I don't think it's possible to debunk this, but please try. Requirements are knowing how airplanes work.
You are flying an old propeller airplane. It has a speed of 200 mph. Now, when the elevator is set to level the plane will fly level, that is how airplanes work.
Now the curvature of the earth is square of the distance multiplied by 8 inches. So since the airplane is moving 200 mph that is 200/60=3.3 miles per minute.
So in 1 minute the distance to the ground should increase 3.3x3.3x8=88 inches. 88 inches in meter is 2.2 meters. After two minutes the airplane have moved 6.6 miles, that is 6.6x6.6x8=348 inches or 8.8 m. After three minutes it's 784 inches or 19.9 meters.
You can see how it adds up, and this plane does not have magical alternate elevators or something that could change it's pitch, so why is it that the airplane continues to stay level with a supposed curved earth?