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How Does VNAV Work?

This story was made in partnership with Envoy Air. Check out the full series here. Ready to apply? Submit your application here.

For new airline pilots and those upgrading to advanced aircraft, VNAV is one of the biggest automation hurdles to understand. You might not have flown an airplane with VNAV before and understanding the basics can be confusing. Beyond your knowledge of an LNAV/VNAV instrument approach, how exactly does VNAV work in the background of your FMC? What guidance does VNAV give you and how can you use it to fly more efficiently?

While VNAV experts may find this article helpful for refreshing the basics, it's primarily intended to teach pilots new to VNAV the fundamentals needed before aircraft-specific training.

Live from the Flight Deck

A Vertical Path: Start To Finish

In advanced aircraft, when you program your route into the FMC you'll have departure, cruise, arrival, and approach segments. Vertical Navigation (VNAV) draws a path from your departure runway all the way to your arrival runway. It takes into consideration SID altitude restrictions, the cruise altitude you programed, STAR altitude restrictions, and approach altitude restrictions.

In many piston airplanes, you climb in Flight Level Change (FLCH - which holds a speed), or Airspeed mode (IAS). In more powerful advanced aircraft, you can now climb in VNAV because performance isn't a massive constraint.

Here's the catch: VNAV operates separately from what you program into the autopilot's flight control panel. For example, you filed 30,000 feet as your cruise altitude. ATC clears you to 30,000 feet, you set that altitude in BOTH the altitude window of your autopilot, and on the FMC cruise page. You can fly in VNAV all the way to that cruise altitude without touching a thing.

Now let's say you have only been cleared to 20,000 feet. If you set an altitude of 20,000 feet in the altitude window, your autopilot will stop your climb at 20,000 feet, even if the VNAV wants you to keep climbing to a cruise altitude of 30,000 feet. To continue following VNAV to your cruise altitude, you'd have to match your altitude window to what's typed into the FMC cruise altitude and select VNAV to start climbing again.

As the pilot, you can override what VNAV "wants" by manually controlling speed via the speed select window and altitude via the altitude select window.

How does VNAV handle multiple climb restrictions along a SID? Keep reading to find out.

Each Airplane Calculates VNAV Differently

If you're familiar with VNAV, you may recognize some of the terminology used in this article, but notice small differences in terminology. That's because each airplane uses VNAV a little differently. For instance... some airplanes capture altitudes in VNAV PTH, and others in ALT CAP. Caution: Different VNAV modes offer varying protections for altitude and speed restrictions, which can change by aircraft type.

What matters is these are the basic principles that you can use to better understand how automation works on your specific airplane.

Boldmethod

VNAV Speed vs. VNAV Path

VNAV Speed flies based on the speeds programmed into your FMC for each specific segment of flight (you'll have different speeds programmed for the climb, cruise, descent, and approach). VNAV Path uses altitude constraints and crossing restrictions from the legs page of your FMC route. In most cases, you'll climb in VNAV SPD and descend in VNAV PTH.

As the pilot, you can override what VNAV "wants" by manually controlling speed via the speed window and altitude via the altitude window. Those are the basics... now let's dig deeper.

When flying in VNAV SPD, the airplane will only attempt to fly at the speed programmed into the FMC for that specific segment of flight. You usually fly in VNAV SPD for climb segments since your goal is to quickly climb to a more efficient altitude. Let's say you're flying a SID with multiple crossing restrictions and you're instructed to "climb via." You'd set the top altitude for the SID in the altitude window, and when selecting "VNAV," the plane would fly in a VNAV Climb Speed mode until hitting an altitude restriction. The aircraft may level off to respect the VNAV Path (the altitude restriction), before climbing again in VNAV SPD.

So, during a climb in VNAV SPD mode, the path still exists lurking in the background until it "wakes up" and realizes you need to level off to meet a restriction. Then it goes back into the background while you continue climbing in the more efficient SPD mode.

Boldmethod

Since VNAV Path uses the altitude constraints and crossing restrictions from the legs page of your FMC route, you may level off at cruise altitude and still see "VNAV Path" and not "ALT CAP."

Before descent, you'll see a top of descent (TOD) point where you'll intercept the vertical path programmed towards your arrival runway. As you intercept the path, you will descend in VNAV PTH. This honors altitude crossing restrictions during descent. But what happens if you're high or fast on the path and the plane just can't keep up?

The plane may revert to a VNAV SPD mode while descending if it's no longer able to descend on the path at the commanded speed. To get back on the vertical path, you'll need to change speed or add drag (speed brakes, configure, etc).

Wikipedia

SIDs + STARs Are Built For VNAV

SIDs and STARs are easy when flying in VNAV. As long as the restrictions are properly programmed into your FMC, a vertical path will be drawn for the plane to follow in order to meet each restriction. It's a whole lot easier than using vertical speed and manual calculations to see what your required descent rate is to meet a crossing!

For VNAV to work, you must become an expert at programming your FMC. VNAV depends on the speeds and altitudes programmed for each flight segment. You also need to become intimately familiar with what VNAV mode you're flying in and what protections it offers you.

When You Shouldn't Use VNAV

VNAV is based on a specific path from takeoff to touchdown, and I'm guessing you've probably never flown a perfect flight without one enroute change, vector, or altitude change. When you deviate from that VNAV Path, using VNAV isn't always the best option. Your autopilot altitude and speed windows are the final control for the plane, so remember to set exactly what you want the airplane to do outside of FMC VNAV planning.

For instance... During descent, you may get vectors to final. While a "path" will still exist to the runway, ATC may want you lower than what VNAV "wants" you to fly. If you try to descend in VNAV, the airplane may stay leveled off until it rejoins the descent path. To fix this, use a descent mode besides VNAV and program the altitude window accordingly.

Boldmethod

What Else Do You Want To Learn?

We want you to be prepared for every step of training and your career. What else would help you feel prepared? Let us know in the comments below!


Ready to launch your airline career? Get started by applying to Envoy Air today.


Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and a First Officer on the Boeing 757/767 for a Major US Carrier. He graduated as an aviation major from the University of North Dakota in 2018, holds a PIC Type Rating for Cessna Citation Jets (CE-525), is a former pilot for Mokulele Airlines, and flew Embraer 145s at the beginning of his airline career. Swayne is an author of articles, quizzes and lists on Boldmethod every week. You can reach Swayne at swayne@boldmethod.com, and follow his flying adventures on his YouTube Channel.

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