PAPI

The Precision Approach Path Indicator is the system in commercial aviation that has generally been identified as the most critical use of color for commercial pilots. The system uses red and white lights on the ground to inform the pilot whether he is too low or too high during a landing approach.

A PAPI Simulator Test can be used to test examinees on their ability to discern the red and white signals. This is generally used as the gold standard reference against which Pseudoisochromatic Plate tests are measured for the aviation industry.