Loss of power on one engine reduces prop wash over the wing. Yaw also affects the lift distribution over
the wing causing a roll toward the "dead" engine. (See Figure E-4.) These roll forces may be balanced by
banking into the operating engine.
The critical engine is that whose failure would most adversely affect the performance or handling qualities
of the airplane. The critical engine on most U.S. light twins is the left engine, as its failure requires the
most rudder force to overcome yaw. At cruise, the thrust line of each engine is through the propeller hub.
But, at low airspeeds and at high angles of attack, the effective thrust centerline shifts to the right on each
engine (see figure E-5) because the descending propeller blades produce more thrust than the ascending
blades (P-factor). Thus, the right engine produces the greatest mechanical yawing moment and requires the
most rudder to counterbalance yaw.