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 the KCAB involved the glideslope receiver; however, the Safety Board concludes that navigation receivers, including glideslope receivers, may be susceptible to spurious radio signals. Therefore, the Safety Board believes that the FAA should disseminate information to pilots, through the Aeronautical Information Manual, about the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter (for example, a glideslope, VOR, or nondirectional beacon [NDB] transmitter) is inoperative. Further, this information should reiterate to pilots that they should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified as unusable or inoperative.

2.4.1.3.2 Confusion About Location of DME

About 0140:37, when the airplane was at 2,400 feet msl and descending at 1,000 feet per minute, the captain stated, "since today's glideslope condition is not good, we need to maintain one thousand fourteen hundred forty [feet]. please set it." This statement suggests that the captain was attempting to comply with the restrictions of the localizeronly approach and believed that he had passed the GUQQY step-down fix. However, the CVR recorded no discussion between the captain and the first officer about DME values or their position in relation to the next step-down fix, the VOR, or the airport.

The Safety Board considered whether the flight crew might have confused the configuration of the runway 6L localizer approach with one in which the DME is located on the airport. A review of the flight crew's training records showed that the nonprecision approaches incorporating DME provided to the flight crew during training and check rides had the DME located on the airport. A countdown/count up DME procedure, which is rarely encountered on a localizer procedure, was not included in any of the Korean Air simulator training scenarios. If the flight crewmembers had the misconception that the DME information referred to the distance from the airport, they might have believed that the airplane was much closer to the airport than it actually was (the DME was located 3.3 nm southwest of the airport) and that the airplane was well above the minimum altitudes for the intermediate step-down fixes and thus ready to descend directly to the MDA. If the captain had this misconception, it could explain why he flew the airplane and commanded altitude selections as though he believed he was at or above the altitude constraint for each navigational fix along the approach. If the other flight crewmembers shared this misconception, it could explain why they failed to challenge the captain's premature descents below 2,000 and 1,440 feet.

However, this scenario suggests strongly that the captain was not noting the definitions of the navigational fixes on the approach chart, which were clearly defined as DME values. Thus, the Safety Board concludes that the captain may have mistakenly believed that the airplane was closer to the airport than its actual position; however, if the captain conducted the flight's descent on this basis, he did so in disregard of the DME fix definitions shown on the approach chart.