![]() 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. It could have been setup as a front course, but then flying it outbound would have required reverse sensing and made its use more difficult.The flight crew was in for a surprise. ![]() It is located on a mountain at 11000 MSL well south of the airport and angled so the front course is oriented to 303 degrees, but since the procedure is to intercept it and fly it outbound 303 degrees, standard CDI sensing is used to fly it. There is a back course used in the missed approach procedure at Aspen, CO. On a departure procedure, tracking a back course outbound from the runway would use standard sensing. then the PT, then tracking inbound on the localizer uses standard CDI sensing. Flying the full ILS procedure using a standard CDI requires tracking outbound in the localizer and using reverse sensing. Anytime you are flying on either the front course or the back course in the direction opposite of the front course, CDI sensing is reversed. Tracking on the back course or the front course is the same, that is any time you are fly the front or back course is the direction of the front course, CDI sensing is normal.
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