Sam - On props - if contra-rotating, then no critical engine. With turbine, apart from possible small torque effect, term does not really apply. Sometimes, if neither effect is a consideration, an engine may be considered 'critical' if, sy, it happens to drive an important system that cannot be driven by another engine. Probably quite rare on modern arcraft - but that is why at one stage, people used to get confused about the term 'critical', which now generally applies to the engine which, if it fails, will cause maximum yaw. So, on a piston or turbo-prop, whichever DOWNGOING prop blade is further from the longitudinal axis of the aircraft, compared to its counterpart on the other side will be 'critical'. If you are in a single - then that is the critical engine but not really for the same reason, although I suppose it is since it does affect the handling carachteristics of the aircraft - if it fails, you go in a downward direction <img border="0" title="" alt="[Big Grin]" src="biggrin.gif" />