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 The flight crew of Ryan International flight 789, which landed shortly after the accident occurred, stated that the visibility was sufficient to see the lights of Guam from about 150 nm away. The first officer stated that on-board weather radar indicated showers northeast of the airport but no thunderstorms.

Additionally, the Ryan flight crew initially requested a visual approach when the flight was about 15 nm from the VOR, but the airplane encountered clouds and rain on approach to runway 6L shortly thereafter. The first officer stated that the airplane remained in the clouds until it was in proximity of the VOR, at which time the airplane broke out and the flight crew was able to acquire and maintain visual contact with the airport. The captain stated that, although clouds and rain were over the island's shoreline, the air around the airport and in the vicinity of the accident site was smooth. Further, the captain, who was also a check airman based at Guam, said that he "noticed that once [flight] crews are given a visual approach [to Guam International Airport] they have a tendency to press on even when they lose visual contact in hopes of regaining visual contact again.... That's because so many approaches are visual and the clouds and rain showers are so localized."

In addition, a witness who was hunting on Nimitz Hill at the time of the accident stated that it was not raining when he observed flight 801 pass over his position (100 feet north of the VOR beacon) and crash a very short distance away. He said that there had been intermittent rain showers shortly before the accident but that, when he saw the airplane crash, he could see stars directly over the accident site. The witness also said that the visibility was "very good" at the time of the accident and that, although he could not see the airport lights, he could see the lights of the town of Tamuning (3 to 4 miles northeast of his location). He said that the wind was "normal" and that no thunder or lightning was in the area.

Guam International Airport is serviced by three navigational aids: the NIMITZ VOR/DME (UNZ); the Mount Macajna nondirectional beacon (NDB), which was not operational at the time of the accident; and the ILS glideslope and localizer.

The colocated VOR and DME transmitters were equipped with a "selfmonitoring" system that samples radiated transmitter signals to ensure that the system is operating within prescribed tolerances and parameters. If these tolerances are exceeded, the monitoring system automatically shuts down the equipment. According to the facility logs, the VOR was not shut down at the time of the accident.

On the day of the accident, the FAA conducted a flight check of the localizer, outer marker, and NDB at Guam. The VOR and DME at Guam were not checked by the FAA until the day after the accident because of rescue operations. The FAA's flight checks determined that the respective systems were functioning properly and within prescribed tolerances. The flight checks did not examine the glideslope because it was out of service and removed at the time (see section 1.10.2 for more information).