Originally Posted by
canadiancow
The aircraft generally flies with a positive angle of attack.
If the seat is parallel to the floor, your head will be lower than your feet, which really sucks.
If the seat is perpendicular to gravity, you get what you want.
Can you clarify what you mean by "truly flat"?
Truly flat means exactly that; The seat should compensate for the flying angle and provide the lie flat experience and not the annoying off angle sensation.
How could the head be lower than the position of the feet if the seat is angled to provide a true 180 degree angle?
The preferable angle is to have the feet elevated. On long flights the blood pools in the legs. That's why many people over the age of 40 have swollen ankles and worse after a few hours in the air. It is unfortunate that the seat manufacturers and the airlines don't pay attention to the published health literature which details the health benefits of elevated legs during long flights. This option could be offered in the premium classes.