PAST and the Critical Engine
PAST is a common memory aid for four aerodynamic effects used to explain why one engine may be critical on a conventional twin: P-factor, accelerated slipstream, spiraling slipstream and torque. The idea is easiest to understand by asking one question: if either engine fails, which failure creates the more unfavorable combination of yaw and reduced control?
P-factor
At high angle of attack, the descending propeller blade can produce more thrust than the ascending blade. On a twin with both propellers rotating clockwise as viewed from the cockpit, the thrust line of the right engine can act farther from the airplane centerline than the thrust line of the left engine. If the left engine fails, the operating right engine can therefore create a larger yawing moment. That helps make the left engine critical on many same-direction twins.
Accelerated slipstream
The operating propeller accelerates airflow over parts of the wing. If the remaining engine's accelerated slipstream produces lift farther from the centerline, that lift can create an additional rolling moment. The exact effect depends on airplane geometry and propeller rotation.
Spiraling slipstream
Propeller wash does not travel straight aft; it spirals. On some twins, the slipstream from one engine interacts with the vertical tail more favorably than the other. Losing the engine that provided the more favorable tail interaction can make directional control more difficult.
Torque
Newton's third law means the airplane reacts opposite the direction of propeller rotation. Depending on which engine remains operating, torque can either add to or oppose the roll associated with the engine-out condition.
Why counter-rotating twins are different
On airplanes designed with counter-rotating propellers, the descending blades can be positioned symmetrically closer to the fuselage. That can make either engine failure aerodynamically similar enough that there is no single critical engine. This is one reason some training twins are designed that way.
How PAST connects to VMC
The critical engine is the engine whose failure most adversely affects performance or handling. Because VMC is fundamentally a directional-control problem, anything that increases the yawing moment after one engine fails can affect the control speed. Continue with VMC factors, critical engine explained, and the multi-engine checkride study guide.
FAA source
The FAA Airplane Flying Handbook discusses critical-engine aerodynamics in the multiengine transition chapter. Open the current handbook.