The critical engine is the engine on a multi-engine airplane whose failure would be the hardest to control. On most twin-engine propeller aircraft with both propellers spinning clockwise (as seen from the cockpit), the critical engine is the left engine. Losing the left engine leaves only the right engine running, and the right engine produces a stronger yawing force that tries to swing the airplane’s nose toward the dead side. Understanding why comes down to four aerodynamic factors that pilots memorize with the acronym PAST.
Why One Engine Failure Is Worse Than the Other
When an engine quits on a twin, the remaining engine keeps producing thrust, but that thrust is offset from the airplane’s centerline. This creates a turning force (a yaw) toward the dead engine. The pilot has to push hard on the rudder pedal to keep the airplane flying straight. The critical engine concept recognizes that this turning force isn’t equal depending on which engine fails. One side is consistently harder to manage.
On a conventional twin where both propellers rotate clockwise, the right engine’s thrust line sits farther from the airplane’s center of gravity than the left engine’s thrust line. That greater distance creates a longer lever arm, which means more yawing force. So when the left engine fails and only the right engine remains, the pilot faces a bigger control challenge than if the right engine had failed instead. The left engine is “critical” because its loss produces the worst-case scenario.
The Four Factors Behind It: PAST
Four aerodynamic effects combine to make one engine’s failure worse. Pilots use the acronym PAST: P-factor, accelerated slipstream, spiraling slipstream, and torque.
P-Factor (Asymmetric Blade Effect)
When an airplane climbs with its nose pitched up, the descending propeller blade (swinging downward on one side) takes a bigger bite of air than the ascending blade on the other side. This means the propeller doesn’t produce thrust evenly across its disk. Instead, the thrust shifts toward the descending blade side. On a clockwise-spinning propeller, the descending blade is on the right side of the disk, so the effective center of thrust moves to the right of the propeller hub.
For the right engine, this shift pushes the thrust line farther from the airplane’s centerline, increasing the lever arm. For the left engine, the shift actually moves the thrust line closer to the centerline, shortening the lever arm. The result: when the left engine fails and the right engine keeps running, P-factor makes the yaw worse than it would be if the right engine had failed. This single effect is the primary reason the left engine is designated critical.
Accelerated Slipstream
The blast of air behind a spinning propeller (the slipstream) flows over the wing section directly behind it. This high-speed airflow increases the lift on that portion of the wing. Research from U.S. Army aviation testing found that propeller slipstream can increase wing lift by as much as 57%, with the swirling air creating differences in local angle of attack of 10 to 20 degrees on either side of the propeller.
When one engine quits, you lose that extra lift on the dead engine’s wing. The wing on the operating engine’s side keeps producing boosted lift, creating a rolling tendency toward the dead side. Because the right engine’s thrust effects are already more pronounced (due to P-factor shifting its thrust outward), the lift imbalance and resulting roll are more severe when the left engine is the one that fails.
Spiraling Slipstream
Air behind a spinning propeller doesn’t just flow straight back. It corkscrews around the fuselage in the same direction the propeller rotates. This spiraling air eventually strikes the vertical tail (the fin and rudder) on one side. On an airplane with clockwise-spinning propellers, the slipstream from the left engine wraps around and hits the left side of the tail, which actually helps push the nose back toward center when the right engine fails. The right engine’s slipstream, by contrast, hits the tail in a way that’s less helpful. This makes controlling the airplane slightly harder when only the right engine is running, reinforcing the left engine’s status as critical.
Torque
Newton’s third law means that for every spinning propeller trying to rotate clockwise, the airplane’s airframe experiences an equal force trying to roll it in the opposite direction. With both engines running, these forces are present but manageable. When one engine quits, only the operating engine’s torque remains, rolling the airplane toward the dead engine’s side. Combined with the other three factors, this rolling tendency is more severe when the right engine is the one still running.
How This Affects Minimum Control Speed
The critical engine directly determines one of the most important speeds a multi-engine pilot needs to know: Vmc, or minimum control speed. This is the slowest airspeed at which the pilot can still maintain directional control after the critical engine suddenly fails. Below Vmc, there isn’t enough airflow over the rudder to counteract the yaw from the remaining engine.
FAA certification standards require that Vmc be determined under worst-case conditions: maximum takeoff power on the operating engine, the most unfavorable weight and center of gravity position, landing gear retracted, and flaps in the takeoff position. The pilot must be able to keep the airplane from turning more than 20 degrees off heading, and the force required on the rudder pedal can’t exceed 150 pounds. A bank angle of no more than 5 degrees toward the operating engine is allowed to help maintain control.
Vmc for takeoff cannot exceed 1.2 times the stall speed at maximum takeoff weight. This ensures the airplane is controllable throughout the takeoff and climb, even if the critical engine quits at the worst possible moment.
Counter-Rotating Propellers Eliminate the Critical Engine
Some twin-engine aircraft are designed with propellers that spin in opposite directions: the left engine’s propeller rotates clockwise while the right engine’s propeller rotates counterclockwise (or vice versa). This is called counter-rotation, and it’s specifically designed to balance out the asymmetric effects described above.
With counter-rotating propellers, P-factor shifts each engine’s thrust line by the same amount in opposite directions, so both engines sit at equal distances from the centerline. The slipstream, torque, and lift effects also mirror each other symmetrically. The result is that losing either engine produces the same yaw and roll forces. Neither engine is more critical than the other, so these aircraft technically have no critical engine. Every multi-engine pilot training scenario becomes identical regardless of which engine fails.
Aircraft like the Piper Seminole use counter-rotating propellers for this reason, making them popular training platforms. Airplanes with both propellers spinning the same direction, like many Beechcraft Barons, retain a true critical engine and require pilots to train specifically for the worse-case left-engine failure.