Twin-engine airplanes certified under FAR 23 do not have the same single-engine performance guarantees as transport category aircraft certified under FAR 25. This distinction is critical for understanding real-world single-engine performance in light twin-engine aircraft.

In particular, light twin-engine airplanes weighing less than 6,000 pounds and having a VS0 of 61 KCAS or less are not required to demonstrate any positive single-engine climb performance. In practical terms, this means that after an engine failure, many light twins may be capable of maintaining control but may not be able to climb at all, even under favorable conditions.

Twin-engine aircraft that weigh more than 6,000 pounds and/or have a VS0 greater than 61 KCAS are subject to additional certification requirements. These aircraft must demonstrate, in still air at 5,000 feet, with the inoperative engine feathered, a minimum climb gradient of 1.5 percent if certified after February 1991. Older aircraft may instead be required to meet a climb rate of 0.027 V2S0.

While these requirements technically satisfy certification standards, the resulting performance is modest at best. The demonstrated single-engine climb capability under FAR 23 is limited and highly sensitive to factors such as aircraft weight, density altitude, engine condition, and pilot technique. As a result, even twin-engine aircraft that meet these standards should not be assumed to have meaningful single-engine climb performance in operational conditions.

This regulatory reality explains why single-engine performance planning is so critical when operating twin-engine aircraft, especially during takeoff, initial climb, and instrument departures. Certification alone does not guarantee obstacle clearance or climb capability after an engine failure—it only defines the minimum standard the aircraft must meet.

Single-Engine Performance, Accident Statistics, and Real-World Risk in Twin-Engine Aircraft

It is often said that losing an engine in a light twin-engine aircraft is more dangerous than losing the only engine in a single-engine airplane. While this statement is frequently repeated, accident data and aerodynamic research provide a more nuanced explanation that places single-engine performance, rather than basic controllability, at the center of the risk.

Melville Byington Jr. (1989, 1993) of Embry-Riddle Aeronautical University conducted an experimental study examining the bank angles required to achieve zero sideslip flight, which in turn produces maximum achievable single-engine performance in light twins. Using NTSB accident data, Byington identified the primary causes of accidents following engine failure in twin-engine airplanes:

• 30% of accidents were caused by loss of directional control (VMCA rollover)

• 43% were caused by insufficient single-engine performance (OEI)

• approximately 26% were stall/spin accidents, which also carried the highest fatality rate

These findings clearly indicate that inadequate single-engine performance planning and understanding account for a greater proportion of accidents than pure control issues alone. In other words, the aircraft may remain controllable, but lack the performance required to avoid terrain or obstacles.

Zero Sideslip Flight and Performance Optimization

One effective way to reduce drag, lower VMCA, and improve single-engine performance in twin-engine aircraft is to neutralize sideslip caused by asymmetric thrust. This is achieved by banking slightly toward the operating engine, a technique commonly remembered as “raise the dead.”

Although rudder input remains necessary, the inclinometer ball will typically be displaced off-center toward the operating engine during zero-sideslip flight. A yaw string provides the most direct and inexpensive method for accurately adjusting to zero sideslip and achieving maximum performance.

The key operational question, however, is often misunderstood:
What bank angle actually produces zero sideslip—and is it always five degrees?

The Optimum Bank Angle in Single-Engine Operations

In steady, zero-sideslip forward flight, the lateral aerodynamic force generated by the rudder is balanced by the lateral component of aircraft weight. The bank angle required to achieve this balance results from aerodynamic relationships that cannot realistically be calculated during flight.

What is frequently overlooked is that the optimum bank angle is not constant.

Referring to aerodynamic relationships illustrated in Figures 1 and 2, the required bank angle increases as the moment arm “a” of asymmetric thrust increases, and decreases as the rudder-to-center-of-gravity arm “b” increases. Consequently, the optimum bank angle depends on several variables, including:

• aircraft weight

• density altitude

• thrust available

• rudder effectiveness

• propeller characteristics and P-factor

For normally aspirated engines, lower density altitudes require larger bank angles to achieve zero sideslip than higher density altitudes. Likewise, lighter aircraft weights require greater bank angles for optimum performance.

This variability demonstrates that no single fixed optimum bank angle exists.

Why “Five Degrees” Is Often Incorrect

For most light twin-engine airplanes, the ratio of thrust arm to rudder arm (“a/b”) typically falls between 0.33 and 0.45, but this ratio varies throughout the flight envelope. It is influenced by angle of attack, sideslip angle, propeller rotation direction, and P-factor.

For aircraft with sea-level, standard-day thrust-to-weight ratios of:

• 0.20–0.25 (both engines operating at VYSE), and

• 0.10–0.125 (single-engine at VYSE),

the actual zero-sideslip bank angle typically ranges between 1.9° and 3.2°, not the commonly taught five degrees.

These values assume new engines and propellers operating with ideal technique. In practice, engine wear and propeller efficiency losses further reduce available thrust and alter the optimum bank angle.

Single-Engine Performance vs Climb Capability

It is critical to understand that achieving zero sideslip while OEI does not guarantee climb performance. Instead, it simply minimizes total drag. This distinction is especially important during drift-down procedures, where optimizing range rather than climb is often the realistic goal.

During descent, the optimum bank angle changes continuously as both weight and density altitude decrease. Treating the bank angle as a fixed value can therefore degrade performance rather than improve it.

IFR Departures, Obstacle Clearance, and Performance Planning

Performance calculations become particularly critical when planning IFR departures into IMC from airports with Obstacle Departure Procedures (ODPs). Many light twin-engine aircraft are incapable of meeting published climb gradients following an engine failure.

When no ODP is published, pilots may assume the FAA minimum climb gradient of 200 ft/NM (≈3.33%), with the requirement to cross the departure end of the runway at 35 feet AGL. For many light twins, this requirement cannot be met with one engine inoperative unless weight is carefully controlled.

Adjusting Takeoff Weight to Improve Single-Engine Performance

Reducucing aircraft weight is one of the most powerful tools available to pilots for improving single-engine performance in twin-engine aircraft.

Weight reduction provides two major benefits:

1. Rate of climb increases inversely with weight

2. Power required decreases, increasing excess horsepower

As altitude increases, normally aspirated engines suffer a further reduction in available power. At 5,000 feet DA, only about 86% of rated power is available, making weight limitation essential for maintaining any meaningful single-engine climb capability.

Putting It All Together: A Practical Example

Using performance data for a 285-hp Cessna 310, a practical scenario demonstrates how weight, wind, and performance interact. With a required climb gradient of 300 ft/NM, careful calculation shows that a maximum allowable takeoff weight (MATOW) of approximately 4,880 lbs is required—over 600 lbs below MTOW.

Despite this limitation, sufficient payload remains to safely conduct the departure, even if an engine fails at the most critical moment. Without wind, MATOW would need to be reduced further, illustrating how sensitive single-engine performance is to environmental conditions.

A final operational consideration is missed approach planning. By adjusting landing weight to maintain a single-engine service ceiling 2,000–3,000 feet above DA or MDA, pilots preserve the option to safely execute a missed approach on one engine.

Conclusion

Understanding single-engine performance in twin-engine aircraft is fundamental for safe flight operations, especially during takeoff, climb, and departure procedures. While certification standards such as FAR 23 establish minimum performance criteria, they do not guarantee strong single-engine climb capability for many light twins. Real-world experience and accident data show that insufficient planning and misunderstanding of single-engine performance contribute to a large share of twin-engine accidents, often more so than basic controllability issues.

Pilots must not only recognize the limitations inherent to their aircraft’s design but also actively plan for these conditions before every flight. Techniques such as zero-sideslip flight, optimizing bank angle, and careful weight management are essential tools in maximizing performance when one engine is inoperative. Likewise, adjustments to takeoff weight and an understanding of obstacle clearance requirements can make the difference between a safe departure and a compromised climb.

By integrating aerodynamic knowledge, performance planning, and disciplined execution, pilots can greatly improve their ability to manage single-engine performance limitations while flying twin-engine aircraft. This foundation prepares aviators not only for emergency scenarios but also for advanced training and certification.

For a deeper look into how training standards approach twin-engine systems and procedures, including fuel management and engine handling, see:
👉 Twin Engine Fuel System: Design, Crossfeed and Redundancy