Mentor Matters: VNAV basics

Thankfully, the vertical navigation (VNAV) function of modern flight decks automates the mental math pilots without VNAV must master to avoid these unpleasant surprises. The precise plotting of a desired descent path, combined with the autopilot’s ability to track that path just as it would a glideslope, makes for a tremendous workload reduction for pilots who understand how to leverage the full strength of the VNAV their airplane features.

Yet many pilots view VNAV with suspicion, and are only comfortable performing the most basic functions, akin to a pilot only using “direct-to” with their GPS navigators. A clear understanding of the ways the VNAV function works, and the nomenclature involved, is critical to fully harness the VNAV systems’ capabilities.

The most important thing to keep in mind is that the VNAV system is, at heart, drawing line segments in three-dimensional space. Each line segment is defined by two pieces of information: where the segment ends and where it begins.

The point in space where the segment ends is called bottom-of-descent (BOD), and is the combination of a waypoint and an altitude that can either be pilot-entered or loaded automatically from the flight management system’s database. If a controller tells a pilot to “cross the ABC VOR at 12,000 feet” and ABC is an en route fix, the pilot most likely would need to enter 12,000 feet next to ABC in the flight plan. In contrast, if ABC is a fix on an arrival or approach, and 12,000 feet is the minimum altitude for the leg terminating at ABC, there’s a good chance 12,000 feet was already associated with the waypoint when the procedure was loaded.

With the BOD defined, the other point needed to define the line drawn by VNAV is where the aircraft would begin descent, called top-of-descent, or TOD. Most VNAV systems calculate a default TOD by starting at BOD and drawing a line backward (toward the aircraft) and upward at a 3-degree angle relative to the ground, until the line intersects the current altitude of the aircraft. That intersection becomes top-of-descent, and when the aircraft reaches it, the VNAV systems will show a centered vertical deviation indicator, just as if an aircraft was perfectly on a glideslope (and the vertical deviation indicator, by design, looks very similar to a glideslope indication).

For various reasons, a 3-degree descent may not always be desirable. Perhaps the pilot is enjoying clear air, and knows the descent will involve ice and/or turbulence. In this case, the pilot may wish to descend more steeply to prolong the time at cruise altitude. Or perhaps the airplane in question is very clean, and picks up speed in the descent in such a way that staying under redline on a 3-degree angle is difficult; here the pilot may wish to select a 2.5-degree angle to better finesse the descent. In both these cases the pilot can input the desired angle, and the avionics will recompute the line segment, relocating top-of-descent to the appropriate location.

A special method of locating TOD also exists, enabled by the VNAV-Direct function of the flight management system. This is used when the pilot wants to place top-of-descent at the aircraft’s current location (although for practicalities of alerting and autopilot capture, the TOD is actually placed slightly in front of the aircraft). A common use of VNAV-Direct is when ATC assigns a crossing restriction too late for the aircraft to descend along a 3-degree path. In this case, the pilot doesn’t care what numeric angle of descent will honor the restriction; he just wants the airplane to start down right now and end up at the desired bottom-of-descent.

Fortunately for pilots, just as flight management systems can do more than navigate directly to a single one fix, but rather can store an entire sequence of waypoints in a flight plan, the VNAV element of the flight management system can also calculate multiple BOD points and either automatically sequence along them as one BOD after another is passed, or, if possible, calculate a single path that complies with multiple restrictions. Common in busy airspace are “descend via” arrivals—those where the pilot is notified on the standard terminal arrival route (STAR) diagram that certain waypoints are associated with altitudes or altitude “windows” at which the airplane must cross. Complying with a sequence of several mandatory crossing altitudes manually would be workload intensive, and require sequential pilot inputs subject to error.

With a coupled VNAV system, however, the entire process is automated. Once the pilot activates the VNAV tracking function of the autopilot and sets the altitude selector to the final, lowest altitude on the procedure, the autopilot will transition back and forth between tracking the appropriate descents between fixes, and leveling off between descents, if needed. Of course altitude is only one form of energy a pilot must manage, and without an autothrottle system (not currently available on any in-production light jets) to maintain speed, pilot action is needed during the transitions between level-offs and descents, as well as when a speed limit is associated with a given fix.

Yet even without autothrottles, here too the VNAV system can offer workload reduction by providing airspeed target prompting. Either by a set of pilot-modifiable rules (e.g. be at 250 KIAS by 10,000 feet), or by a specific speed limit associated with a waypoint (“cross the ABC VOR at 210 KIAS”), newer-generation VNAV systems (such as those found in Garmin’s G3000/G5000 or Rockwell Collins’ ProLine systems) can provide a speed bug prompt that smoothly transitions with descent so as to reach the desired speed at the specified altitude or waypoint. Akin to an “airspeed flight director,” the pilot simply needs to adjust the thrust levers to keep airspeed in the bug; the planning and calculation is taken off the task list.

Newer-generation VNAV systems offer one other advanced feature, which is more in demand as complex RNAV departure procedures proliferate. That’s the ability to calculate VNAV guidance not just in descent, but in climb as well. “Climb via” standard instrument departures have mandatory altitudes associated with fixes, just as a STARs would, and the pilot is responsible for transitioning between climb and level-off as necessary for compliance.

Just as VNAV descents automate transitions, so do VNAV climbs, albeit in a slightly different manner. Because climbing at a set angle is not needed or desirable, VNAV climbs don’t typically involve a path to track, but instead permit multiple level-offs by automatically capturing successive altitudes. Even so, VNAV climbs in some avionics suites still require the pilot to manually advance power or otherwise trigger each climb segment. That’s because it can be dangerous to neglect power. The autopilot may execute a climb, but if the pilot doesn’t follow up with climb power then airspeed is certain to decay.

Neil Singer is a Master CFI with more than 8,500 hours in 15 years of flying.