Why does a light twin lose so much climb performance when one engine fails?

Two factors combine to devastate climb performance: the failed engine produces no thrust while its windmilling propeller creates enormous drag, and the operating engine must overcome that drag before any net climb force is available. The result is that many light twins have very little—or even zero—single-engine climb performance at gross weight in warm or high-altitude conditions, far less than the 50% many pilots intuitively expect.

Why is the left engine the critical engine on a conventional twin, not the right?

On a conventional light twin, both propellers rotate clockwise as viewed from the pilot's seat. P-factor causes the descending blade of each engine to produce more thrust, and on the right engine that descending blade is farther from the aircraft's centerline—giving it a longer yawing moment arm. Losing the left engine leaves the powerful right engine operating with maximum leverage, creating the worst asymmetric yaw. This makes the left engine the critical engine whose failure most adversely affects directional control and handling.

At what speed should I climb after an engine failure on a twin, and what bank angle helps most?

Accelerate to and maintain Vyse—best single-engine rate of climb speed, marked by the blue line on your airspeed indicator—as it gives the greatest altitude gain per minute with one engine out. For the best actual climb gradient, establish approximately 2 degrees of bank into the operating engine combined with enough rudder to eliminate sideslip (the zero-sideslip technique); this minimizes total drag better than wings-level or larger bank angles, squeezing the maximum performance out of the available single-engine power.

Engine-Out Climb Performance and the Windmilling Propeller Drag Penalty

Flying a light twin with both engines running feels like a capable, redundant aircraft. Lose one engine, however, and the airplane transforms almost instantly into a different machine—one with far more drag, asymmetric thrust, and a dramatically reduced ability to climb. The windmilling propeller on the failed engine is a major villain in that story, and understanding its drag penalty alongside the key one-engine-inoperative (OEI) speeds is the foundation of sound multi-engine airmanship.

This article examines why engine-out climb performance is so limited, what a windmilling propeller actually does aerodynamically, how pilots can optimize whatever performance remains, and the key speeds and rules every multi-engine applicant must know cold.

What Happens the Moment an Engine Fails

When an engine loses power in flight, the propeller attached to it does not simply stop. Ram air pressure keeps it spinning—often at surprisingly high RPM—in what is called a windmilling condition. A large, flat spinning disc presents tremendous aerodynamic resistance to the airflow. According to the FAA Airplane Flying Handbook (FAA-H-8083-3C, Chapter 13), a windmilling propeller creates so much drag that it can be compared, in terms of total drag increase, to a very large barn door suddenly opened into the slipstream. The drag from a windmilling prop is dramatically greater than the drag of a feathered propeller, whose blades are rotated edge-on to the wind to minimize resistance.

At the same time, the airplane has lost roughly half its thrust—but not half its performance. Because drag increases so steeply at low speeds and total thrust is now asymmetric, the actual climb performance available with one engine out in a light twin is far less than 50 percent of the two-engine capability. Many light twins, when loaded near gross weight on a hot day, have zero or even negative single-engine climb rate. This sobering reality is what pilots mean when they say a twin's second engine sometimes only flies you to the scene of the accident.

The Drag Penalty: Windmilling vs. Feathered

The difference in drag between a windmilling and a feathered propeller is not subtle. A windmilling propeller can generate parasite drag equivalent to or exceeding the drag of the entire airframe at cruise speed. Feathering the propeller—rotating the blades so their chord line is parallel to the relative wind—reduces that drag by a dramatic margin, recovering a significant portion of lost climb performance.

This is why the memory items for engine failure in most multi-engine aircraft include feathering the propeller on the failed engine as a high-priority step, after confirming which engine has actually failed and securing it. Leaving the propeller windmilling while troubleshooting for an extended period extracts a severe performance penalty at the moment when every foot-per-minute of climb rate is precious.

A fixed-pitch propeller cannot be feathered—on aircraft so equipped, the pilot must accept the windmilling drag as a permanent penalty, which makes the performance situation significantly worse than on a constant-speed, featherable-propeller installation. Most training twins and all transport-category twins use constant-speed, full-feathering propellers for this reason.

Asymmetric Thrust and the Critical Engine

Beyond the raw drag penalty, one-engine-inoperative flight introduces a powerful yawing and rolling moment. The operating engine continues producing full thrust off to one side of the centerline while the failed engine side produces only drag. On a conventional light twin, both propellers rotate clockwise as viewed from the pilot's seat. Because of P-factor, the descending blade of each engine produces more thrust than the ascending blade. On the right engine, the descending (thrust-producing) blade is farther from the aircraft centerline, giving it a longer moment arm and a stronger yawing effect.

This asymmetry designates the left engine as the critical engine—the one whose failure most adversely affects performance and handling. Losing the left engine leaves the right engine operating with its maximum yawing leverage, creating the worst-case directional control problem. Pilots must never assume the right engine is critical on a conventional twin; the left engine is always the critical one. Counter-rotating propellers (where the left and right engines turn in opposite directions) eliminate the critical-engine concept entirely, because both engines have equal and opposite yawing moments.

Vmc: The Directional Control Boundary

Vmc is the calibrated airspeed at which, with the critical engine suddenly made inoperative, directional control can be maintained and straight flight held with a bank angle of not more than 5 degrees into the operating engine. Vmc is a directional control standard only—it says nothing about climb capability. It is established under 14 CFR Part 23 certification requirements.

Critically, Vmc is determined at the most unfavorable weight and CG. The most unfavorable weight is the lightest weight—not maximum gross weight. This surprises many students, but the explanation is straightforward: when banked up to 5 degrees into the operating engine, the vertical component of the lift vector helps the rudder counteract yaw. A heavier aircraft generates more lift and therefore more of this helpful vertical component, making control easier. A lighter aircraft generates less lift, so the rudder must work harder, and Vmc is higher. The most unfavorable CG is the aft limit, which reduces the rudder's moment arm.

Vmc also increases with: maximum available takeoff power on the operating engine, landing gear retracted, and decreasing bank angle. Banking 5 degrees into the operating engine can lower Vmc by approximately 3 knots per degree of bank between 0 and 5 degrees. Zero bank (wings level) yields the highest Vmc; 5 degrees of bank into the good engine yields the lowest published Vmc. Standard published Vmc assumes a windmilling (not feathered) critical-engine propeller and operations out of ground effect.

If airspeed decays below Vmc with an engine out and full power on the operating engine, the rudder can no longer maintain directional control. The correct immediate response is to reduce power on the operating engine to eliminate the asymmetric thrust, and simultaneously lower the nose to accelerate—this is not a last resort but the primary immediate action to regain control.

Key OEI Performance Speeds

Vyse (Blue Line): Best single-engine rate of climb speed. Marked with a blue radial line on the airspeed indicator, it is the speed that produces the greatest altitude gain per unit of time with one engine inoperative. Always accelerate to and maintain Vyse when climbing OEI.
Vxse: Best single-engine angle of climb speed. Used when obstacle clearance is the priority; provides the greatest altitude gain per unit of distance. Lower than Vyse.
Vsse: Safe single-engine speed. The minimum speed for intentional engine cuts in training, providing an adequate margin above Vmc to allow safe demonstration of OEI procedures.
Zero-sideslip technique: For best OEI climb performance, establish approximately 2 degrees of bank into the operating engine combined with just enough rudder to eliminate the ball deflection and ball-induced sideslip. This zero-sideslip configuration minimizes total drag and yields the best available climb gradient—better than either wings-level (which requires heavy rudder and creates sideslip drag) or a large bank angle (which reduces lift).

Why It Matters: Performance, Not Just Control

Pilots tend to fixate on the control challenge of engine-out flight, but the performance challenge may be more consequential. A light twin at maximum gross weight on a warm, high-elevation airport may have a published single-engine ceiling below field elevation—meaning it cannot sustain level flight at all with one engine out, regardless of technique. Knowing this ahead of time through weight-and-performance planning is the only real mitigation; there is no in-flight solution if the numbers do not work.

Proper technique—feather immediately, identify before you verify, accelerate to Vyse, establish zero-sideslip—extracts the maximum available performance from a difficult situation. Even small errors, like leaving the propeller windmilling too long or flying with excessive bank, can consume the narrow margin between a successful single-engine climb and a controlled descent into terrain.

Key Numbers and Rules

Critical engine on a conventional twin: the left engine (right engine's descending blade has the longest moment arm).
Vmc bank: not more than 5 degrees into the operating engine; each degree lowers Vmc by roughly 3 knots.
Most unfavorable weight for Vmc certification: lightest weight, not maximum gross.
Most unfavorable CG: aft limit.
Best OEI climb speed: Vyse (blue line on ASI).
Optimal OEI climb bank: approximately 2 degrees into the operating engine (zero-sideslip).
Loss of control below Vmc: immediately reduce power on the good engine and lower the nose.

Common Test Traps

Vmc is not a climb speed. It governs directional control only. Maintaining Vmc does not guarantee any positive climb rate—Vyse is the climb speed.
Vmc is highest at light weight. Many students incorrectly believe Vmc is highest at heavy weight. The opposite is true because the helpful lift-vector component is smaller when the airplane weighs less.
The left engine is critical, not the right. Examiners specifically probe this. The right engine's operating thrust line has a longer moment arm, so losing the left is always worse on a conventional twin.
Windmilling vs. feathered drag. A windmilling propeller creates far more drag than a feathered one. Do not underestimate how severely this limits single-engine climb—feathering is urgent, not optional.
Response to loss of control below Vmc is to reduce power, not add back-pressure. Pulling back at or below Vmc worsens the situation; the correct response is to unload the asymmetric thrust by reducing power on the good engine while lowering the nose to accelerate.