It can seem almost impossible when you look at a huge passenger aircraft sitting on the runway. A modern commercial plane can weigh hundreds of tonnes, yet moments after take-off it can rise into the sky and travel thousands of kilometres above the ground.
So, how does something that heavy actually fly?
The answer comes down to lift, thrust, weight and drag — four forces that work together to determine whether an aircraft can take off, remain in the air and eventually land.
The most important force is lift.
As an aircraft moves forward, air flows around its wings. The wings are specially designed to interact with that moving air and produce an upward aerodynamic force known as lift.
At the same time, gravity is constantly pulling the aircraft toward the Earth. This downward force is known as weight.
For a plane to climb, the lift produced by the wings must become greater than the aircraft’s weight for a period of time. Once the aircraft is cruising steadily at a constant altitude, lift and weight are approximately balanced.
But the wings cannot generate significant lift if the aircraft is simply sitting still.
That is where the engines come in.
Jet engines produce thrust, pushing the aircraft forward through the air. As the aircraft accelerates along the runway, air moves faster over and around the wings.
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Once the aircraft reaches an appropriate take-off speed, the wings can generate enough lift to overcome its weight, allowing the aircraft to leave the ground.
This is why large airports require long runways.
The aircraft needs enough distance to accelerate to a speed at which its wings can generate the required lift.
The shape and angle of the wings are also extremely important.
An aircraft wing is not simply a flat piece of metal. Its aerodynamic design allows it to interact with the surrounding airflow in a way that creates an upward force.
The pilot can also change the aircraft’s configuration during take-off and landing.
Flaps and other movable surfaces on the wings can alter the wing’s aerodynamic characteristics, allowing it to generate more lift at relatively low speeds.
This is particularly important during take-off and landing, when an aircraft is travelling much slower than it does during cruising flight.
Another force working against the aircraft is drag.
Drag is aerodynamic resistance that acts against the aircraft’s forward movement.
The engines therefore have to produce enough thrust to overcome drag and maintain the aircraft’s speed.
During normal cruising flight, the four forces are generally balanced: lift acts upward, weight acts downward, thrust acts forward and drag acts backward.
When the pilot wants the aircraft to climb, descend, accelerate or slow down, these forces become temporarily unbalanced.
So why doesn’t the aircraft simply fall because it is so heavy?
Because weight alone does not determine whether something can fly.
The crucial issue is whether the aircraft can generate enough aerodynamic lift while moving through the air.
A 300-tonne aircraft is certainly extremely heavy, but its enormous wings are specifically designed to generate a corresponding amount of lift.
The engines provide the necessary thrust to keep the aircraft moving through the air, while the wings convert that forward motion into aerodynamic lift.
There is also an important misconception about the engines.
Jet engines do not simply “push the plane upward.”
Their primary job is to provide forward thrust. The wings then interact with the airflow created by the aircraft’s forward motion to produce lift.
This relationship between thrust and lift is one of the reasons aviation is such an impressive example of engineering.
A passenger aircraft may weigh hundreds of tonnes when fully loaded with passengers, luggage, fuel and cargo, but its designers have calculated the wing size, engine power, aircraft weight and aerodynamic characteristics so that the forces can work together.
And once the plane is cruising thousands of metres above the ground, the aircraft is not defying gravity.
Gravity is still pulling it downward every second.
The difference is that the wings are continuously producing enough upward aerodynamic force to counteract that weight.
That is the secret behind how something as enormous as a commercial airliner can fly.
It is not because the aircraft is light.
It is because air, wings and forward motion can produce an upward force powerful enough to support hundreds of tonnes.
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