The Critical Engine

The engine whose failure has the most adverse effect on directional control (AFH ch. 13).

That's the whole definition — it's about controllability, not about which engine drives the hydraulics or which one you'd miss more. The AFM/POH-published VMC is determined with the critical engine inoperative.

The left engine is critical on conventional twins, where each engine rotates clockwise as viewed from the pilot's seat (AFH ch. 13).

The reason is P-factor and the moment arm it produces:

• The descending propeller blade of each engine produces greater thrust than the ascending blade when operating under power at positive angles of attack
• The descending blade of the right engine is a greater distance from the CG — a longer moment arm — than the descending blade of the left engine
• So the right engine's thrust acts farther out on the wing

Therefore failure of the left engine produces the most asymmetrical thrust (adverse yaw), because the remaining thrust comes from the right engine, acting on the longer arm (AFH ch. 13, PHAK ch. 2).

Asymmetric loading of the propeller — one of the four left-turning tendencies of a propeller-driven airplane, along with torque reaction, the corkscrew effect of the slipstream, and gyroscopic action (PHAK ch. 5).

At positive angles of attack the descending blade meets the relative wind at a higher effective angle of attack than the ascending blade, so it produces more thrust, and the propeller's thrust center shifts toward the descending-blade side. Multiengine airplanes are subject to P-factor just as single-engine airplanes are (AFH ch. 13).

No — the asymmetry appears when the airplane is operated under power and at positive angles of attack (AFH ch. 13). That's exactly the regime of takeoff and initial climb, which is why the critical engine matters most at the moment it matters most.

Not through the CG — out on the wing, and displaced further by P-factor toward the descending blade side of that propeller disc.

• Right engine operating — descending blade is on the right side of the right propeller, the side farther from the CG. Long arm, large yawing moment
• Left engine operating — descending blade is on the right side of the left propeller, the side nearer the CG. Shorter arm, smaller yawing moment

Longer arm means more yaw for the same thrust. Losing the left engine leaves you fighting the larger moment (AFH ch. 13).

Many twins use a counter-rotating right engine, which removes the critical engine entirely:

• The degree of asymmetrical thrust is the same with either engine inoperative
• No engine is more critical than the other
• A VMC demonstration may be performed with either engine windmilling (AFH ch. 13, PHAK ch. 2)

The descending blades of both propellers end up equidistant from the CG, so neither engine's thrust enjoys the longer moment arm.

No. It still has a published VMC and a red radial line — it simply has no critical engine, because either failure is equally adverse. Everything in the certification-conditions discussion still applies, and losing either engine still costs 80 to 90 percent of climb performance (AFH ch. 13).

Dead foot — dead engine.

In maintaining directional control, rudder pressure is exerted on the side of the airplane with the operating engine. So the foot that isn't working is on the same side as the engine that isn't working (AFH ch. 13).

Identification should be primarily through the control inputs required to maintain straight flight, not the engine gauges — depending on the failure mode, confirmation on the gauges may or may not be possible (AFH ch. 13). Variations on the phrase: "idle foot — idle engine," "working foot — working engine."

Because the recovery from a misidentification is worse than the original failure. The PHAK's case study: a pilot on final assumed a left-engine failure, feathered the good left engine, and set the right at zero thrust — restricting the aircraft to a controlled glide. Realizing he wouldn't make the runway, he added power to both, which caused an enormous yaw to the left with the left propeller feathered, and the aircraft began to turn (PHAK ch. 2).

The first step is identify. There is no shortcut past it.

No — those are systems consequences, and they matter operationally, but they are not what makes an engine critical. The definition is strictly about directional control: the engine whose failure has the most adverse effect on directional control (AFH ch. 13).

That said, know your airplane's systems asymmetries too. On some airplanes with a single engine-driven hydraulic pump, failure of that engine means the only way to raise the landing gear is to allow the engine to windmill or use a hand pump — which the AFH calls not a viable alternative during takeoff, and which can cost hundreds of feet of altitude (AFH ch. 13).

The four (PHAK ch. 5):

1. Torque reaction from engine and propeller
2. Corkscrewing effect of the slipstream
3. Gyroscopic action of the propeller
4. Asymmetric loading of the propeller (P-factor)

Of these, both the AFH's critical-engine discussion and the PHAK's tie the identification of the critical engine specifically to P-factor and the resulting difference in moment arm (AFH ch. 13, PHAK ch. 2). If you offer a longer list on the oral, be ready to defend it — and lead with P-factor, because that is the explanation the handbooks give.

The high-speed rotation of the propeller gives a corkscrew or spiraling rotation to the slipstream. At high propeller speed and low forward speed — takeoff, approaches to power-on stalls — the spiral is very compact and exerts a strong sideward force on the vertical tail, causing a yawing moment about the vertical axis.

As forward speed increases, the spiral elongates and becomes less effective. The corkscrew flow also causes a rolling moment about the longitudinal axis — to the right, while torque reaction's yaw is to the left, so one may partly counteract the other (PHAK ch. 5).

Newton's third law applied to the airplane: as the internal engine parts and propeller revolve one way, an equal force tries to rotate the aircraft the other way. Airborne, this acts about the longitudinal axis, tending to roll the aircraft (PHAK ch. 5).

On the ground during the takeoff roll it adds a yawing moment: as the left side is forced down, more weight is on the left main gear, producing more ground friction on the left tire and a further turning moment to the left (PHAK ch. 5).

Note: most US-built aircraft engines rotate the propeller clockwise as viewed from the pilot's seat — the whole left-turning discussion assumes that (PHAK ch. 5).

The procedure doesn't change; the symmetry does. A VMC demonstration may be performed with either engine windmilling, since the degree of asymmetrical thrust is the same either way (AFH ch. 13).

What does not change: VMC still rises with reduced bank, aft CG, lighter weight, a windmilling propeller, and high power on the operating engine — every certification condition from the VMC section still governs your actual red line (AFH ch. 13).

Slightly, yes — if the airplane is not equipped with counter-rotating propellers, the precise zero-sideslip condition varies slightly with the engine failed, due to P-factor (AFH ch. 13).

The AFH also notes these variations are difficult to detect without more sensitive testing equipment, so the practical target remains the same: roughly 2 degrees of bank toward the operating engine with the ball one-third to one-half out toward it.