The FAA is proposing an airworthiness directive for variants of the Bombardier Challenger 600 and 601. There have been several reported incidents of jamming/malfunctioning of the left-hand engine thrust control mechanism on the affected airplanes. The investigation has shown that an improperly stowed or dislodged upper core cowl door hold-open rod can impede a fuel control unit function by obstructing the movement of the FCU actuating lever arm, rendering the left-hand engine thrust control inoperable.