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Pulse Light Approach Slope Indicator (PLASI)
From SKYbrary Wiki
A visual approach aid for use in visual flight conditions which assists the execution of a stabilised approach by transmitting visible beams up the approach from a position adjacent to the intended touchdown point of a runway or helipad rather like a PAPI. A PLASI system usually has an effective visual range of 5 nm during the day and up to 20 nm at night. When on the correct vertical profile, a steady white light will be seen, if above it, the white light will flash and if below it, a steady red light will show. This red light will begin to flash when much too low. Suitably configured PLASI systems may be used to assist either fixed wing or rotary wing approaches.