ILS critical areas are “protected” by airport traffic controllers only under the specific conditions spelled out in the AIM.
The dimensions of their designated critical areas vary according to such factors as the size of the aircraft that operate at the airport. The localizer antenna is located beyond the departure end of the runway the glideslope antenna is off the side of the runway, close to the approach end. “ILS critical areas are established near each localizer and glideslope antenna.” “Most ILS installations are subject to signal interference by … surface vehicles, aircraft or both,” the AIM says.
#LOCALIZER ANTENNA LOCATION MANUAL#
The notice recommended that pilots review the guidance contained in the Aeronautical Information Manual (AIM) and be “continually aware of the conditions under which 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.” Partial Protection “This well-known phenomenon may occur when aircraft or vehicles are moving through the ILS localizer and/or glideslope critical areas and is due to interference with the ILS signals,” the notice said, adding that in several of the reported incidents, pilots were conducting coupled approaches, and the autopilots tracked the distorted ILS signals, causing excessive pitch and roll excursions. The notice said that there had been several recent reports by pilots and air traffic controllers about fluctuations of glideslope and/or localizer indications in aircraft on ILS approaches.
Federal Aviation Administration (FAA) in April to issue a notice “to remind operators of the potential for erroneous glideslope and/or localizer indications caused by movement of aircraft or equipment through ILS critical areas.” 2 Tower personnel told the crew that, because of the weather conditions - 2 1/2 mi (4,000 m) visibility in snow and a 1,500-ft overcast ceiling - they were not required to protect the ILS critical area. The anomaly likely was caused by disruption of the glideslope signal by a large cargo aircraft holding for takeoff on Runway 28. “While leveling, I saw the glideslope indicator go back to the correct indication of full-up,” the pilot said. The airplane pitched nose-down and descended 100 ft before the pilot flying disengaged the autopilot and hand flew the airplane back to the appropriate altitude. They had established their large air carrier aircraft on the localizer during a coupled instrument landing system (ILS) approach to Chicago O’Hare International Airport’s Runway 28 and were awaiting glideslope interception when their glideslope course deviation indicators (CDIs) abruptly moved from the full-up position to full-down.
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