How To Fly Twin Engine Aircraft At Single Engine Performance Levels

Twin-engine airplanes certified under FAR 23 do not have the same performance guaranteed for transport category airplanes certified under FAR 25. Especially, light twins weighing less than 6000 lbs and have VS0 equal to or less than 61 KCAS are not required to have any positive single-engine performance. Twins weighing more than 6000 lbs and/or having VS0 above 61 KCAS must demonstrate, in still air at 5000 ft, with the inoperative engine feathered, a climb gradient of 1.5 percent (if certified after February 1991), or a rate of climb of 0.027 V2S0-not exactly earth-shaking performance. We often hear that losing an engine in a light twin-engine airplane is far more dangerous than losing the only engine in a single-engine airplane. Melville Byington Jr. (1989 and 1993) of Embry-Riddle Aeronautical University conducted an experimental study of the bank angles required to obtain the zero sideslip flight, and consequently, the maximum performance in light twins. Based on the NTSB accident data, Byington concluded that 30 percent of twin-engine airplane accidents occur due to the loss of directional control (VMCA rollover), while 43 percent can be traced to insufficient performance with one engine inoperative (OEI). The remaining 26 percent or so are stall/spin accidents, which also carry the highest fatality rate. Evidently, more accidents happen due to the inadequate SE performance planning or understanding, than due to the control problems.

Key Takeaways:

• Light twin-engine airplanes certified under FAR 23 often have limited or no guaranteed positive single-engine performance, with a majority of accidents attributed to insufficient performance rather than loss of control.
• The optimum bank angle for zero-sideslip flight with one engine inoperative (OEI), which minimizes drag and maximizes performance, is not a constant 5 degrees but varies significantly with factors like aircraft weight, density altitude, and engine/rudder arm ratios.
• Ensuring adequate single-engine climb performance is critical for safe operations, particularly for IFR departures and obstacle clearance, as light twins may struggle to meet required climb gradients.
• Pilots can significantly improve single-engine climb performance and safety by proactively adjusting the aircraft's takeoff weight, often needing to be substantially below the maximum allowed to meet specific climb requirements.

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