I have seen many cockpit videos of airplanes landing, and nearly none of them have their nose down for losing altitude. How does this happen? and how does an airplane, such as A320, descend without its nose pointing down? And how does this happen in a controlled manner?
A plane descends when it does not have enough thrust to maintain its altitude. A plane can descend with its nose pointed up or down so long as there is not enough thrust to maintain altitude. Altering the pitch of an aircraft is used to control airspeed: pitching up slows an aircraft down (and may cause a climb if done from a level attitude) and pitching down generally speeds up an aircraft (and causes a descent if done from a level attitude). So if you want to descend at a slow speed you reduce power and pitch the nose up.
Since you want to land at a slow speed you will need to pitch up and slow the aircraft down while reducing power to descend nicely. This article covers it nicely in terms of flying an approach. It's also covered in this question.
When an aircraft ascends or descends, its inertial velocity vector (let's simplify by saying the ground is inertial) will be pointed upward or downward. The angle with respect to the horizon, or ground, is called the flight path angle ($\gamma$).
There are two more angles relevant to this story:
- The pitch angle ($\theta$) is the aircraft attitude with respect to the horizon.
- The angle of attack (AOA) ($\alpha$) is the air flow incidence against the aircraft.
When aircraft is not turning and there is no wind with respect to the ground, the relationship between the three angles are:
So as you can see, the plane can descend with a positive pitch angle. Or it can do level flight with a non-zero pitch angle. Pitch angle has no direct relation to the flight path.
The extremely coarse "explain it like I'm 5" answer:
You know how they talk about in school, how the curved top part of the wing has faster air across it at lower pressure, and that creates lift? They make a big deal about the Bernoulli Principle and how it's not just a wedge pushing through the air. Yeah, they didn't tell you the whole story. Airplanes also do the "wedge" thing. They make lift by angling the wing upward. In fact if you've built a paper or stamped balsa wood airplane, you may notice that it flies even though its wings are obviously flat. You could build a 737 with flat wings and it would fly. It would not get very good fuel economy.
The faster a plane goes, the better the wings use the Bernoulli Principle. The slower, the worse; so at slower speeds they must tilt the wings upward and using them in wedge fashion.
The nose is connected to the wings, so they must lift the nose to tilt the wings.
Really short version:
"How does an aircraft descend without its nose pointing down?"
By descending with its nose pointing level or up.
Thrust is not required to do this.
Here's an example:
Now for a tiny bit more detail:
How does an aircraft descend without its nose pointing down?
By flying along a shallow glide path at a moderate to high angle-of-attack.
Angle of glide path (relative to airmass, and expressed as a negative number) plus angle-of-attack minus angle-of-incidence equals pitch attitude of fuselage.
The resulting pitch attitude may be positive (nose up), negative (nose down), or flat, depending on the values of the other three angles involved.
If we ignore the angle-of-incidence, which is rather small in most airliners, then we can say that to a first approximation, the fuselage will be in a nose-up pitch attitude whenever the angle-of-attack is larger than the angle of the glide path measured with respect to the surrounding airmass, which in still air, is the same as the angle of the glide path measured with respect to the ground.
Draw a sketch that shows the wing's angle-of-attack in relation to the glide path through the airmass, and you'll see. For normal climb or glide angles a heavily-loaded jet needs to fly at a high angle-of-attack to generate enough lift when flying slowly. Also, airliner-style approaches to landing typically involve maintaining enough power to create a rather shallow glide path. The result is a nose-high pitch attitude.
In a descent, gravity is supplying power to the aircraft, which decreases the thrust requirement associated with any given airspeed, i.e. which decreases the thrust required to offset drag and keep the airspeed constant, i.e. which increases the airspeed associated with any given thrust setting, all compared to level (horizontal) flight. (The opposite happens in a powered climb.) But this "boost" from gravity doesn't depend on whether the aircraft's pitch attitude is nose-up or nose-down. It only depends on how steeply the flight path through the airmass is inclined downward.
One way to more comprehensively answer this question would be to create a 3 x 3 or 4 x 4 grid of 9 or 16 panels. Each column would show a fixed angle-of-attack (and therefore also a fixed lift coefficient and drag coefficient), progressively decreasing from left to right. Each row would have a fixed thrust setting, progressively increasing from top to bottom. Each panel would have a sketch showing angle-of-attack, direction of flight path through airmass (i.e. direction of airspeed velocity vector), pitch attitude, and airspeed. Some of the aircraft would have a nose-up pitch attitude, and others a nose-down pitch attitude.
For more, these related answers to related questions:
The angle at which the nose of the aircraft points has less to do with descending or climbing than your intuition would lead you to think. Airspeed affects the rate of climb, not attitude. The faster the air flows over the wings, the more rapidly the aircraft will climb. Slower air flow causes the aircraft to descend.
The airplane flies because lift created by the wings counteracts its weight created by gravity. To create lift the surface of the wing must be in relative forward motion through the mass of air. Thrust created by the engine generates the forward motion which is opposed by drag. As long as thrust is greater than drag, the wing can move forward and generate lift. Gravity is universal and operates in all objects. Lift, on the other hand applies only to whatever is attached to a wing. To generate lift the wing must have the air moving around it following certain rules, otherwise it is incapable of generating lift. These rules are conceptualized as the angle of attack or the angle between the cord of the wing and the relative motion of the aircraft. At high angles of attack the wing is incapable of generating lift. Gravity, experienced by the airplane as weight does not follow any particular rules, all objects fall from the sky in the absence of lift. An airplane can have its nose up or down and gravity will still apply and if the lift is less than the weight, the airplane will go down. In fact, during landing most airplanes will maintain a nose-high attitude while going down into the runway.