Critical Engine:
That engine in a twin, which when failed, causes the most control difficulties.
It depends on which way the props rotate. For various aerodynamic reasons if the props both rotate clockwise when viewed from the cockpit - as in US manufactured a/c - failing the left engine will produce much greater control problems than the right. For Props rotating counterclockwise - older UK/European a/c - the right engine is Critical.
Some training twins, eg the Seminole have counter rotating props and thus no Critical Engine.
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