Imagine the plane at Point A, gravity is straight down perpendicular to its path.
Plane travels to Point B, 60 nautical miles further on. While gravity still points perpendicularly straight down, measurement will show that it’s 1° different from the previous gravity, converging on the centre of the planet.
Of course, it’s a continuous change as you fly, it doesn’t just jump every degree.
If you do this with two weights on strings (say) a metre apart, a rigorous scientific measurement will also show the convergence, and the amount is exactly as predicted by a spherical Earth. These experiments have of course been done many times.
In other words, “down” is always towards the centre of the planet. You standing next to your flat earther friend and you’re always leaning away from each other.
Because it is almost never flying in a true straight line. It just “looks straight” because we’re so incredibly small and the Earth is suuuuuper big by comparison. When it looks like it’s flying “level” it’s flying on a very shallow curve that maintains 1G, the force of the Earth’s gravity. So flying “level” isn’t straight, it’s level to the surface of the Earth, which is curved, just not in a way that a normal person can perceive (unless you’re way up high, like in low orbit).
Some simplifications here ignoring how altimeters work and the fact aircraft usually fly constant altitudes vs reference the horizon, but the point still stands.
Edit: not very ELI5 I guess. So let’s try again… it isn’t flying straight, it’s flying in a very very shallow curve that matches the shape of the Earth. Same way a boat is slowly following the curve as well, you can just see it because that curve is the surface of the ocean.
When gravity is pulling everything towards the center of a sphere, propelling something in a “straight” line results in a circle, with the center of gravity at the center (well technically an ellipse, with the center of gravity being one of the focii).
In space, this is easily seen. Not enough forward force, and the object falls into the planet. Enough forward force, and it misses the planet and travels in an ellipse. Even more, and you break free of the gravity and fly off away from the planet.
So yes, with enough force the plane could break the force of gravity and go “straight” away from the surface. But planes aren’t designed for space travel and it likely won’t have enough thrust to do so, even if the rest of the plane could survive the attempt.
Simple answer. It is not flying in a straight line.
If boat sails across the sea on a straight course away from shore it maintains a consistent altitude of 0 which is a curve over the surface of the earth. It’s straight course is curved to maintain contact with the earth.
The aeroplane is doing the same thing but higher up. It maintains a steady altitude and it’s path therefore is a curve around the earth. It’s straight course is curved towards the earth just like the boat.
Basically, it’s not going in a straight line.
When an airplane is maintaining altitude and speed, all the forces acting on it are canceling each other out. To simplify, those forces are gravity, lift, drag, and the engines. Altitude staying the same means that gravity and lift cancel each other out. Lift pushes the plane away from Earth. Gravity pulls it toward Earth. But this is where it get complicated. If it was going in a straight line, the surface would get slowly further away. That means that the forward motion of the engines, separate from the lift they provide, also add an “upward” force to the plane. So, the actual lift provided is slightly less than the gravitational force. That makes it a little bit like orbit. You’re essentially falling at the same rate that the earth recedes.
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