The FAA has issued an airworthiness directive (AD) effective Aug. 13, 2009, for Pratt & Whitney Canada PW305As and PW305Bs installed on, but not limited to, Bombardier Learjet 60s and Hawker 1000s. The AD stems from mandatory continuing airworthiness information (MCAI) to identify and correct an unsafe condition on an aviation product. The MCAI states that P&WC has determined that the Post-Service Bulletin PW300-72-24287 high-pressure compressor (HPC) drum rotor assemblies on PW305A and 305B engines with single-stage coated labyrinth seals are susceptible to developing significant cracks in the region of the labyrinth seal, which may result in an uncontained failure of the drum rotor assembly and damage to the airplane. The AD calls for a borescope inspection of the interior of affected HPC rotor assemblies for cracks within 500 flight hours. The FAA estimates compliance will affect about 540 U.S.-registered engines, take approximately 10 work-hours and require about $5,000 in parts.