Based on a study of 11 accidents that occurred between December 2002 and February this year involving primarily general aviation aircraft, the NTSB expressed "serious concern" about the effectiveness of ATC's minimum safe altitude warning system (MSAW) and conflict-alert systems. According to the Safety Board, in several of the accidents–including the Oct. 24, 2004, accident in which a Learjet 35A hit mountainous terrain, killing all five aboard–MSAW and conflict-alert software provided timely warnings, "but controllers did not provide appropriate safety alerts." In other accidents, the Safety Board said the alerts were not effective because of "software configuration and other [technical] issues." In the Learjet accident, the Safety Board said, "The controller failed to use available information to recognize that he had left the pilot with no viable options." As a result of the study, the NTSB made several recommendations asking the FAA to "redesign" MSAW and conflict-alert systems to implement software changes and to improve controller training.