What happened
During the final approach phase, the aircraft with registration N416MC, a Boeing 787 (B787), intercepted the ILS localiser and glideslope. Simultaneously, another Boeing 787 was holding on taxiway A1. This taxiway is situated within the glideslope critical area for runway 16R. The presence of the stationary aircraft in this zone likely caused the incoming aircraft to receive a distorted or fluctuating glideslope signal. This signal instability led to a glideslope failure indication, which the captain identified while crosschecking the two primary flight displays. As a result of the signal disturbance, the aircraft descended below the standard 3° approach path.
The investigation
The investigation focused on the integrity of the ILS signal and the positioning of aircraft within the runway environment. Investigators determined that at the time of the event, the manual of air traffic services did not require the glideslope critical area to be protected under the prevailing weather conditions. The investigation examined how the presence of the aircraft on taxiway A1 influenced the signal strength and accuracy received by the approaching flight crew.
Findings
- The primary cause of the descent below the nominal 3° glideslope was the autopilot capturing a distorted glideslope beam.
- The distortion of the glideslope beam was likely caused by the presence of a Boeing 787 in the glideslope critical area.
- Under the weather conditions present during the incident, protection of the glideslope critical area was not a regulatory requirement.