From the Horses mouth!
Extract from the FAA Aeronautical Information Manual
Aeronautical Information Manual
Official Guide to Basic Flight Information and ATC Procedures
Chapter 1. Navigation Aids
Section 1. Air Navigation Aids
K. ILS Course Distortion
1. All pilots should be aware that disturbances to ILS localizer and glide slope courses may occur when
surface vehicles or aircraft are operated near the localizer or glide slope antennas. Most ILS
installations are subject to signal interference by either surface vehicles, aircraft or both. ILS CRITICAL
AREAS are established near each localizer and glide slope antenna.
2. ATC issues control instructions to avoid interfering operations within ILS critical areas at controlled
airports during the hours the Airport Traffic Control Tower (ATCT) is in operation as follows:
(a) Weather Conditions. Less than ceiling 800 feet and/or visibility two miles.
(1) Localizer Critical Area. Except for aircraft that land, exit a runway, depart or miss
approach, vehicles and aircraft are not authorized in or over the critical area when an
arriving aircraft is between the ILS final approach fix and the airport. Additionally, when
the ceiling is less than 200 feet and/or the visibility is RVR 2,000 or less, vehicle and
aircraft operations in or over the area are not authorized when an arriving aircraft is
inside the ILS MM.
(2) Glide Slope Critical Area. Vehicles and aircraft are not authorized in the area
when an arriving aircraft is between the ILS final approach fix and the airport unless the
aircraft has reported the airport in sight and is circling or side stepping to land on a
runway other than the ILS runway.
(b) Weather Conditions. At or above ceiling 800 feet and/or visibility two miles.
(1) No critical area protective action is provided under these conditions.
(2) A flight crew, under these conditions, should advise the tower that it will conduct an
AUTOLAND or COUPLED approach to ensure that the ILS critical areas are protected
when the aircraft is inside the ILS MM.
EXAMPLE - Glide slope signal not protected.
3. Aircraft holding below 5,000 feet between the outer marker and the airport may cause localizer signal
variations for aircraft conducting the ILS approach. Accordingly, such holding is not authorized when
weather or visibility conditions are less than ceiling 800 feet and/or visibility 2 miles.
4. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS
course or glide slope signals. Also, critical areas are not protected at uncontrolled airports or at airports
with an operating control tower when weather or visibility conditions are above those requiring
protective measures. Aircraft conducting coupled or autoland operations should be especially alert in
monitoring automatic flight control systems.
Note: Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are
not flight inspected below 100 feet AGL. Guidance signal anomalies may be encountered
below this altitude.