I am not about to consider the flat earth theory. I can tell you as an airline pilot for 31 years with 18000 flight hours this is why airplanes do not appear to tilt down in flight.

Imagine a black rectangle on the wing of the jet you are flying. This rectangle is perfectly centered between the leading and trailing edges of the wing. Imagine now that there is a Blue Circle in that rectangle. As the jet climbs higher and higher you will notice that the center of lift denoted by the Blue Circle moves towards the trailing edge of the wing. Finally, when at cruise altitudes and the jet levels off at the assigned cruising altitude the nose of the aircraft must be raised above the horizon slightly (usually about 3 degrees) to maintain the altitude and best economy for the engines. This keeps the airplane flying safely and within it's safety envelope portrayed in this exercise by the black rectangle. The Blue Circle will always be in the black rectangle.

Bottom line is the aircraft flies with a slightly nose-up attitude at cruise. Then when the pilots begin the descent for landing the engines are powered back (most modern jets this occurs simply by resetting the altitude alerter to a lower altitude.)Autothrottles do the work of managing speed along with the autopilot. The airspeed remains the same and the angle of attack remains about the same but with less power the plane descends downhill without gaining airspeed as managed by the autothrottles.

Once out of Mach speed range the speed does increase noticeably. And the aircraft attitude is managed by the autothrottles. Attitudes below the horizon on the attitude indicator do occur but normally 2 degrees or less and it all depends on what the flight is directed to do in consequence to conflicting traffic. A quick descent assigned by the ground controller will result in as much as a few degrees down. Most passengers rarely notice this.

Jets are not flown either to slow or too fast both are dangerous. Thus, the angle of attack or the angle of the aircraft if you could see it from the outside is within a small number of degrees in climb, cruise and descent. Basically, the attitude of the airplane stays the same once above 18,000 feet or so. Unless one is in the cockpit looking at the attitude indicator, it is nearly impossible to know what the pitch angle of the aircraft actually is.

Have a good evening,

CapnGriff