On December 8, 1998, about 1725 eastern standard time, a Boeing 737, C-GCPX, operated by Canadian Airlines International Ltd. as Flight 528, experienced a near midair collision while maneuvering to land at La Guardia Airport, New York, New York. The airplane was not damaged, and the 2 flight crewmembers, 4 flight attendants, and 46 passengers were not injured. Instrument meteorological conditions prevailed and an Instrument Flight Rules (IFR) flight plan had been filed for the scheduled passenger flight that was conducted under 14 CFR Part 129. According to the duty chief dispatcher for Canadian Airlines, the Boeing 737 was on downwind for La Guardia's runway 4, at 4,000 feet msl, when the crew caught a glimpse of three lights. The lights were red, white, green, and aligned vertically with the red light on the bottom. The duty chief dispatcher added that the first officer saw the other airplane and had enough time to realize they were not going to collide, but not enough time to execute an evasive maneuver. In addition, the airplane's traffic alert collision avoidance system went from "TRAFFIC" to "MONITOR VERTICAL SPEED" to "CLEAR OF CONFLICT" in approximately 2 seconds. According to the Federal Aviation Administration's Preliminary Pilot Deviation Report, the other airplane was a Cessna 172, IFR, at 5,000 feet msl. The pilot of that airplane apparently saw the Boeing 737 at his 12 O'clock position and perceived it to be at the same altitude. To avoid a perceived collision, the Cessna pilot executed a descent, and at 4,000 feet MSL, the two airplanes passed 0 feet vertically and 500 feet horizontally from one another.