Hi Bob,
I just want to clear my doubts about the Pressure Height. Why do we find Pressure height inorder to calculate our landing distance required/ Takeoff distance required?
Thanks alot for your help.

The take-off and landing performance of an aircraft is profoundly affected by the condition of the atmosphere. If the air is dense (heavy or thick), the wings will make much more lift for a given true air speed than when it is less dense (light or thin). Also, when the air is dense, the engine will develop more power and the propeller will generate more thrust than it will in less dense air. Air density is determined by the current pressure and temperature of the air. Because atmospheric temperature and pressure are constantly changing, the designer of the aircraft cannot publish the take-off or landing distance required for every possible condition of atmospheric density. To allow the pilot to assess the take-off and landing performance, the designer publishes figures for a 'standard atmosphere'. The pilot can then allow for factors such as pressure (QNH) and temperature that apply at any one location at a particular time. Pressure Height is necessary to allow the pilot to extract meaningful figures for take off or landing distance required BEFORE he/she attempts to take off or land. Good idea - don't you think!!

Thank you sir.