Singles vs. Twins: A Case Study (Part 2)

In a single-engine airplane engine failure introduces relatively few decision steps. The airplane’s tendency during the emergency is to continue ahead in a straight line, descending. This characteristic helps prevent either a stall or a spiral. In a twin, engine failure introduces a large number of sequential pilot decisions, each with potentially adverse consequences … all while the airplane (under the influence of asymmetric thrust) is attempting to radically diverge from a controlled path in all three axes. It takes regular, intense training (ideally in a simulator where such things can be realistically presented and safely practiced) to be proficient in overcoming aircraft tendencies, and making safe and proper decisions.

Pilots buying or flying multiengine airplanes should include annual, simulator-based training in their flight training regimen. Without it, statistics suggest strongly that they are less safe with the second engine.

Counter-rotating Propellers
Some more recent multiengine designs like the Piper Seminole and Beech Duchess tout counter-rotating propellers as a safety improvement in the event of an engine failure. Because of the left-turning tendency of American engines (and propellers), there is more thrust developed on the right, descending side of the propeller disc (when viewed from behind). Further out from the centerline of the fuselage, the right propeller’s descending side has more mechanical leverage to push the airplane out of controlled flight than the downward side of the left-hand propeller. American airplanes with counter-rotating propellers have a propeller on the right engine that rotates “backwards,” so the descending side of its prop disc is closer to the fuselage and has no more or less mechanical leverage than the left engine’s propeller.


There is a slight safety advantage to counter-rotating propellers. In a conventional (non-counter-rotating) twin if the left (critical) engine fails, the rate of departure from controlled flight is slightly greater than the rate if the right (non-critical) engine should quit. With counter-rotating propellers the rate of departure from controlled flight is the same regardless of which engine has failed, and would be roughly equal to the rate for non-critical engine failure in the conventional twin.


VMc, or the indicated airspeed at which full deflection of the flight controls is insufficient to counter this rate of divergence, is also higher for a failure of the critical engine in a conventional twin than it is if the “non-critical” engine has failed — although the difference is measured in a very few knots. With counter-rotating propellers the VMc speed is the same regardless of which engine has quit, and would be the same as the VMc speed for non-critical engine failure in an otherwise identical airframe.


Centerline Thrust
Why not align the engines fore and aft so as to eliminate any “asymmetric thrust” if an engine fails? Lose an engine and the airplane continues straight ahead, just like a single, albeit with greatly reduced power.  That’s a philosophy pioneered very early in aviation history, successfully employed by Cessna Aircraft with its Skymaster “push-pull” twins and now being resurrected by Adam Aircraft with its A500.  Trouble is, the Skymaster has a rate of engine-related accidents indistinguishable from conventional twins … in many cases Skymaster pilots did not detect that the out-of-sight, rear engine had failed. Again, the “stats” call for pilot training and practice in emergency procedures, even in centerline-thrust twins.

THE BOTTOM LINE: The choice of a single-engine airplane or a twin, and if a twin whether you fly with a conventional, counter-rotating or centerline thrust arrangement, revolves mainly around which airplane meets your budget, mission and pride-of-ownership needs. In any multiengine airplane you’ll need significant training and recurrent work for a true increase in safety; without that training you’re kidding yourself if you think a twin is safer than a single.