4 October 2000

Caution: Erroneous ILS Signals

Pilots have come to believe that ILS equipment is extremely accurate and reliable. Normally this is so; so much so that some pilots have been tempted to use an ILS when cockpit indications suggest the ILS is working, despite being NOTAMED as on test ("NOT AVBL, ON TEST") or unserviceable ("U/S") or not monitored ("OPR BUT CTN DUE NOT ATS MNT"). IFR pilots need to be fully aware of the risks involved when they use equipmentunder these conditions.

A recent overseas incident where an aircraft operator experienced significant glide-slope errors while conducting an ILS approach provides a valuable lesson. The operator conducted a detailed investigation and reached the following conclusions:

ILS equipment, if operating with an abnormal mode selected (eg, incorrectly switched, or left in test mode), is capable of transmitting an incorrect localiser and/or glide-slope, which will result in an erroneous approach track or glide-slope that does not terminate at the source. If flown like a normal ILS, this could result in the aircraft approaching at the wrong angle, above or below the correct 3 degree glide-slope, or paralleling the correct glide-slope, or on the wrong track (within approximately 40 degrees of arc either side of the localiser) and not reaching the airfield.

There will be no instrument unserviceability flags, and the localiser and glide-slope indications will appear correct. The ILS ident will be normal (it is broadcast only with the localiser signal). Note that the ILS presentation will be fixed at more or less on localiser or on glide-slope without any indicated deviation.This situation will exist even if the aircraft is not on the localiser or glide-slope.

Flight director and autopilot functions will also be normal. In approach mode, the flight director will automatically capture the incorrect glide-slope beam without a prior fly-up or fly-down indication.

This erroneous situation is not the same as a "false" capture, as the abnormal localiser track or glide-slope will not terminate at the airfield source.

This situation can arise only if the ILS equipment is operating with an abnormal mode selected. Tower status monitoring of the ILS equipment should correctly identify that the ILS is unserviceable if the mode selection is abnormal.

Operating Procedures

Extra caution is required when briefing and flying ILS approaches. The following procedures will provide an extra measure of safety assurance and should be adopted immediately. They should also be incorporated in operating manuals.

  1. Briefings should include the requirement for a glide-slope reference check of altitude in relation to DME, Marker beacon or NDB locator. This check should be used to confirm that the glide-slope altitude at the reference point is accurate with respect to the published approach chart information, eg, ILS 16 WN 3000 feet at 9.2 ILS/DME (final approach fix).
  2. Briefings should contain a reference to the expected point (both altitude and distance) at which the glide-slope will be intercepted. Consider using the level segment prior to glide-slope capture to verify the fly-up indication – this indication will not occur if the glide-slope beam is incorrectly switched or on test.
  3. During the approach, a reasonableness check of all ILS precision approach tracks and glide-slopes should be conducted using appropriate alternate means of verification, eg, Marker beacon, NDB, ILS/DME or VOR/DME.
  4. If an ILS is advised as "on test" or "unserviceable", it must not be used for approach guidance even if it is observed to be operating.
  5. If the ILS is advised as being "not monitored", consider the wisdom of trusting the glide-slope at all. Fly a localiser approach, descending in accordance with chart profiles where DME is available, and confirm the localiser with VOR/NDB track information. If in doubt, consider an alternative type of approach.
  6. If there is any doubt at any stage of an approach as to the accuracy or validity of the ILS equipment go around.