ILS Signal Interference

 

Recently a commercial large jet was cleared for an ILS approach to Runway 28 at Chicago O’Hare airport. While inbound to intercept the Glide slope, the cockpit indications were initially full up. Abruptly the display changed from full up to full down position and the aircraft pitched down and descended to stay on the glide slope. The pilot reacted to disconnect the autopilot, but not before the aircraft had descended a 100 ft. The display restored to full up deflection soon thereafter.

 This anomaly was most likely caused by disruption of the Glide slope signals caused by a large cargo aircraft holding for take-off. ATC controller had advised the crew of this aircraft that they were not required to protect the ILS critical area.

Most ILS installations are subject to signal interference by surface vehicles, aircraft or both. ILS critical areas are established near each localizer and glideslope antenna. While the localizer antenna is located beyond the departure end of the runway, the glideslope antenna is off to the side of the runaway, close to the approach end. The critical areas are protected by the ATC only under specific conditions spelt out in the Aeronautical Information Manual (AIM). Chief among them is that visibility must be less than 2 miles and ceiling less than 800 ft. and arriving aircraft is has crossed outer marker fix or FAF. The protection is achieved by ATC advising aircraft to hold short of CAT II holding point/ ILS critical area.

Interestingly, when the visibility is less than 2 mi and ceiling less than 800 ft., and an aircraft is inside the FAF, protection may not be assured against aircraft that have landed and are exiting the runway or are on missed approach or departure. Controllers are required to keep critical areas clear in this case, only whenever RVR is 600 m or less or the ceiling is less than 200 ft. and the arriving aircraft is inside the ILS middle marker.

 At uncontrolled airports, there is no protection of ILS critical areas. The AIM, recommends that pilots be alert when conducting a coupled approach to an uncontrolled airport, but it provides no guidance for ground operations. Vehicles can also disrupt ILS signal as a business airplane pilot found to his dismay. The spurious and random oscillations experienced in this case, were the likely result of a large grass cutting mower operating near the localizer antenna.

The AIM also warns of false courses generated outside the ILS service area (40 to 60 deg outside the Localiser), as a normal by product of ILS signal generation. Depending on the ILS installation, an aircraft may give spurious normal on course indications without any failure warnings.  Erroneous ILS indications may also occur during maintenance or testing of the ILS ground equipment. However, in these cases, the Morse code identification is removed and also NOTAM action is taken.

Incidents like these prompted FAA to issue a notice to remind operators of potential for erroneous indications of localizer/ glideslope caused by movement of aircraft or equipment through ILS critical area.

Curtsey and reference to Airsafety World