical deviation scale is displayed on the EFIS. When the altitude preselector is not at a lower altitude, the message RESET ALT SEL? is displayed.

  • The flight guidance computer does a normal level off switching from VPATH to VASEL (5) to VALT (1).

The VNAV angle is always displayed on MCDU for path descents. The path is always followed except when the aircraft speed approaches the following:

  • Vmo/Mmo

  • Landing gear or flap placard speed

  • VREF.

VNAV tries to satisfy both the altitude preselector and the waypoint constraint altitude. However, VNAV never flies through the altitude preselector.

Selecting VNAV prior to Top of Descent with a target altitude in the altitude preselector and entered in the FMS will allow the FMS to reduce power and pitch to initiate a descent at TOD and control both power and pitch to maintain a predetermined descent angle all the way to the new altitude. At that point the aircraft will level off and resume cruise. This always works, except when it doesn't:

  • If the computed angle is outside 1° and 6° the VNAV should tell you in the scratch pad but it might just give its best effort and not let you know it will not make the required altitude at the required point in space.

  • The VNAV might be waiting for a VERT ALERT that never came because it was inside the one minute warning once it figured out where the VERT ALERT should occur. You may or may not get a warning.

  • An input error may result in completely a valid VNAV computation that is not what you had intended and will allow you to exceed an ATC clearance.

You can save yourself a lot of trouble if you simply make a quick descent calculation after you've told the VNAV what to do. If your FMS has the default 3° VNAV angle entered, all default descents should occur 3 miles per 1,000' prior to the constraint. (Why? Refer to Flight Lessons / 60 to 1.) A few examples in the following scenarios.


VNAV Early Descent to Capture Path (VPATH)

Figure: VNAV Early Descent to Capture Path, from G450 Aircraft Operating Manual §2B-28-40 Figure 20.

[G450 Aircraft Operating Manual §2B-28-40 ¶9.E.]

This scenario is typical, should ATC instruct a descent before the established TOD point is reached. The steps are as follows:

  • An altitude constraint is entered at away point (1). The FMS calculates an angle and TOD (2) for the path descent.

  • The altitude preselector is set to the ATC cleared altitude (3).

  • Push the FLCH button (4) to begin the descent. The flight guidance computer switches to VFLCH to begin the descent.

  • As the path is approached, the vertical deviation is displayed on the EFIS. When the path is intercepted, the flight guidance computer switches to VPATH (5).

  • The flight guidance computer does a normal level off, switching from VPATH to VASEL to VALT (1).

This scenario is what you hope to see, in fact you expect it. But if the descent is entered just as the default 3° top of descent is reached, or if you've made a entry error, or if something else is out of place, the descent can be missed or miscalculated. You can ensure you catch these errors by some very simple math:

  • Determine the thousands of feet to be lost

  • Multiple that by three

  • The result is your normal top of descent point.

If, for example, you are at FL450 and told to cross the JHW VOR at FL200, your TOD should be at (45 - 20) x 3 = 75nm. If the VNAV hasn't started down by 75nm, you need to override it.


VNAV Direct-To Path (VPATH)

Figure: VNAV Early Path Descent Using Vertical DIRECT-TO, from G450 Aircraft Operating Manual §2B-28-40 Figure 21.

[G450 Aircraft Operating Manual §2B-28-40 ¶9.F.]

  • An altitude constraint is entered at a waypoint (1). The FMS calculates an angle and TOD (2) for the path descent.

  • The altitude preselector is set (3) to the ATC cleared altitude (4).

  • A vertical direct-to is performed (5) to the constrained waypoint (1). The FMS calculates the new angle and the flight guidance computer transitions to VPATH.

  • The flight guidance computer does a normal level off, switching from VPATH to VASEL (6) to VALT (1).

If you want to start down early, or if you are beyond the default 3° top of descent and need to start down late, you can press the DIR key on the MCDU and then the RSK key abeam the altitude constraint to force the VNAV to figure its descent from that point and to use the result as the target descent angle. This should work as long as the computed angle is between 1° and 6°. You can check the system's math by dividing the altitude to lose (in hundreds of feet) by the distance to lose (in nautical miles). More on this at: 60 to 1.


VNAV Late Descent Direct-To (VPATH)

Figure: VNAV Late Path Descent Using Vertical DIRECT-TO, from G450 Aircraft Operating Manual §2B-28-40 Figure 22.

[G450 Aircraft Operating Manual §2B-28-40 ¶9.G.]

In this scenario, descent clearance is not received before the TOD is reached. The following applies:

  • An altitude constraint is entered at a waypoint (1). The FMS calculates an angle and TOD (2) for the path descent.

  • One minute before reaching the TOD, a vertical track alert (VTA) is given (3) and the vertical deviation scale is displayed on the EFIS. When the altitude preselector is not at a lower altitude, the message RESET ALT SEL? is displayed.

  • At the TOD (2), the flight guidance computer remains in VALT and remains level through the TOD.

  • The altitude preselector is set (4) to the ATC cleared altitude (5).

  • A vertical DIRECT-TO is performed (6) to the constrained waypoint (1). The FMS calculates the angle and the flight guidance computer transitions to VPATH.

  • The flight guidance computer does a normal level off, switching from VPATH to VASEL to VALT (1).

If the VNAV computes an angle in excess of the programmed value, defaulted to 3°, your only warning may be the "V" on the PFD sliding above you while the aircraft maintains altitude. Using an Direct-To command should make things right unless the angle is increasing quickly or the angle already exceeds 6°. You should point the nose down using FLCH, VS, or TCS. If the angle is still within limits, you should be able to re-accomplish the Direct-To.


VNAV STAR

Figure: RIIVR TWO ARRIVAL, Los Angeles Jeppesen Terminal Chart, 10-2J, 2 APR 10.

The VNAV works well for most Standard Arrival Routes but you need to understand what the VNAV wants to do, as opposed to what you want to do. The RIIVR TWO is an example of where you can allow the VNAV to control the flight path so long as you are able to configure the aircraft at the right time.

Figure: From Eddie's notes.

The VNAV examines the procedure one step at a time and comes to the following conclusions:

  • It can make the entire descent at 3° or less by targeting 14,000' at RIIVR.

  • Therefore it begins an en route descent using a 3° descent from altitude that crosses GRAMM at 19,400', well inside the 17,000' - FL 210 criteria.

  • From RIIVR at 14,000' it can fly a 2.85° glide path all the way to the runway, so it plans on doing just that.

If you allow the VNAV to control the descent from altitude at 0.80 Mach, it should be able to control the airspeed to decelerate to 250 KCAS by 10,000'. Sometimes it does, sometimes it doesn't. From 10,000' flying a 2.85° glide path you should be able to get the airspeed low enough to extend flaps and landing gear, fully configured in time for landing. But you cannot wait for the autospeed to move from 250 KCAS to 200 KCAS on its own, it will be too late.

Understanding that a 3° glide path requires about 4,000' to decelerate from 300 KCAS to 250 KCAS at 10,000' allows you to monitor the VNAV's performance and to manually select 250 KCAS if it hasn't done so it time. The target speed should change from 250 KCAS to 200 KCAS about 20 nm out, and that works well. (If it hasn't, you should select it manually.) But you will need 10° flaps for the deceleration, the airplane won't get there clean. If your goal is to be stabilized on approach by 1,000' AGL — and it really should be — you need to be fully configured not later than 1,200' AGL. That's 4 nm from touchdown. To make that happen, you should have:

  • 10° Flaps no later than 9 nm

  • 20° Flaps no later than 6 nm

  • Landing gear no later than 5 nm

  • 39° Flaps no later than 4 nm

Armed with this knowledge, you can use the VNAV profile descent without having to resort to using the speed brakes to configure or having to fly an unstable approach.


VNAV Approach

The rules for VNAV down to minimums have changed over the years and we are not all created equal:

Approach

[G450 Aircraft Operating Manual §2B-27-90 ¶1. (Pg 46.)] Approach.

Vectors to Final

[G450 Aircraft Operating Manual §2B-27-90 ¶3. (Pg 51.)] Vectors to Final. LNAV/VNAV Engagement. For guidance in the terminal/approach area, the FMS is used to fly LNAV/VNAV coupled up until sequencing onto a vectored approach transition intercept leg or until activation of a vectored approach transition. At that point, LNAV disengages (if engaged). VNAV remains engaged (if engaged, except in VPATH). From that point on, the pilot must fly vectors (as directed by ATC) to re-establish an intercept to the FAF and arm LNAV before LNAV is allowed to capture.

VNAV TOD Computation

[G450 Aircraft Operating Manual §2B-27-90 ¶3. (Pg 51.)] VNAV TOD Computation. If the aircraft is in level flight and the FAF is the active (TO) waypoint and Vectors to Final is active, a TOD is computed along the heading-select-intercept leg.

VNAV Offset

[G450 Aircraft Operating Manual §2B-28-30 (Pg. 15)] VNAV Offset. Discussed at G450 Systems / FMS Insert VNAV Offset.

VGP

[G450 Aircraft Operating Manual §2B-28-40 ¶5.]

See G450 Normal Procedures & Techniques / RNAV(GPS) Approach for more.


References

Advisory Circular 90-105A, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System and in Oceanic and Remote Continental Airspace, 3/7/2016, U.S. Department of Transportation

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013