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The FAA adopted a new airworthiness directive (AD) requiring operators of affected Diamond aircraft to revise their flight manuals, inspect door-latching and safety-hook systems, correct any problems found, and keep up with repetitive inspections going forward. The rule takes effect Dec. 7, 2026. The Diamond DA40 is a four-seat, low-wing, piston aircraft built mainly from fiber-reinforced plastic composite materials. Diamond Aircraft first flew the prototype on November 5, 1997, and the first production aircraft entered service in 2000. It is mainly used for personal flights and flight training. According to the agency, the AD was prompted by multiple reports of passenger door separation in flight. Diamond Aircraft Industries determined some occurrences involved improperly latched doors and attempts to secure them after takeoff, while other events did not have a definitive root cause. The FAA concluded the issue is a safety concern that could damage the aircraft and potentially contribute to loss of control. The directive applies to all DA40, DA40 F and DA40 NG aircraft on the U.S. registry. The FAA estimates the directive affects 987 U.S.-registered aircraft. Because the DA40 is widely used in training environments, utilization rates may be higher than rates on many owner-flown aircraft, increasing the practical impact of compliance requirements. The effect is amplified because the aircraft fly frequently and maintenance tracking systems must ensure the inspections remain current. The directive also includes potential replacement requirements if inspections identify problems involving components such as the gas spring, hook assembly, retaining block, guide blocks, or door sealant. What Operators Need to Know DA40 owners now have an additional recurring inspection requirement to budget for, track, and document. Flight schools and high-utilization operators will likely feel the greatest operational impact, while buyers and brokers should expect AD-compliance reviews involving door-latching inspections and associated corrective actions to become a routine part of DA40 transactions.