Counter-Rotating Twins and the Absence of a Critical Engine

Many modern training twins are equipped with a counter-rotating right engine. In this configuration, the propellers rotate in opposite directions, eliminating the traditional “critical engine” effect seen in conventional twins.

With counter-rotating engines, the degree of asymmetric thrust is essentially the same regardless of which engine becomes inoperative. No engine produces a disproportionately greater yawing moment, and therefore no engine is aerodynamically more critical than the other. From a certification and training standpoint, a VMC engine inoperative demonstration may be conducted with either engine windmilling.

This design significantly improves controllability characteristics and reduces the severity of asymmetric yaw following an engine failure. However, it does not eliminate the need to understand VMC factors. Even without a critical engine, VMC engine inoperative limits still vary with configuration, power, and CG position.

Certification Conditions Under 14 CFR §23.149

The published VMC in the AFM/POH is determined under specific certification conditions historically defined in 14 CFR Part 23, §23.149. These conditions intentionally create a worst-case controllability scenario. Understanding them is essential to interpreting the meaning of the red line on the airspeed indicator.

Maximum Available Takeoff Power

During certification, maximum available takeoff power is initially set on each engine. When the critical engine is made inoperative, the operating engine remains at full takeoff power.

VMC engine inoperative speed increases as power on the operating engine increases. The greater the asymmetric thrust, the greater the yawing moment that must be counteracted by rudder and bank.

For normally aspirated engines:

• VMC is highest at sea level and maximum takeoff power.

• As altitude increases, engine power decreases, and VMC decreases accordingly.

For turbocharged engines:

• Takeoff power can be maintained up to the engine’s critical altitude.

• VMC remains relatively constant up to that altitude.

• Above critical altitude, VMC decreases similarly to normally aspirated engines.

Because VMC varies with altitude and power, certification testing is conducted at multiple altitudes. The results are then extrapolated to produce a single sea-level published VMC value.

Propeller in Takeoff Position (Windmilling Drag)

During VMC determination, all propeller controls are placed in the recommended takeoff position. The inoperative engine’s propeller is allowed to windmill in low pitch/high RPM configuration unless an autofeather system is installed.

This configuration creates maximum drag on the failed engine. Increased drag raises VMC engine inoperative speed because additional rudder force is required to counteract the combined effects of asymmetric thrust and windmilling resistance.

A feathered propeller significantly reduces drag and therefore reduces real-world controllability speed. This is why feathering is not just a checklist action — it directly improves single-engine controllability.

Most Unfavorable Weight and Center of Gravity

Certification standards require VMC to be determined at the most unfavorable weight and center-of-gravity position.

VMC engine inoperative speed increases as:

• Center of gravity moves aft

• Aircraft weight decreases

An aft CG reduces the rudder’s effective moment arm. With less leverage available to counteract yaw, directional control is degraded and VMC increases.

Weight influences VMC in the opposite direction many pilots expect. As weight decreases, the airplane requires less lift. Because less lift is required, less bank angle is needed to maintain altitude. Reduced bank decreases the horizontal lift component available to counteract asymmetric thrust. As a result, VMC increases as weight decreases.

For most light twins, the aft-most CG limit represents the most unfavorable controllability condition.

[Figure 2]

Stall Risk During VMC Engine Inoperative Conditions

If a stall occurs while the airplane is under asymmetric power, a spin entry becomes highly likely. The yawing moment produced by asymmetric thrust closely resembles the yaw induced by full rudder in an intentional spin entry in a single-engine aircraft. The difference, however, is critical: in a multiengine airplane operating under VMC engine inoperative conditions, the aircraft will depart controlled flight toward the idle engine — not necessarily in the direction of applied rudder.

This distinction is vital. Most light twins are not required to demonstrate spin recovery during certification, and their spin recovery characteristics are often poor. Once a twin departs into a spin under asymmetric thrust, recovery may be extremely difficult or impossible.

When stall speed (VS) is encountered before VMC, the departure from controlled flight may be abrupt and violent. Strong yawing and rolling moments can rapidly develop into an inverted attitude and spin entry. This risk increases with altitude, as VMC engine inoperative speed decreases with altitude (normally aspirated engines), while stall speed remains relatively constant.

During any VMC demonstration, the appearance of stall warning indications — horn, light, buffet, or sudden degradation in control effectiveness — requires immediate recovery. The correct response is to:

• Reduce angle of attack

• Simultaneously reduce power on the operating engine

• Regain airspeed and reestablish directional control

It is essential to understand that the VMC demonstration is a controllability exercise, not a performance exercise. Allowing the maneuver to degrade into a single-engine stall under high asymmetric thrust creates a severe and potentially fatal loss-of-control scenario.

Modified VMC Demonstrations in High Density Altitude Conditions

Under certain density altitude conditions, or in aircraft where VMC equals or is lower than VS, a full VMC engine inoperative demonstration may not be possible or safe.

In such cases, instructors may use a rudder-limiting technique. This method artificially restricts rudder travel to simulate the point of maximum available rudder authority while maintaining airspeed well above stall speed — typically at least 20 knots above VS.

This approach avoids the spin hazard associated with asymmetric stall conditions while still demonstrating the concept of directional control limits.

To reduce risk further, VMC demonstrations should never begin from a high pitch attitude with both engines producing high power followed by an abrupt power reduction on one engine. This technique resembles dynamic certification testing and dramatically increases loss-of-control risk.

OEI Climb Performance and Zero Sideslip

Understanding OEI (One Engine Inoperative) climb performance is where many multiengine pilots either gain true proficiency — or develop misconceptions.

Best OEI climb performance is achieved at VYSE (blue line speed) with:

• Maximum available power on the operating engine

• Failed engine propeller feathered

• Landing gear retracted

• Flaps retracted

• Minimum drag configuration

• Zero sideslip

Minimizing sideslip is one of the most important elements of engine-out performance.

Zero Sideslip: The Hidden Key to Performance

In coordinated two-engine flight, zero sideslip occurs when the ball is centered. The airplane presents its smallest frontal area to the relative wind, minimizing drag.

However, during VMC engine inoperative operations, a centered ball no longer guarantees zero sideslip. Asymmetric thrust distorts the relationship between ball position and relative wind.

There is no cockpit instrument that directly indicates zero sideslip during OEI flight.

A yaw string — a simple piece of yarn taped to the windshield — is actually the most accurate indicator of relative wind direction. When the string aligns with the aircraft centerline, true zero sideslip is achieved.

In the absence of a yaw string, pilots must use a predetermined combination of bank angle and rudder input specific to the aircraft type. This technique replicates the conditions under which AFM/POH OEI performance charts were derived — at zero sideslip.

Why Rudder Alone or Bank Alone Is Incorrect

There are two forces available to counteract asymmetric thrust:

1. Rudder-generated yaw force

2. Horizontal component of lift produced by bank

Used independently, neither method is optimal.

Scenario 1: Wings Level, Ball Centered

Maintaining wings level while centering the ball requires significant rudder input toward the operative engine. This creates moderate sideslip toward the inoperative engine.

Consequences:

• Increased drag

• Reduced OEI climb performance

• Higher VMC engine inoperative speed

Without bank, no horizontal lift component assists rudder in opposing yaw. All directional correction comes from rudder deflection, increasing drag and reducing performance margin.

[Figure 4]

Отлично, продолжаем — усиливаем объяснение, добавляем больше анализа и аккуратно внедряем ключевые фразы VMC engine inoperative, OEI climb performance, zero sideslip technique.

2. Excessive Bank Without Rudder Input

Engine inoperative flight using ailerons alone typically requires an 8–10° bank angle toward the operative engine to counteract asymmetric thrust. In this configuration, little or no rudder input is used. The inclinometer ball will be displaced significantly toward the operative engine.

At first glance, this may appear to stabilize the airplane. However, this technique produces a large sideslip toward the operative engine and dramatically increases drag. As a result, OEI climb performance deteriorates severely.

The aerodynamic consequences are substantial:

• Increased parasite drag due to fuselage misalignment with the relative wind

• Increased induced drag from excessive bank angle

• Reduced vertical lift component due to bank

• Higher effective VMC engine inoperative speed

Because climb performance in a twin depends on excess power, even modest increases in drag can eliminate the small performance margin available after engine failure. In many light twins, that margin is already narrow.

Instructors generally avoid demonstrating this configuration because:

• Large bank angles increase stall speed

• Asymmetric power combined with high bank increases loss-of-control risk

• The configuration does not represent real-world best performance technique

This method illustrates an important principle: bank alone is not a solution to asymmetric thrust.

[Figure 5]

3. Proper Rudder and Aileron Coordination — Establishing Zero Sideslip

When rudder and ailerons are applied together in the correct aerodynamic balance, the aircraft settles into a slight bank of approximately 2° toward the operative engine. In this configuration, the inclinometer ball is displaced roughly one-third to one-half of its width toward the operating engine.

This attitude establishes zero sideslip, the condition in which the longitudinal axis of the airplane aligns with the relative wind despite asymmetric thrust. Zero sideslip is the configuration that produces maximum OEI climb performance (One Engine Inoperative climb performance) or, when climb is not possible, the minimum achievable rate of descent.

Any deviation from zero sideslip increases aerodynamic drag and reduces performance. If the pilot maintains wings level using excessive rudder, the airplane slips toward the inoperative engine, increasing drag and raising effective VMC engine inoperative. If the pilot banks excessively without sufficient rudder, induced drag increases and controllability margins shrink.

It is important to understand that under true zero sideslip conditions, VMC may actually be higher than the published value. Published VMC is determined during aircraft certification using up to 5° of bank toward the operative engine, which artificially lowers the minimum controllable airspeed by adding a horizontal lift component to assist directional control.

In real-world engine failure performance scenarios, pilots typically use less than 5° of bank to optimize climb performance rather than minimize controllability speed. Because less horizontal lift is available to assist the rudder, the practical loss-of-control speed can occur above the red radial line if wings are held too level.

This distinction is critical:

• Published VMC reflects certification controllability limits.

• Zero sideslip configuration reflects best performance technique during OEI climb.

They are related, but not identical.

For advanced multiengine operations, understanding this difference is essential. Safe twin-engine flying requires the pilot to continuously balance asymmetric thrust, rudder effectiveness, bank angle, and airspeed—especially near VMC and during single-engine climb performance scenarios.

When bank angle is plotted against climb performance in a typical light twin, a clear aerodynamic relationship appears: zero sideslip produces the best available OEI climb performance — or, when climb is no longer possible, the least rate of descent. The improvement may be marginal, especially at high weight or density altitude, but in single-engine operations even a small performance gain can be operationally significant.

Whether the airplane can actually climb on one engine depends on three primary variables: aircraft weight, density altitude, and pilot technique. Even a properly configured twin at VYSE may only achieve a shallow climb gradient when heavy or operating in hot-and-high conditions. If the pilot maintains wings level and counters yaw exclusively with rudder, a moderate sideslip develops toward the inoperative engine. That sideslip increases drag and reduces OEI climb performance, sometimes turning a marginal climb into a descent. Conversely, banking excessively without coordinated rudder input creates a large sideslip toward the operative engine, dramatically increasing induced drag and further degrading performance.

The exact zero sideslip condition is not universal. It varies slightly between aircraft models and even between operating conditions in the same aircraft. Power setting, airspeed, weight, and whether the airplane uses counter-rotating propellers all influence the precise rudder and bank combination required. In conventional twins with clockwise-rotating propellers, the presence of a critical engine means the required correction may differ slightly depending on which engine has failed due to P-factor effects. These zero sideslip recommendations apply primarily to reciprocating-engine multiengine airplanes operating at VYSE with the inoperative engine feathered.

Typically, the required bank angle for zero sideslip ranges between approximately 1.5° and 2.5° toward the operative engine. The inclinometer ball is displaced about one-third to one-half of a ball width toward that same engine. This configuration aligns the aircraft’s longitudinal axis with the relative wind despite asymmetric thrust, minimizing drag and optimizing single-engine performance.

During flight training, instructors often simulate feathering through a “zero thrust” technique. In this scenario, power on one engine is reduced until the drag produced by its windmilling propeller equals the drag of a fully feathered propeller. When the airplane is stabilized at VYSE with maximum available power on the operating engine, the correct combination of rudder and slight bank becomes apparent. If a yaw string were installed, it would align vertically on the windshield — a visual confirmation of zero sideslip.

Minor variations in zero sideslip attitude occur depending on airspeed, available power, density altitude, and aircraft weight. These differences are subtle and often imperceptible without precise instrumentation. The most noticeable change across operating conditions is not lateral attitude, but pitch attitude. As density altitude increases or weight rises, a higher pitch angle may be required to maintain VYSE, even though the lateral zero sideslip relationship remains essentially the same.

Understanding and consistently applying the zero sideslip technique is fundamental to safe multiengine airplane engine inoperative flight principles. It bridges the gap between theoretical VMC certification limits and practical OEI climb performance, reinforcing the central lesson of twin-engine operations: control and aerodynamic efficiency determine survivability far more than raw horsepower.

Conclusion

Understanding multiengine airplane engine inoperative flight principles goes far beyond memorizing VMC or reciting certification conditions. A professional twin-engine pilot must recognize that published VMC is simply a certification reference point — not a guarantee of controllability under all real-world conditions.

In actual operations, VMC varies with power, weight, center of gravity, configuration, altitude, and pilot technique. Bank angle selection, rudder coordination, and disciplined airspeed management directly influence controllability and climb performance during OEI (One Engine Inoperative) flight. The difference between wings-level flight and proper zero sideslip technique can determine whether the aircraft climbs, maintains altitude, or descends.

Equally critical is stall awareness. A stall under asymmetric thrust is one of the most dangerous situations in twin-engine flying and may lead to an unrecoverable loss of control. That is why VMC demonstrations emphasize control recognition and prompt recovery — never performance chasing.

Ultimately, safe multiengine operations require:

• Precise rudder coordination

• Proper bank toward the operative engine

• Strict airspeed protection above VMC

• Immediate reduction of asymmetric thrust when control degrades

• Deep understanding of OEI climb limitations

Twin-engine flying is not about “having two engines.” It is about understanding what happens when one of them fails.

For a deeper understanding of how environmental factors influence single-engine performance and controllability margins, continue reading:

👉 https://melibrary.pro/article/density-altitude-and-aircraft-performance/

Mastering the relationship between VMC, VYSE, zero sideslip, and density altitude is essential for building true multiengine competence and professional-level decision making.