climb rate after an engine failure, MU-2 takeoff accident rates were reduced. Additionally, the MU-2 is sensitive to trim settings, and it is critical to promptly trim the aircraft properly in all phases of flight. The absence of adverse yaw eliminates the need to use rudder for coordinated flight, but proper and prompt use of rudder is vital to counter the aircraft's tendency to roll in reaction to engine torque; at low airspeed, the aircraft will rapidly roll and enter an accelerated stall if the pilot applies full power without adequate preparation, and safe recovery from this condition is very difficult at low altitude. Most crashes early in the type's service life were attributable to pilot error; however, in 1981, four uncontrolled descents from altitude prompted the United States National Transportation Safety Board (NTSB) to initiate separate investigations into the cabin pressurization system and autopilot, but the outcome was inconclusive. In 1983, after more crashes, the NTSB convinced the Federal Aviation Administration (FAA) to perform a more comprehensive study of various aircraft systems; the study ultimately did not fault the MU-2's design, but the repeated investigations had damaged the aircraft's reputation by this time. This prompted Mitsubishi, in cooperation with FlightSafety International, to initiate the Pilot's Review of Proficiency (PROP) seminar program to better educate MU-2 pilots about the aircraft's characteristics. The PROP seminars were suspended following the end of MU-2 production in 1986 but were reinstated in 1994 due to continuing accidents. The aircraft's accident rate caused resale values to fall and prompted Mitsubishi to campaign the FAA to require an MU-2 type rating. In 2005, the FAA undertook another safety evaluation of the MU-2 and concluded that a properly maintained MU-2 was safe when operated by well-trained pilot; however, the study also found that training standards were inconsistent, and sometimes recommended unusual or unauthorized procedures. In 2006, the FAA issued a Special Federal Air Regulation (SFAR) directed at MU-2 operations, with standards exceeding a type rating in some aspects. The training curriculum was standardized and pilots required to receive type-specific initial and recurrent training. A fully functional autopilot became a requirement for single-pilot operations, and FAA-approved checklists and operating manuals must be on board at all times. Unusual for this SFAR, pilot experience in other aircraft types cannot be used to comply with MU-2 operational requirements. By 2011, the SFAR was credited with lowering MU-2 accident rates below