Why do temperature or weight changes not affect Takeoff or Landing Performance results?

Weight and temperature inputs that are less than those published are calculated at the published low weight (or temperature) limit. As a result, there will be no change in performance values for weights and temperatures below the published limits. This allows the values that are provided by the Takeoff and Landing Performance tool to be conservative (longer takeoff and landing distances and lower rates of climb). 

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Image 1. Example Performance Chart with limited published weight information