Turbine Edition: Power Play

Supplanting pistons with turbine wheels

October 1, 2008

A quick survey of the new turbine aircraft market reveals a stable of highly capable performers, but be ready for sticker shock. Even the least expensive, entry-level single-engine turboprop can rapidly surpass $2 million as options accumulate, and that’s just the price of admission. There is another way to reap the benefits of turbine power, however—take a proven piston-engine aircraft and put a turboprop up front.

Virtually any piston airplane can be converted to turbine power, but the unforgiving mathematics of payload and range make the transition impractical for all but a few designs. These days, the GA airplanes most often earmarked for turbine conversions include the 200-series Cessna singles, the Beech Bonanza, the Piper Malibu/Mirage family, and the de Havilland Beaver and Otter bush planes. In the twin realm, turbines can be grafted onto various types, including the Cessna 421, the Beech Duke, and the Douglas DC-3.

The number of converted aircraft have always been small, as the pluses and minuses exist in tenuous balance. First, the bad news. Turbine engines are considerably thirstier than their piston counterparts, and must be flown at high altitudes to keep fuel consumption in check. Jet fuel is about a pound per gallon heavier than avgas, and airplanes designed for pistons typically must be fitted with additional fuel tanks to provide an equivalent level of endurance. These extra tanks (and the fuel inside them) can consume the payload gained by switching to a lighter turbine engine.

A converted airplane is the lowest-cost single turbine on the market. But conversion packages typically begin in the $500,000 range—a considerable sum when the host aircraft can, in many cases, be acquired for far less. Potential buyers should also compare the acquisition cost of a converted airplane with that of the used turbine inventory, which includes various 1970s-era twin turboprops (Merlins, King Airs, MU-2s, etc.) that can be had for well under $1 million in good condition.

Integration is another challenge with turbine conversions, and receiving airplanes usually have their fuselages lengthened and reinforced to accommodate a smaller, lighter engine that nonetheless exerts much greater twisting force on the airframe than the original designers envisioned. Stability, flutter, and other aerodynamic issues must be resolved before taking low-altitude designs into the flight levels. A converted airplane may also lack pressurization, which some see as a prerequisite for high-altitude flight.

Parts for turbine engines have never been inexpensive and, while turbine time between overhaul is typically much longer than the piston interval (3,500 hours versus 2,000 hours are good ballpark figures), a “hot section” inspection at the halfway point can run $40,000 or more. Unscheduled maintenance can also be painful with turbines, as a single botched “hot start” procedure typically calls for an expensive engine teardown.

On the plus side, a turbine engine offers more power in a much lighter package, far greater reliability, higher cruise speeds at altitude, better short-field takeoff and landing performance, and improved gliding (engine-out) characteristics. Also, outside the United States, jet fuel is generally less expensive than avgas, and easier to find in remote areas. Assuming the aircraft is flown consistently at high altitudes at optimal power settings, switching to turbine power can cut mission block times by 20 to 30 percent.

The three engine types most typically used for turbine conversions—the Rolls-Royce 250, the Pratt & Whitney PT6A, and the Honeywell TPE331—are in use all over the world aboard a wide range of fixed-wing aircraft and helicopters. Parts for all of these engines are plentiful and easy to find, as is expertise in their care and upkeep.

While it’s a stretch to place turboprops in the “quiet” category, their slower-turning propellers effectively reduce their overall noise footprint. The Rolls-Royce 250 on the Soloy Cessna 206 “Mark II” conversion, for example, turns a three-blade Hartzell propeller at just 2,000 rpm—far slower than the 2,750-rpm screech of the piston engine’s propeller. An optional five-blade prop from Germany’s MT is quieter still.

Do the advantages of a turbine conversion outweigh the disadvantages? It depends on the mission, and on the operator’s expectations. One thing we do know is that a few hundred hours in a converted airplane is excellent training for the larger, single-engine turboprops. “A lot of people use our A36 Bonanza conversion to build turbine time so they can move up to a TBM or something,” says James Whatley, service director of Amarillo, Texas-based Tradewinds Turbines. “In comparison to a new single-engine turboprop, it’s really inexpensive to operate.”