Page:KAL801Finalreport.pdf/156

 airport requiring special pilot qualifications. Therefore, the Safety Board believes that the FAA should consider designating Guam International Airport as a special airport requiring special pilot qualifications.

2.4.1.3 Possible Explanations for the Approach Conducted 2.4.1.3.1 Confusion About Status of Glideslope

Despite several indications that the flight crew was aware that the glideslope was inoperative, in the last 2½ minutes of the flight (beginning shortly after the airplane was established on the approach), the CVR recorded a series of conflicting flight crew comments concerning the operational status of the glideslope. About 0139:55, the flight engineer asked, "is the glideslope working?" The captain responded, "yes, yes it's working." About 0139:58, an unidentified voice in the cockpit stated, "check the glideslope if working?" One second later, an unidentified voice in the cockpit stated, "why is it working?" About 0140:00, the first officer responded, "not useable." About 0140:22, an unidentified voice in the cockpit stated, "glideslope is incorrect," followed by the captain's statement, "since today's glideslope condition is not good, we need to maintain one thousand four hundred forty [feet]." However, about 0141:46, after the airplane crossed the GUQQY outer marker (1.6 DME from the VOR), the captain again stated, "isn't glideslope working?"

The Safety Board considered whether the flight crew might have misinterpreted some cockpit instrumentation indications as a valid glideslope capture signal. During the localizer approach into Guam, both pilots' HSIs would have appeared centered; the captain's would have captured the localizer, and the first officer's would have captured the VOR radial. With VOR/LOC selected, the localizer captured, and the pitch commands set to VERT SPEED (the most likely setting), the captain's FD command bars would have shown some vertical and horizontal movement, similar to an FD that was responding to a captured localizer and glideslope. However, the raw data glideslope needles on the attitude director indicator (ADI) and HSI would not have been affected by the VERT SPEED setting; therefore, the captain's ADI and HSI glideslope needles should have been covered by "off" flags. Further, there would have been no glideslope capture annunciator on the GS bar of the FMA on top of the captain's and first officer's instrument panels.

The Safety Board also considered whether the flight crewmembers might have observed intermittent movement of the glideslope needles during the approach, thereby creating or adding to their confusion about the glideslope. An FAA navigation expert testified at the Safety Board's public hearing that spurious radio signals could cause a sporadic or intermittent glideslope indicator deviation in the absence of a valid glideslope signal. However, he stated that the glideslope off flag would still appear on the HSI and ADI glideslope needles and that, when the off flag appears, any movement of the glideslope needle should be considered unreliable. Postaccident testing by Korean Air and the KCAB confirmed that an airplane's glideslope receiver could be affected by spurious