The FAA issued an Information for Operators (InFO) bulletin to remind aircraft operators of the potential for erroneous glideslope and/or localizer indications caused by interference from aircraft or equipment moving through ILS critical areas. The agency said it has received “several reports from both pilots and air traffic controllers of instances of localizer and/or glideslope fluctuations while the aircraft is navigating on the ILS.” In several cases, it said, the aircraft automation/autopilot followed ILS fluctuations, causing the aircraft to pitch and roll excessively.

ATC protects these critical areas when arriving aircraft are inside the outer marker/final approach fix (FAF) on an ILS approach and the reported ceiling is less than 800 feet or visibility is less than two miles. However, the ILS critical areas are not protected for aircraft outside the FAF.

“Pilots should be continually aware of the conditions under which the critical area protections are imposed, and whether or not the ILS fluctuations are likely caused by movement through the ILS critical area, or an actual equipment malfunction,” the FAA said in the InFO. “In all cases it is imperative that the pilot maintain positive aircraft control whether or not the autopilot is engaged and coupled to the ILS.”