Narrative:

While approaching the airport we were given a clearance direct to the bog VOR for the ILS Z runway 13L approach with a speed assignment of 230 knots. While our altitude was on path for a standardized approach; our speed was higher than anticipated. The first officer was flying. As we approached the bog VOR he began to slow the aircraft to 210 knots. I suggested that the first officer slow the aircraft. He elected to use speed brakes and I further suggested landing gear extension to slow at a faster rate; because there would also be an altitude change passing the VOR. The first officer elected to utilize LNAV/VNAV and armed the ILS with the autopilot engaged as we were cleared for the approach. As we crossed the VOR the aircraft turned; captured the localizer and glideslope and the aircraft began to descend on the glideslope from the 12;000 foot altitude with the gear extending and the speed brakes deployed. I shifted my focus to the before landing checklist flow; ATC calls and finding the preceding aircraft and airport visually. Those tasks seemed to be done in the usual time frame. When I spotted the airport we seemed to be low. I looked back inside at the flight instruments to check our progress and found we were full scale deflection low on the glideslope; the command bars were centered; the autopilot was still engaged; and the localizer and GS were still captured. This made no sense so my call of 'glideslope deviation' was slowed by another 200 foot loss. I immediately called 'too low; climb now'; pushed the power up and pulled on the yoke. The first officer also came on the controls; I'm not sure which of us disconnected the autopilot; but I retracted the speed brakes while we climbed back to the glideslope and what looked to be an appropriate visual glide path. There was no GPWS or glideslope (or any other warning) warning. The flight landed without further incident.after arriving at the gate; I called maintenance control to try to understand what failure could have transpired since we had no warning/failure indications. It was very possible that the preceding aircraft created interference with the ILS signal. But there is no way for us to gauge to what level that may have occurred. The airport briefing guide mentions possible localizer signal deviations but not glideslope deviations. The mechanic performed some maintenance fault scans. He was scrolling through the long list of faults too fast for me to take good notes; but there were faults which said; 'mcdp flight fault; main data supply fault; several no land 3 faults; ILS beam error; and several VNAV failures.' in the final analysis; the mechanic thought that it was a ground based signal error.first; given the nature and history of this airport's flight operations; it should be a captain only arrival and departure. Since it is not a captain only airport; first officers have every expectation they will either fly into or out of this airport; especially since this is only a two leg day. Several days prior the first officer had already proclaimed that he had been here at least 8 times in very recent history and was thoroughly familiar and prepared to fly one of the legs in or out. Sometimes even those who think they are prepared have difficulty with this airport. Second; the briefing guide is very comprehensive but should also state that the ILS has possible glideslope signal issues as well. Third; I would recommend not using LNAV/VNAV to get to localizer/GS capture; but rather fly the step down segments as published.

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Original NASA ASRS Text

Title: A B767 Captain reported the First Officer descended below the glideslope even though they were using autopilot and were receiving the localizer and glideslope signal.

Narrative: While approaching the airport we were given a clearance direct to the BOG VOR for the ILS Z Runway 13L Approach with a speed assignment of 230 knots. While our altitude was on path for a standardized approach; our speed was higher than anticipated. The First Officer was flying. As we approached the BOG VOR he began to slow the aircraft to 210 knots. I suggested that the First Officer slow the aircraft. He elected to use speed brakes and I further suggested landing gear extension to slow at a faster rate; because there would also be an altitude change passing the VOR. The First Officer elected to utilize LNAV/VNAV and armed the ILS with the autopilot engaged as we were cleared for the approach. As we crossed the VOR the aircraft turned; captured the LOC and glideslope and the aircraft began to descend on the glideslope from the 12;000 foot altitude with the gear extending and the speed brakes deployed. I shifted my focus to the before landing checklist flow; ATC calls and finding the preceding aircraft and airport visually. Those tasks seemed to be done in the usual time frame. When I spotted the airport we seemed to be low. I looked back inside at the flight instruments to check our progress and found we were full scale deflection low on the glideslope; the command bars were centered; the autopilot was still engaged; and the LOC and GS were still captured. This made no sense so my call of 'glideslope deviation' was slowed by another 200 foot loss. I immediately called 'too low; climb now'; pushed the power up and pulled on the yoke. The First Officer also came on the controls; I'm not sure which of us disconnected the autopilot; but I retracted the speed brakes while we climbed back to the glideslope and what looked to be an appropriate visual glide path. There was no GPWS or glideslope (or any other warning) warning. The flight landed without further incident.After arriving at the gate; I called Maintenance Control to try to understand what failure could have transpired since we had no warning/failure indications. It was very possible that the preceding aircraft created interference with the ILS signal. But there is no way for us to gauge to what level that may have occurred. The airport briefing guide mentions possible LOC signal deviations but not glideslope deviations. The mechanic performed some maintenance fault scans. He was scrolling through the long list of faults too fast for me to take good notes; but there were faults which said; 'MCDP flight fault; Main Data Supply fault; several no land 3 faults; ILS Beam Error; and several VNAV failures.' In the final analysis; the mechanic thought that it was a ground based signal error.First; given the nature and history of this airport's flight operations; it should be a Captain only arrival and departure. Since it is not a captain only airport; first officers have every expectation they will either fly into or out of this airport; especially since this is only a two leg day. Several days prior the First Officer had already proclaimed that he had been here at least 8 times in very recent history and was thoroughly familiar and prepared to fly one of the legs in or out. Sometimes even those who think they are prepared have difficulty with this airport. Second; the briefing guide is very comprehensive but should also state that the ILS has possible glideslope signal issues as well. Third; I would recommend not using LNAV/VNAV to get to LOC/GS capture; but rather fly the step down segments as published.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.