Thank you for informing us that you never dip your airplane to be constant with the supposed Earth Curvature.
I did no such thing. We don't fly "straight" as in your frame of reference. The aircraft naturally follows the curve of the earth by maintaining a constant pressure altitude. The pressure level is following the curve of the earth. I informed you that no "dip" is necessary because it is both exceptionally gradual and already maintained by the autopilot keeping a constant pressure altitude.
Your supposition that I am flying a perfectly straight line tangential to the curve of the Earth is flawed for a number of reasons. The lift generated by a wing depends entirely on air pressure. With no pressure, no lift. Aircraft cannot continue to climb out of the atmosphere because the air will become too thin to generate lift. Likewise, the engines need air pressure. No air, no combustion, no thrust. This is why all aircraft have a maximum altitude. (I have seen posted elsewhere the erroneous figure of less than 40,000. This is not true, most aircraft are capable of flying in the range of 40-45,000 feet. Not a significant difference, but facts are important.)
The only instrument we have telling us our altitude measures air pressure. The rate at which atmospheric pressure decreases is a well known constant. Airplanes maintain a constant air pressure, meaning that the altitude in feet can actually vary a bit. But since we are all using the same standard, the variation is irrelevant. Want to be at about 18,000 feet? Maintain a pressure of 500mb. 35,000 ft? Maintain 250mb. This level, 250 millibars for example, will curve around the Earth, and so will the plane.
You have agreed that air pressure decreases with altitude. Therefore, in order to maintain a constant pressure altitude, the aircraft would have to curve with the Earth. You are mistaken in thinking there is a "dip" that the pilots have to perform. As has been pointed out mathematically a few posts up (thanks, Rabinoz), the angle of "dip" is exceedingly small, and since the autopilot maintains a constant pressure altitude, no further adjustment is required.
Gravity decreases as we go up? Okay, true enough, but by how much? Is the ISS in a null gravity environment? Or is zero gravity a localized relative sensation because the ISS is free falling towards the Earth at exactly the same speed it is moving around it? (Hint: it's the latter. If the ISS were to cease its orbital velocity it would plummet to the Earth.) An airplane at any altitude is under the force of gravity. The ISS is under the force of gravity. The Moon is under the force of gravity. At what point will the gravitational field of the Earth be zero? Only when the gravitational influence of the Sun overpowers it. There is no way any airplane can fly high enough to feel any significant change in gravity. Measurable, yes, but not significant.
I will happily provide a diagram as soon as I can figure out how to insert one into the message. I had drawn up some better diagrams for my first post, but I think they have to be on a server somewhere, so I pilfered the one from somewhere. So please show me how to include a non-internet based diagram in my posts.
I'd count that as successful trolling inFlatEarth. I thought your OP was a waste of typing with the obvious factual flaws, but you've managed to elicit considered replies - and even a dignified response from an airline captain. 9/10.
I probably have better things to do with my time, but this is like watching a train wreck in slow motion. I just can't look away. Having said that, FE theory does have some interesting observations and questions, and I think they deserve a dignified factual response. I can easily understand why "my personal and physical observations say differently to what I'm taught" and if I can add to their education, I will. Tread on my turf, and I'm going to respond.
I understand, however, that you all have been doing this for a long time. Give me a few weeks, I'll be spitting venom like a rabid puff adder.