 In April 2007 , the NTSB made four further recommendations , three measures to avoid fatigue affecting the performance of air traffic controllers , and one to prevent controllers from carrying out non @-@ essential administrative tasks while aircraft are taxiing under their control . Although these recommendations were published during the course of the NTSB 's investigation into the accident to Comair Flight 5191 , they were in part prompted by four earlier accidents , and the Board was unable to determine whether fatigue contributed to the Comair accident . 
