PAPIs Explained

So what exactly is a PAPI?  A PAPI is a Precision Approach Path Indicator, a visual aid to pilots that allows them to maintain a proper flight angle into a runway.  A PAPI is a very simple mechanical system that uses red and white lights to tell a pilot if he's too high in his approach, or too low.  



Outside PAPI unit

 

PAPI systems can be set up in sets of either two units or four.  In a two unit system the unit closest to the runway is set to direct the light slightly above the glide angle.  The unit furthest from the runway will direct its light slightly below the glide angle.  One red light and one white light tells the pilot he is coming down on the proper angle.  Two white lights means he's too high and two red lights means he's too low.



Lens red filters



PAPI systems are checked by the FAA about once a year by flying a special aircraft thru the airport and verifying the angles.  Setup and adjustments are made with a clinometer (a very accurate digital level) and a couple of wrenches.  The units are first leveled horizontally, followed by setting the proper vertical angle.  Finally a mercury switch is set to automatically shut the system off if the unit is somehow disturbed and goes out of level. 



Inside look



The equipment seen in the photos here are brand new units recently installed at one of our airports.  They are currently adjusted and working, but not yet commissioned by the FAA.  So they will remain out of service until a flight check can be made.