4
Mastering the Vertical Profile 10:54 Lena: We’ve talked a lot about the lateral side—staying on the line on the map. But the vertical side—VNAV—seems much more complex. I mean, the plane isn't just flying at one altitude the whole time, right?
11:06 Miles: Not at all. VNAV is probably the most sophisticated part of the entire system. It’s responsible for managing the plane’s energy from takeoff all the way to touchdown. Think of it as a continuous calculation of potential and kinetic energy. In the climb, it’s optimizing for the best rate or best angle; in cruise, it’s looking for the most efficient altitude as the plane gets lighter; and in the descent, it’s trying to achieve the "holy grail" of aviation: the idle descent.
11:34 Lena: An "idle descent"? That sounds like the plane is basically gliding.
11:38 Miles: It is! Pilots call it a "green descent." The idea is to pull the throttles to idle at exactly the right moment so that the plane just slides down a three-degree path all the way to the airport without needing to add power or use speed brakes. To do that, the FMGC has to "fly the descent backwards."
11:57 Lena: Backwards? How do you fly backwards?
12:00 Miles: Well, the computer starts at the runway. It knows the elevation and the required speed at touchdown. Then it works up the arrival procedure, accounting for every altitude constraint—like "must be at 7,000 feet at this waypoint"—and every speed limit. It calculates how much air resistance and gravity will affect the plane, factoring in the predicted winds at different altitudes, until it finds the exact spot in space where the descent should begin. That spot is called the Top of Descent, or TOD.
12:28 Lena: And if you hit that spot perfectly, you just... glide?
12:32 Miles: In a perfect world, yes. But the FMGC is constantly adjusting. If the wind is stronger than predicted, the plane might start to get "high on profile." The FMGC will see this and maybe increase the speed to get back down to the path. Or, if it can't fix it with speed alone, it’ll pop a message on the screen like "DRAG REQUIRED" or "MORE DRAG" on an Airbus. That’s the computer’s way of saying, "Hey, I’m too high, I need you to use the speed brakes because gravity is winning."
12:58 Lena: It’s interesting that it tells the pilot to do it. It doesn't just deploy the speed brakes itself?
13:04 Miles: Most current systems don't. The FMGC manages the "brain" and the "muscles" like the throttles and the elevators, but things like landing gear, flaps, and speed brakes are still very much in the pilot's hands. It’s a collaborative effort. On the Boeing side, they even have different modes for this—VNAV PTH, which follows the calculated path exactly, and VNAV SPD, which focuses on maintaining a specific speed even if it means drifting off the path.
13:30 Lena: And then there’s the "Step Climb" during cruise. I was reading about how as the plane burns fuel, it actually wants to go higher.
13:38 Miles: Right, because the plane gets lighter, the engines don't have to work as hard to keep it in the air, and the air is thinner higher up, which reduces drag. The FMGC monitors this constantly. It’ll calculate exactly when it becomes more efficient to climb another 2,000 feet. It might show a "Step Climb" point on the navigation display. If the pilot gets clearance from ATC, they just dial in the new altitude, and the FMGC manages the transition to the new, more efficient cruise level.
14:04 Lena: It’s like a constant game of optimization. But I'm curious about the human interface. Pilots aren't typing on a standard keyboard, right? They’re using those little boxes with the screens—the MCDUs.
14:17 Miles: Exactly—the Multipurpose Control and Display Unit. This is the primary way the crew talks to the FMGC. It’s got a screen, a bunch of "Line Select Keys" on the sides, and an alphanumeric keypad. It looks a bit like a bulky calculator from the 80s, but it’s incredibly powerful. You type in the "scratchpad" at the bottom and then "up-select" that data into a field.
14:40 Lena: I’ve seen those in flight sims! And there’s a specific flow to it, right? You don't just jump around; there's a logical order to getting the plane ready.
14:49 Miles: There is, and it’s a bit different between Airbus and Boeing. On an Airbus, pilots usually follow a flow across the MCDU pages: INIT A for the basic flight info, then the Flight Plan page to build the route, then INIT B for the weights and fuel, and finally the PERF page for the takeoff speeds. It’s a systematic way of making sure the "brain" has every piece of the puzzle before you even push back from the gate.
15:11 Lena: And if you get one of those numbers wrong—like the weight of the plane—what happens?
15:16 Miles: That’s a huge safety risk. If the FMGC thinks the plane is ten tons lighter than it actually is, it’ll calculate takeoff speeds that are way too low. You could try to lift off and find the plane simply doesn't want to fly yet. That’s why there’s a "Takeoff Config" test. Before the throttles go forward, the computer checks the flaps, the trim, and the weights. If anything doesn't match the plan, it’ll scream at you with a warning chime. The FMGC is basically the final safety check against human error.