Can you tell if this aeroplane is landing or taking off? Here are the hints in the picture

Take off or landing?

Though most people who responded, got the answer correct, many of them didn’t know why it was a ‘landing’ photograph and not ‘take off’ photograph. Those who knew got some basic terms wrong. Taking a cue, I decided to write a full explainer for this. Here is a brief answer to the above question

But before I can begin to explain you the above terms, I have to explain, very briefly, the fundamentals of aerodynamics at play for an aircraft to fly.

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Aerodynamics acting on an aircraft

There are 4 fundamental forces acting on an aircraft while it’s airborne. Drag pulls it back due to air resistance as it moves through the air while thrust helps it move forward. The weight of the aircraft pulls it down due to gravity and lift keeps it flying. For an aircraft to fly, thrust has to be greater than drag and lift higher than weight. How the lift is generated at the wings, is beyond the scope of this article. However, for the sake of clarity, a brief explainer would be in order. 

Thrust – The engine, which is a set of turbine fans rotating at high speed, sucks in air, compresses it, mixes it with fuel and burns it. The hot gas then exhausts through the rear of the engine generating thrust which propels the aircraft forward.

Turbojet engine. Image courtesy – https://commons.wikimedia.org/w/index.php?curid=460106

Lift – As the aircraft moves forward it travels through the air. The unique aerofoil shape of the wing makes it possible for the air over the wing to move faster than the air below it. This creates a pressure difference and the air below the wing travels upward to equalise the pressure difference. This creates the lift. 

Now, this is important to understand.

More the air velocity, more the pressure difference, more the lift. So, lift is a direct coefficient of velocity. The faster you travel, the higher you will fly.” 

Now, let’s come back to the answer and explain it point by point.

Spoilers – Here is how spoilers look like while deployed. 

Spoilers Deployed

Spoilers serve 2 purposes. They aid in aerodynamic braking. They also help by killing the lift to prevent the aircraft from leaving the ground, once it touches down. Perform this simple experiment with a paper plane at home. Fly the plane in the first pic, observe the smooth flight. Deploy the spoilers as shown in the 2nd pic and see how the plane falls down. That’s because it obstructs the air flying over the wing of the aeroplane hence killing the lift.

Paper plane with spoilers

Thrust Reverser – As I said at the beginning of this article, thrust is used to move the plane forward. Now imagine, if I guide the turbine output in the reverse direction. Instead of speeding up the plane, it will actually slow down, isn’t it? How do we achieve this? By opening the reverse door as shown in the pic.

Thrust reverser. Image courtesy: YouTube

These are guiding vents in the midsection of the bypass channel of the turbine. They guide the accelerated air from the turbine fans into the opposite direction i.e. same direction as the plane is moving. This also means the more engine power we apply at reverse thrust, it will further slow down the plane. As weird as it may sound, but this is how it is. At snow or wet condition, pilots are advised to apply full reverse thrust that is, engine power is increased upto 70% to achieve higher reverse thrust and slow down the plane. On a normal day, what we see is called “Idle reverse thrust”, engines outputting power at idle RPM. The small switches below thrust levels deploy reverse thruster doors.

Thrust Reverse levers. Image courtesy: aviationpanel.com

Flaps – Now, this is interesting. Remember what I said in the beginning, “more speed, more lift and higher you fly”. But what if the air traffic control tower tells you to slow down because you are about to land but don’t descend because you may hit the ground early. So, basically, you have to maintain your flight level while decreasing speed. Now a decrease in speed will automatically decrease altitude. To compensate this, what we need is more lift in less speed. How do we achieve that? How about increasing the surface area of the wings? That’s exactly what flaps do. When they come out of the wings, they increase the surface area giving more lift at the same speed because more amount of air is now acting beneath the wings.

Flaps extended

So why not deploy it during the full duration of the flight and achieve more lift by burning less fuel? Because this also increases the total area of the aircraft increasing the drag significantly. It further slows down the plane (aids in landing), hence can’t be deployed for full duration but only when we need more lift. Flaps are deployed during at landing and take off. However, during takeoff, we can’t afford to increase drag and hence slow down. Therefore, flaps are deployed at 1 or 2 depending on aircraft load and weather conditions. During landing, they are deployed at full. Here is the flap lever.

Flap levers. Image courtesy: Here

I hope that is a good explainer for the average layman. So next time you fly out, grab a window seat and watch these amazing aerodynamics at play while a 70-tonne metal bird sores into the sky and touches the tarmac exactly at the spot where we want it to.

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