Boeing Occurrences of false localiser 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 localiser captures can occur at certain airports due to the radiated signal characteristics of the localiser 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 localiser where the signal is normally saturated. False capture occurs when these signal variations satisfy the automatic flight control system’s localiser 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 localiser or approach mode selected, the airplane will initially turn to acquire the inbound course, just like a normal localiser capture.
With the flight director turned on and localiser or approach mode selected, the flight director will initially command a turn to track the inbound course, just like a normal capture.
The localiser 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 localiser.
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 localiser capture with the autopilot engaged, a localiser mode fail may be indicated by an autopilot disconnect.
For 747-400/757/767 series models, following localiser capture with the autopilot engaged, a localiser mode fail may be indicated by an amber line through the ADI/PFD localiser 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 localiser mode fail may be indicated by the removal of the roll flight director command bar from view.
The Boeing recommended procedure of arming localiser or approach mode after the localiser and glideslope pointers appear on the ADI/PFD and the indication is steady, will not preclude false localiser capture. When on an intercept heading within range of the localiser transmitter, the normal indication is a full scale deflection of the localiser 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 localiser appears normal and the flight director commands or autopilot turns the airplane to final approach heading. Then, the localiser 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 localiser which in fact is not the case. It should be noted that excessive delay in selecting localiser or approach mode can result in late capture, overshoot, and unnecessary bracketing manoeuvres.
During any ILS approach, whether flown automatically or manually, correct localiser capture should be verified by other means available (e.g., cross checking raw data to include localiser course deflection indicator and RDMI/RMI ADF bearing pointers, further localiser beam activity, tracking heading, etc.) and any incorrect capture dealt with as the particular situation requires.
Boeing Flight Operations Review 35, 15th May 1995