The NTSB said last week its investigation into 11 recent aircraft accidents -- 10 involving apparent controlled flight into terrain and one involving a midair collision -- has raised "serious concern" about the FAA's effectiveness in ensuring that air traffic controllers properly respond to imminently hazardous situations. In several of the accidents, alert systems provided timely warning of impending conflicts with terrain and aircraft, but controllers did not provide safety alerts to pilots. In other accidents, the alert systems themselves were ineffective. Some accidents occurred when, in the absence of automated alerts, controllers did not use available information to identify and warn pilots of hazardous situations, the NTSB said. The 11 accidents under review occurred between December 2002 and February 2006. The most recent one involved a Cessna 172RG and a 182Q that collided in flight about 3 miles south of Gillespie Field Airport in El Cajon, Calif., killing all three on board. Radar replay data indicates that the aural conflict-alert alarm at the Southern California TRACON activated twice and a continuous visual alert was displayed on the scope. The pilots were not provided with traffic advisories or a safety alert. Controllers told the NTSB they didn't hear or see a conflict alert at any time before the accident. The investigation is ongoing.