Precision Visual Landing Aids at SFO


While the electronic glideslope of the instrument landing system on Runway 28L at SFO was not operational on Saturday morning, when Asiana Flight 214 undershot Runway 28L and hit the sea wall, a visual-based precision system called the “PAPI” was available, according to the National Transportation Safety Board.

The Precision Approach Path Indicator consists of an array of four lights, generally placed to the left side of the runway and approximately 1,000 ft. from the end of the runway, that give inbound pilots an indication of their position relative to 3-degree glideslope for landing.
The lights appear in the cockpit as either white or red, depending on the aircraft’s relative position with respect to the glideslope. Two white lights next to two red lights (see picture above) indicate the aircraft is on the correct glideslope; Four white lights means the aircraft is too high; Four red lights mean the aircraft is too low. Other combinations, such as three white lights and one red light (slightly high), indicate variations between the extremes.

Did the pilots of the Asiana 777-200ER see all-red lights? 

For Runway 28L, the PAPI system appears to be located approximately 1,400 ft. from the runway threshold, and about 1,900 ft. from the sea wall, based on the touchdown zone markers that are painted every 500 ft. along the runway.

CORRECTION (7/9/2012) - The glideslope for the PAPI is actually 2.85 degrees rather than 3 degrees, which will change the numbers slightly, but not significantly)

Based on a 3-degree glideslope, my calculations show that an aircraft on glideslope (two white and two red lights) should be approximately at 100 ft. altitude when crossing over the sea wall, and that’s from the pilot’s perspective in the cockpit.

For a long aircraft like the 777-200ER (209 ft. length), the height of the tail section can be quite different from that of the cockpit, depending on the pitch angle. But even at a nose-high pitch angle of 15 degrees, my back-of-the-envelope calculations show that the tail would have cleared the sea wall by 50 ft. or so had the pilots seen two white and two red lights on the PAPI.

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