Six times the control tower at San Francisco International Airport ordered an incoming Air Canada plane to abort its landing, fearing another plane might be on the runway. Each time, the order went unanswered.Finally, air traffic controllers Sunday night took out an emergency red light and aimed it outside their window toward the jet to try to get the pilots’ attention. That didn’t work either, the plane landed and one of the pilots then radioed that he was having problems with the radio.“That’s pretty evident,” the controller responded.The Federal Aviation Administration said Tuesday it is investigating the second serious safety issue in three months involving an Air Canada passenger jet landing at San Francisco airport.In July, an Air Canada jet with 140 people on board nearly landed on a taxiway where four planes were waiting before takeoff, prompting the FAA to issue new rules for nighttime landings and control tower staffing at the airport.Sunday’s incident involved a flight from Montreal. The Airbus A320 was given initial clearance to land when it was about 6 miles (10 kilometers) from the airport, FAA spokesman Ian Gregor said. The cockpit acknowledged the instruction.But air traffic control then reversed its clearance, concerned a plane that had just landed would not be off the runway in time.“Air Canada 781, go around,” the controller said, using terminology telling the pilot to abort the landing.Audio from the control tower indicates the Air Canada flight was told six times in less than 35 seconds to “go around” for another landing attempt. The controller’s voice gets slower and more emphatic.The instructions were met with silence, according to the audio clip posted at LiveATC.net.The air traffic control supervisor then used a flashing “red light gun” shined from the control tower windows toward the plane to alert the crew, Gregor said. Doing so is a standard procedure when an air crew does not respond to radio instructions.“Air Canada did not respond to the verbal instructions or to the light gun instruction,” Gregor said.The flight landed about 9:30 p.m.He said that a radar replay showed the runway was in fact cleared of the earlier arrival by a Southwest Airlines jet when the Air Canada plane landed.Air Canada spokesman Peter Fitzpatrick said the company was investigating the incident.“After receiving proper clearance to land it proceeded to do so and landed normally. Upon landing the crew was informed the tower had attempted unsuccessfully to contact the aircraft, however the message was not received by the crew,” Fitzpatrick said.Pilots say monitoring radio traffic is especially important in San Francisco because its layout of two sets of parallel runways means controllers often give them last-minute instructions on airspeed and other issues. They say that silence from the tower at the busy airport should have been a clear message to the Air Canada crew.“The hair on the back of my neck would raise up if I’m not hearing anything on the tower frequency for 10 seconds in San Francisco,” longtime American Airlines pilot Chris Manno said.John Cox, a former airline pilot and now a safety consultant, said now and then tower instructions are lost because of other chatter on the radio. But “It is uncommon for a communications difficulty to go on as long as this one did,” he said.Pilots, he said, develop a sixth sense telling them when radio silence indicates a problem.Pilots are trained to respond to the flashing-light warnings the FAA said were used in the airport tower. Manno said the light signal is obvious — green means land, red means go around — but he had never encountered such a warning while flying an airliner, only while piloting Air Force planes.Cox said he was not surprised the Air Canada crew did not react to the warning. “If they are not looking at the tower — which is not something you normally do — you won’t see it,” he said.On July 7, Air Canada pilots mistook the taxiway for the runway next to it and flew their jet to just 59 feet (18 meters) above ground before pulling up to attempt another landing, according to the National Transportation Safety Board. That’s barely taller than the four planes that were on the taxiway.Air Canada has not said whether the pilots were disciplined.In response to the incident, the FAA issued new procedures in August for when a runway parallel to a plane’s designated runway is closed, as it was July 7.Air traffic controllers may no longer let pilots make so-called visual approaches to land. Instead, they must use instrument landing systems or satellite-based systems to line up for the correct runway.The FAA also stipulated that two controllers must be in the airport tower during busy late-night periods. Only one controller was working during the Air Canada incident.