Until this stage we have only been dealing with two dimensions and determining if obstacles or terrain are inside or outside the constructed obstacle protection areas. If an obstacle is inside the protection area the aircraft must over-fly it by the required buffer. This buffer is called the Minimum Obstacle Clearance and varies with the phase of flight.
Generally a pilot must see the runway to land and if he cannot see the runway he will have to divert to another aerodrome where the weather is better. So the lower that the aircraft can descend to, without conflicting with obstacles the greater the chance that the pilot will see the runway and land.
On an approach to a runway an obstacle may determine the minimum altitude that an aircraft can descend to. If a fix was placed in a position that was positively past the obstacle the aircraft could descend lower. The minimum altitudes and the position of the fixes should be such that the aircraft is not required to level off his descent and wait for the next fix. The aircraft should descend smoothly, at approximately 3 degrees, to the minimum altitude clear of obstacles.
In a departure varying climb-out gradients are widely used. However the calculation of the optimal configuration is quite complex and must be completed for each obstacle in the departure obstacle protection area.