What Is A Light Twin, Anyway?

The Commander was, and still is, a lot of airplane in a compact package. Photo by Alan Lebeda (GFDL 1.2 OR GFDL 1.2) via Wikimedia Commons

As a generalization, "light twins" are more or less defined by their 6,000-pound maximum takeoff weight. Different certification standards apply if the airplane weighs more than 6,000 pounds or has a landing-configuration stall speed in excess of 61 knots. In that case, it must produce a single-engine climb rate at 5,000 feet MSL equivalent to its Vso squared times .027 (not much) and, for post-1991 certifications, a climb gradient of 1.5%. If the twin-engine airplane weighs no more than 6,000 pounds or stalls at less than 61 kts (70 mph), the FAA does not require ANY capability of sustained level flight at 5,000 feet with one of the two engines failed---merely that engine-out performance be determined, good or bad. This is, needless to say, a much lower certification bar for manufacturers to shimmy under, so 6,000 pounds is a bit of a magical figure.

In addition, the airplane must be controllable with the "most critical" engine windmilling at Vmc(a). Except for very early twins, this Vmc speed is marked with a red radial on the airspeed indicator, and a similar blue radial is depicted to show Vyse, the speed that produces the best single-engine rate-of-climb, or at least the minimum descent rate, depending on conditions.

During the 1970s, it became fashionable to furnish turbocharged engines on twins, not so much to enable high-altitude flight as to enhance their single-engine rate of climb and service ceiling.

All of these foregoing distinctions aside, for discussion purposes, we often simply consider a light twin to be a multi-engine plane used for personal and business travel, one probably intended to be flown by its owner rather than crewed.

While the manufacturers' marketing departments and design engineers will have determined, sometimes through creative flight testing, a positive single-engine rate of climb and generous engine-out service ceiling, in order to show some favorable brochure figures, the experienced factory test pilot will have had a new well-running airplane to generate these numbers; be assured that your actual mileage may vary.

So long as both engines are healthy, light twins fly with Ã©lan and feed a pilot's ego quite nicely. Control forces may be somewhat heavy and response a bit ponderous, but the heavier wing loading and necessarily larger tail feathers make for a stable, comfortable ride. Most of the upgrading pilot's transition training will focus on handling the airplane with one engine failed, learning how to maintain control, and maximizing any remaining performance.

Used Light Twins Roundup

J BeckettWriter