What causes an airplane to stall? It's important for pilots to understand what causes a stall because a stall can cause a rapid loss in lift. If you remember from lesson two, we need lift in order to fly. In order to understand this principle, let's take a look at the cross section of a wing.

This cross section is known as an airfoil. On the front of the wing, you have what's known as the leading edge, and then on the back, it's called the trailing edge. In order to talk about this next part, let's draw an imaginary line from the leading edge to the trailing edge. We call this line a chord line.

Anytime an airfoil is in flight, It creates an opposite moving force known as relative wind. It's this relative wind, along with the shape of the wing, that causes the airplane to be able to fly, just like we learned in the last lesson. When an aircraft changes its direction of travel, relative wind also changes.

So relative wind acts on the airfoil at different angles. This angle is known as the angle of attack. The most important thing to keep in mind here is that the angle between the chord line and the relative wind is the angle of attack. Anytime you increase the relative wind or the angle of attack, the more lift an airfoil will produce, as long as you have good laminar flow.

What exactly is laminar flow? In order for an airfoil to create an area of low pressure on top of the wing, Air must flow smoothly over the top of the wing and conform to the shape of the wing. When air is able to move smoothly over the surface of the wing, this is called laminar flow. As the angle of attack is increased, it forces these air molecules on top of the wing to travel faster, ultimately decreasing the pressure on top of the wing.

This will cause an increase in lift, but if you increase the angle of attack even more, you'll begin to see separation from the wing. This is called boundary layer separation. This separation, along with turbulent air on the upper surface of the wing, is known as poor laminar flow. Even with poor laminar flow, the airfoil can still produce lift, and as long as lift is more powerful than weight, the airplane will still fly.

If we increase the angle of attack even more, there'll come a point in time when the lift that a wing produces can no longer support the weight. When that happens, you'll get a rapid loss in lift, and this loss of lift is known as a stall. One very important thing to keep in mind as pilots is that each airplane has a specific angle of attack that it stalls at.

Altitude and gross weight can both affect the airspeed at which an aircraft stalls at, but the angle of attack at which that specific airplane stalls is always exactly the same. That maximum angle right before an airplane stalls is known as its critical angle of attack. That'll be important for you to remember.

Remember, anytime you increase the angle of attack beyond the critical angle of attack, the airfoil will stall, and an unintentional stall can cause a really dangerous situation.