A space shuttle is actually falling but its path matches the curvature of the earth so it falls "around" the earth. Jet Planes are not falling so their is gravity effects.  Its not the height that causes weightlessness, its falling.  A jet plane that is crashing, i.e. Falling out of the sky, will also have weightlessness inside.

In states that have lethal injection as a method to execute criminals, potassium is used to stop their hearts.  High levels of potassium can kill.  Thats why people who have renal disease and are on dialysis have to watch their potassium levels since their kidneys are no longer good at removing potassium from you blood.