Boeing 747-300 accident at Nimitz Hill, Guam, 6 Aug 1997

Accident narrative

On August 6, 1997, about 0142:26 Guam local time, Korean Air flight 801, a Boeing 747-300, crashed into high terrain at Nimitz Hill, Guam. The airplane, registered HL7468, had been cleared to land on runway 6 Left at A.B. Won Pat Guam International Airport, located about 3 miles northeast of the crash site. Of the 254 persons on board, 228 were killed, and 23 passengers and 3 flight attendants survived the accident with serious injuries. The airplane was destroyed by impact forces and a postcrash fire.

### The flight

Flight 801 was a regularly scheduled international passenger flight operating under instrument flight rules from Kimpo International Airport in Seoul, Korea, to Guam. Aboard the flight were 2 pilots, 1 flight engineer, 14 flight attendants, and 237 passengers.

The captain, age 42, had accumulated 8,932 hours of flight time, including 1,718 hours as a 747 captain. The first officer, age 40, had 4,066 hours of flight time, with 1,560 hours as a 747 first officer. The flight engineer, age 57, had 13,065 hours of flight time, including 1,573 hours as a 747 flight engineer.

### Sequence of events

The flight departed Seoul about 2153 on August 5. Upon arrival to the Guam area, the first officer made initial contact with the Guam Combined Center/Radar Approach Control (CERAP) controller about 0103:18.

About 0111:51, the cockpit voice recorder recorded the captain briefing the approach. The captain noted that the instrument landing system (ILS) glideslope was out and that the minimum descent altitude (MDA) was 560 feet. About 0121:59, the first officer stated, "Guam condition is no good." Shortly after, the CERAP controller informed the crew that the automatic terminal information service (ATIS) information Uniform was current. This ATIS broadcast included a Notice to Airmen stating that the runway 6L ILS glideslope was out of service. Between 0124 and 0131, the crew requested and received radar vectors to deviate around weather.

About 0139:44, the CERAP controller cleared flight 801 for the ILS runway 6L approach, stating "glideslope unusable." The first officer acknowledged the clearance but did not acknowledge that the glideslope was unusable. About 0139:55, the flight engineer asked, "is the glideslope working?" The captain responded, "yes, yes, it's working." An unidentified voice asked to check if it was working, and about 0140:00, the first officer responded, "not useable."

Flight data recorder information indicated the airplane began to descend about 0140:13 from an altitude of 2,640 feet mean sea level (msl) at a point approximately 9 nautical miles (nm) from the runway threshold. About 0140:22, an unidentified voice stated, "glideslope is incorrect." About 0140:37, the captain stated, "since today's glideslope condition is not good, we need to maintain one thousand four hundred forty [feet]. please set it."

About 0140:42, the CERAP controller instructed the flight to contact the Agana control tower. The tower controller cleared flight 801 to land about 0141:01.

About 0141:42, the ground proximity warning system (GPWS) issued a radio altitude callout of "one thousand [feet]." Four seconds later, the captain asked, "isn't glideslope working?" About 0141:59, the first officer stated "not in sight?" followed one second later by a GPWS callout of "five hundred [feet]."

About 0142:14, as the airplane descended through 840 feet msl, the GPWS issued a "minimums minimums" annunciation followed by a "sink rate" alert. The first officer responded, "sink rate okay." About 0142:19, the flight engineer stated, "two hundred [feet]," and the first officer said, "let's make a missed approach." The flight engineer stated, "not in sight," and the first officer repeated, "not in sight, missed approach."

About 0142:22, the control column position began increasing nose up, and the flight engineer and captain stated "go around." The airplane's engine pressure ratios and airspeed began to increase. About 0142:26, the airplane impacted hilly terrain at about 660 feet msl, 3.3 nm from the runway threshold.

### What the investigation found

The main wreckage was located in a gully 2,000 feet southwest of the NIMITZ VOR. The landing gear was extended, and the flaps were extended approximately 25 degrees. Examination of the engines revealed evidence consistent with them producing power at the time of impact. The Board found no evidence of an in-flight fire or preimpact structural failure.

Weather radar and observation data indicated that the flight encountered instrument meteorological conditions and rain showers as it continued on its approach.

The ILS glideslope was out of service and removed for reconstruction. The localizer-only approach required a step-down technique using the NIMITZ VOR and distance measuring equipment (DME). The procedure restricted descent below 2,000 feet msl until crossing the outer marker, and below 1,440 feet msl until passing the VOR. However, the airplane descended below 2,000 feet about 1.9 nm before reaching the outer marker, and below 1,440 feet about 2.1 DME before reaching the VOR.

Postaccident testing demonstrated that an airplane's glideslope receiver could be affected by spurious radio signals when no valid signal was being transmitted. However, the Board concluded that the flight crew had sufficient information to be aware the glideslope was unusable and should have ignored any glideslope indications. The Board concluded that the captain lost awareness of the airplane's position and improperly descended below the intermediate approach altitudes as a result of his confusion and preoccupation with the glideslope's status, his failure to properly cross-check the airplane's position and altitude with the approach chart, and his continuing expectation of a visual approach.

The Board concluded that the first officer and flight engineer failed to properly monitor and/or challenge the captain's performance. Although the first officer properly called for a missed approach 6 seconds before impact, he failed to challenge the captain's premature descents earlier in the approach. The Board also concluded that the captain's failure to react properly to the GPWS minimums callout and the direct challenge from the first officer precluded action that might have prevented the accident.

The captain had been awake for 11 hours, and the accident occurred after midnight in the crew's home time zone. The Board concluded the captain was fatigued, which degraded his performance and contributed to his failure to properly execute the approach.

The Board concluded that Korean Air's training in the execution of nonprecision approaches was ineffective. Simulator training relied on a single nonprecision approach scenario in which the DME was located on the airport, unlike Guam where the DME was not colocated with the localizer.

The CERAP controller failed to provide a position advisory when clearing the flight for the approach, did not inform the crew of a rain shower on the final approach path, and failed to monitor the flight after the frequency change to the tower controller. The Board concluded his performance was substandard.

The FAA had intentionally inhibited the Guam ARTS IIA Minimum Safe Altitude Warning (MSAW) system from providing low-altitude alerts inside a 54-nm radius to reduce false alerts. A simulation indicated that if the system had not been inhibited, a visual and aural warning would have activated about 64 seconds before impact. The Board concluded the FAA's intentional inhibition of the system contributed to the accident.

The first emergency response equipment arrived at the access road 52 minutes after the accident. The response was delayed by the air traffic controllers' delayed discovery of the accident, a fire truck needing its brake system recharged, and a severed pipeline blocking the access road. The Board concluded the delayed response hampered the timely evacuation of injured persons, and at least one passenger might not have died if responders had reached the site sooner.

The Board concluded the Korean Civil Aviation Bureau was ineffective in its oversight of Korean Air's operations and pilot training programs. The Board also concluded the FAA's International Aviation Safety Assessment program was not adequate to determine whether foreign air carriers operating into the United States are maintaining an adequate level of safety.

### Probable cause

The National Transportation Safety Board determined that the probable cause of the accident was the captain's failure to adequately brief and execute the nonprecision approach and the first officer's and flight engineer's failure to effectively monitor and cross-check the captain's execution of the approach. Contributing to these failures were the captain's fatigue and Korean Air's inadequate flight crew training.

Contributing to the accident was the Federal Aviation Administration's intentional inhibition of the minimum safe altitude warning system at Guam and the agency's failure to adequately manage the system.