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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.
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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
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