hazards or heavy physical or mental strain" to an average of eight hours in any 24-hour period. The EU directive allows for limited derogations from the regulation, and special provisions allow longer working hours for transportation and offshore workers, fishing vessel workers, and doctors in training (see also medical resident work hours). Aircraft traffic flight controllers and pilots For fewer operational errors, the FAA goal calls for Flight Controllers to be on duty for 5 to 6 hours per shift, with the remaining shift time devoted to meals and breaks. For aircraft pilots, the actual time at the controls (flight time) is limited to 8 or 9 hours, depending on the time of day. Industrial disasters Fatigue due to shift work has contributed to several industrial disasters, including the Three Mile Island accident, the Space Shuttle Challenger disaster and the Chernobyl disaster. The Alaska Oil Spill Commission's final report on the Exxon Valdez oil spill disaster found that it was "conceivable" that excessive work hours contributed to crew fatigue, which in turn contributed to the vessel's running aground. Prevention Management practices The practices and policies put in place by managers of round-the-clock or 24/7 operations can significantly influence shift worker alertness (and hence safety) and performance. Air traffic controllers typically work an 8-hour day, 5 days per week. Research has shown that when controllers remain "in position" for more than two hours, even at low traffic levels, performance can deteriorate rapidly, so they are typically placed "in position" for 30-minute intervals (with 30 minutes between intervals). These practices and policies can include selecting an appropriate shift schedule or rota and using an employee scheduling software to maintain it, setting the length of shifts, managing overtime, increasing lighting levels, providing shift worker lifestyle training, retirement compensation based on salary in the last few years of employment (which can encourage excessive overtime among older workers who may be less able to obtain adequate sleep), or screening and hiring of new shift workers that assesses adaptability to a shift work schedule. Mandating a minimum of 10 hours between shifts is an effective strategy to encourage adequate sleep for workers. Allowing frequent breaks and scheduling 8- or 10-hour shifts instead of 12-hour shifts can also minimize fatigue and help to mitigate the negative health effects of shift work. Multiple factors need to be considered when developing optimal shift work schedules, including shift timing, length,