Gravity changes with altitude and we know that gravitational force is proportional to 1/R^2. This is explained with the help of an example.
Consider the radius of the earth at the equator to be 6400km. If an aeroplane flies at a distance of 40km above the earth’s surface then the gravity, g2 is calulated by equating the ratio of acceleration due to gravity, g1 and g2 to square of ratio of the radius. This gives g2 as 9.67 m/s^2/.
Therefore, it can be said that the gravitational force decreases by 1.2% for an aeroplane which is flying at 40km distance.