Boeing Occurrences of false localizer captures during autopilot and flight director approaches have been reported at a number of airports throughout the world. It is believed that a large number of occurrences are never reported since flight crews are generally trained to detect this anomaly, and false capture events are often not recorded in pilot flight logs.
Flight crews should be aware that repeated false localizer captures can occur at certain airports due to the radiated signal characteristics of the localizer facility even when the airplane systems are functioning normally.
The false capture occurs due to significant scalloping, or signal reversals, in the region of the localizer where the signal is normally saturated. False capture occurs when these signal variations satisfy the automatic flight control system’s localizer capture logic.
False captures can produce the following indications depending on the flight deck instrumentation. Some of these indications are applicable to all airplanes, while some are unique to airplanes equipped with EICAS and/or EFIS displays.
With the autopilot engaged and localizer or approach mode selected, the airplane will initially turn to acquire the inbound course, just like a normal localizer capture.
With the flight director turned on and localizer or approach mode selected, the flight director will initially command a turn to track the inbound course, just like a normal capture.
The localizer deviation displayed on the ADI/PFD or the HSI/ND ILS display, after capture, may become erratic or may maintain a continuous offset with corresponding unpredictable autopilot control or flight director guidance.
The HSI/ND map display of the approach path and approach waypoints may indicate improper aircraft positioning or aircraft track in relation to the path necessary to acquire the localizer.
A cross check of the RDMI/RMI ADF bearing pointer may indicate improper airplane position relative to a tuned NDB associated with the approach.
For 737 series models, following localizer capture with the autopilot engaged, a localizer mode fail may be indicated by an autopilot disconnect.
For 747-400/757/767 series models, following localizer capture with the autopilot engaged, a localizer mode fail may be indicated by an amber line through the ADI/PFD localizer mode annunciation, a master caution light, a master caution aural, an EICAS caution message, and a dedicated autopilot caution light (757/767 only).
With the flight director turned on, a localizer mode fail may be indicated by the removal of the roll flight director command bar from view.
The Boeing recommended procedure of arming localizer or approach mode after the localizer and glideslope pointers appear on the ADI/PFD and the indication is steady, will not preclude false localizer capture. When on an intercept heading within range of the localizer transmitter, the normal indication is a full scale deflection of the localizer and glide path pointers. These full scale deflections appear steady and satisfy the criteria for approach mode selection. Often, movement and capture of the false localizer appears normal and the flight director commands or autopilot turns the airplane to final approach heading. Then, the localizer returns to a full scale deflection; however, the flight mode annunciator and flight director commands may not change. As a result, the flight director commands a constant heading, and the annunciator indicates tracking of the localizer which in fact is not the case. It should be noted that excessive delay in selecting localizer or approach mode can result in late capture, overshoot, and unnecessary bracketing maneuvers.
During any ILS approach, whether flown automatically or manually, correct localizer capture should be verified by other means available (e.g., cross checking raw data to include localizer course deflection indicator and RDMI/RMI ADF bearing pointers, further localizer beam activity, tracking heading, etc.) and any incorrect capture dealt with as the particular situation requires.
Boeing Flight Operations Review 35, 15th May 1995