The FAA proposes to adopt a new airworthiness directive (AD) for certain Honeywell TPE331-5, -5A, -5AB, -5B, -10, -10R, -10U, -10UF, -10UG, -10UGR, and -10UR turboprop engines. It was prompted by engine propeller shaft coupling failures, leading to unexpected propeller pitch changes and resulting in high aerodynamic and asymmetric drag on the airplanes using these engines. This proposed AD would require removing from service certain part number propeller shaft couplings. It would also require inserting a copy of Honeywell International Operating Information Letter OI331-26, dated March 2, 2010, into the applicable airplane flight manuals. The agency is proposing this AD to prevent loss of airplane control, which could lead to an accident.