20.2 Multiengine Knowledge: VMC, VYSE, and Engine-Out Fundamentals

The Private Pilot Airplane Category ACS includes AMEL and AMES, so Area IX Tasks E, F, and G and all four Tasks of Area X carry multiengine knowledge elements. Even a candidate who will never touch a twin should own this vocabulary — it recurs on every later FAA knowledge test.

The two radial lines and the speed that is not marked

The critical engine

The critical engine is the one whose failure most adversely affects the performance or handling of the airplane. On a conventional U.S. twin with both propellers rotating clockwise as seen from the cockpit, the left engine is critical.

The reason is P-factor (Section 10.2). At a positive angle of attack, each propeller's descending blade generates more thrust, and the descending blade of a clockwise-turning propeller is on the right side of that propeller. So each engine's effective thrust line is displaced to the right of its own centerline. That puts the right engine's thrust on a longer moment arm about the airplane's center of gravity, and the left engine's on a shorter one. Lose the left engine and the remaining — right — engine produces the larger yawing moment, which is the more critical case.

Twins with counter-rotating propellers have no critical engine, because the moment arms are symmetric.

What raises and lowers VMC

VMC is a certification number determined under deliberately unfavorable conditions, and the test asks which way it moves.

• Density altitude up → VMC down (normally aspirated engines). Higher up, the operating engine makes less power, so there is less asymmetric thrust to counter. This is the trap: at altitude the airplane may reach its stall speed before it reaches VMC, and a stall with asymmetric thrust rolls toward the dead engine and can spin.
• CG moves aft → VMC up. An aft CG shortens the moment arm from the CG to the rudder, so the rudder has less authority.
• Propeller windmilling → VMC up. A windmilling propeller adds drag on the dead side, increasing the yawing moment. Feathering lowers it.
• Bank up to 5 degrees toward the operating engine → VMC down. Wings level with the ball centered gives the highest VMC.

Zero sideslip — the configuration that actually climbs

After an engine failure, three configurations are possible, and only one performs:

1. Wings level, ball centered. The airplane flies in a large sideslip. Huge drag penalty; often no climb at all.
2. Wings level, rudder holding heading. Still sideslipping badly.
3. Zero sideslip: approximately 2 degrees of bank toward the operating engine, with the inclinometer ball about one-half width toward the operating engine. Drag is minimized and the airplane delivers the single-engine climb performance the book promises.

Two degrees is barely perceptible — it is a sliver of bank, not a turn.

Identify, verify, feather, secure

The engine-failure flow in Tasks IX.F, IX.G, X.A:

1. Identify. "Dead foot, dead engine" — the foot that is not pushing rudder is on the side of the failed engine.
2. Verify. Slowly retard the throttle of the suspected engine. If nothing changes, you identified correctly.
3. Feather. Move that propeller control to feather — the blades rotate to a high, streamlined pitch nearly parallel to the relative wind, which converts an enormous drag disc into a thin one. Most light twins have a centrifugal latch that blocks feathering below roughly 800 RPM, so the propellers do not feather on every normal shutdown.
4. Secure. Mixture, fuel, magnetos, alternator, cowl flaps per the checklist.

Drag reduction (PA.IX.F.K5, PA.IX.G.K4) is the whole game: gear up, flaps up, propeller feathered, and the airplane held at zero sideslip. A windmilling propeller alone can cost more drag than the entire rest of the airframe.

Accelerate-stop, accelerate-go, and the honest performance picture

• Accelerate-stop distance is the total distance to accelerate to a specified speed and then, with an engine failure at that speed, brake to a full stop on the remaining surface.
• Accelerate-go distance is the distance to accelerate to that speed, lose an engine, and continue the takeoff to a height of 50 feet on the remaining engine. It is normally far longer than accelerate-stop, and many light twins cannot do it at all at high weight or high density altitude.

Losing one of two engines does not cost 50 percent of climb performance. Climb comes from excess power, and the excess is a small fraction of total power — so losing half the installed power typically costs roughly 80 percent or more of the climb capability. That is why a light twin's single-engine service ceiling can be below the terrain it is crossing, and why "the second engine takes you to the scene of the accident" is a saying rather than a joke.

Loss of directional control below VMC (PA.X.B.K3) has exactly one immediate answer: reduce power on the operating engine and lower the nose to accelerate. Reducing the asymmetric thrust restores control. Adding power to climb away from the ground makes the roll worse.