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 On the basis of weather data and witness statements, flight 801 was likely to have initially encountered variable scattered to broken cloud layers below 5,000 feet msl during the final approach to Guam. Ground lights were likely occasionally visible along the coastline, and it is probable that only scattered clouds existed below the airplane in the vicinity of the FLAKE intersection, located 7 DME from the NIMITZ VOR (UNZ).

Doppler radar data indicated that a heavy to very heavy rain shower was centered over higher terrain about 4 nm southwest of the airport (along the approach corridor) about the time of the accident. Weather data indicated that, although the Apra Harbor area (about 5 DME on the approach course) would likely have been visible to the flight crew as the airplane descended through 2,000 feet msl, the airplane would have entered clouds and light precipitation shortly after passing Apra Harbor. Radar data indicated that the flight likely experienced rain of continuously increasing intensity as the airplane proceeded inbound toward the airport and that the flight encountered very heavy precipitation for a short time near the outer marker (GUQQY). About 0141:48, when the airplane was near the outer marker, the CVR recorded the captain stating "wiper on."

Although a hunter on Nimitz Hill stated that it was not raining at the time he observed the flight overhead, Doppler radar data indicated light to moderate rain shower activity between Nimitz Hill and the airport. Therefore, on the basis of weather radar and observation data, the Safety Board concludes that, after the flight crew made an initial sighting of Guam, Korean Air flight 801 encountered instrument meteorological conditions (IMC) as the flight continued on its approach to Guam International Airport. Further, the Safety Board concludes that, although flight 801 likely exited a heavy rain shower shortly before the accident, the flight crew was still not able to see the airport because of the presence of another rain shower located between Nimitz Hill and the airport.

The instrument landing system (ILS) glideslope (GS) inoperative, or localizer (LOC)-only, approach to runway 6L at Guam International Airport required the flight crew to maintain at least 2,000 feet from the FLAKE intersection (7 DME from the UNZ VOR) to the GUQQY (outer marker) final approach fix (FAF), which was located 1.6 DME from the UNZ VOR. After passing GUQQY, the crew was required to maintain at least 1,440 feet msl until passing the UNZ VOR. After passing the UNZ VOR, the next step-down fix was to 560 feet (the minimum descent altitude [MDA]), and the flight crew was required to maintain at least this altitude while counting up to 2.8 DME (the missed approach point [MAP]) from the UNZ VOR.

To properly fly this approach, the flying pilot's navigation receiver would need to be tuned to the localizer frequency, and the nonflying pilot's navigation receiver would