Occurrence
On June 14, 2001, a Bombardier CL-600-2B16 aircraft, registration C-FSJR, was on radar vectors for an instrument landing system (ILS) approach to Runway 27 at Victoria International Airport, British Columbia. When cleared for the approach, the aircraft was approximately 10 nautical miles from the airport, about 5 nautical miles north of the localizer course, and descending through approximately 4000 feet above sea level. The pilot selected the autopilot to approach mode, and the aircraft turned right; navigation equipment indicated a localizer capture. The crew recognized the autopilot had taken an inappropriate intercept, directing the aircraft away from the correct localizer, which they knew to be approximately 5 nautical miles south. Upon breaking out below a scattered cloud layer, the crew visually confirmed the false capture was heading the aircraft toward Mount Tuam on Saltspring Island. They switched to heading mode, re-established an intercept for the Runway 27 localizer, and landed safely. No injuries or damage resulted.
Aircraft and Flight Information
The aircraft was a Bombardier CL-600-2B16. No further details about operator, flight number, or exact date were provided beyond the occurrence date of June 14, 2001.
ILS and False Capture Background
An ILS uses VHF signals for localizer and UHF signals for glidepath. Coverage is normally verified within 35° of the front or back course to 10 nm and 10° to 18 nm. Automated flight control systems (AFCS) compute steering commands for localizer interception and capture. Under certain conditions, an AFCS may capture an incorrect localizer course, causing the autopilot to turn away from the correct beam. Transport Canada identified this incompatibility between ground and airborne ILS standards in the early 1990s, issuing advisories and procedural defences. The 1994 Air Carrier Advisory Circular (ACAC) 0066 noted the problem could occur even when both ground and airborne equipment meet specifications. False captures are most likely when the aircraft is 8° to 12° azimuth from the localizer course; in this event, the aircraft was 26° north when the false capture occurred.
Findings
The Transportation Safety Board (TSB) found that when the crew selected approach mode, the aircraft was inside the recommended distance from the airfield but near or beyond the limits at which the integrity of the ILS is verified. The TSB also noted known incompatibility between some ground and airborne ILS equipment can cause false captures. Recent recurrences may indicate a need to remind pilots of guidance in the Aeronautical Information Publication (AIP), especially for foreign carriers. The crew followed required reporting procedures, and the localizer was checked and confirmed serviceable. Safety action included distributing Transport Canada's 1993 aviation notice to involved operators.