I have limited flying experience (two introductory flight lessons in a Cessna and several commercial flights, including some Transatlantic trips), so please excuse my ignorance.
I was wondering; since the earth is spherical and the curvature is approximately 6 foot "drop" for every three miles i.e. a 6 foot person will disappear over the horizon 3 miles away, does a jet aircraft need to be constantly adjusted nose down to adjust for the curvature of the earth?
In other words, if a plane was trimmed for straight and level flight, would it "gain altitude" while flying as the earth surface "fell away" due to the curvature?
Thanks for your time and consideration.
This is an interesting question. A plane will fly at a constant altitude and will follow the curvature of the earth and would not gain altitude during a level flight.
For instance, if a plane is cleared to maintain 35,000 feet, by regulations, the pilot must maintain that level based on a standard barometric pressure setting (29.92 inHg or 1013 millibars) Hence it would stay at that altitude (FL350) because the pilot is either controlling the plane manually or has engaged the autopilot to achieve that.
There are two basic instruments that enable this procedure - an altimeter and a vertical speed indicator (VSI). The VSI provides short term changes in pressure and indicates whether the plane is climbing or descending. These changes will give an indication to the pilot so that he would level the plane to maintain 35,000 feet. He will adjust the controls very slightly by use of the elevator and trims. This can be performed automatically by the autopilot as well. As such, the flight controls are constantly moving very subtly to maintain the correct attitude.
You said that, if the plane was trimmed for a straight and level flight, it would ‘gain altitude’ while flying as the earth surface ‘fell away’ due to the curvature of the earth. Well, that would probably happen in a perfectly motionless atmosphere where the plane would fly dead ahead, and over time gain altitude (provided it has sufficient thrust) as the earth curves away from under the airplane.
In reality, a constant altitude must be kept using the standard pressure and that means a fixed distance to the earth center of gravity is maintained, making the path of the plane a curved one.
So, a plane is not flying a straight line - geometrically speaking!
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