The National Transportation Safety Board has called the FAA's response "unacceptable" to four out of six NTSB safety recommendations addressing fatigue and duty-time limitations. "The Safety Board is very concerned about reducing accidents and incidents caused by human fatigue," NTSB chairman Mark Rosenker said, adding that fatigue was a probable cause or contributing factor in numerous accidents. Among the recommendations: requiring that training, check flights, ferry and repositioning flights be included in a pilot's total revenue time; and ensuring that flight and duty-time limitations take into consideration research findings about fatigue and sleep. Current policy is based on a 2001 FAA notice that calls for pilots to have eight hours of rest in any 24-hour period that includes flight time.