Engine swaps for turbine airplanes are, as Lane Wallace pointed out in her overview of the turbine re-engining marketplace, a popular way for airplane owners to get more value out of an existing airplane. For those owners who are looking to get more hauling power or improved performance without trading up, an engine swap is often just what the doctor ordered.
For potential customers who want to see how much extra performance or economy a different engine could provide in their Caravan or Citation, the specifications are only a few keystrokes away. And while there's some understandable skepticism about just how accurate those numbers are, my experience has been that they are dead on. I've flown a number of airplanes with new turbine engines, and in every instance, I've seen the advertised numbers or better.
As good as numbers are, nothing beats the experience of actually getting into the airplane and witnessing firsthand the real-world result of that additional horsepower.
That's exactly what I did recently, when I went flying with the good folks from Texas Turbine Conversions Inc. in the company's Grand Caravan demonstrator. I'd seen the numbers going in, but nothing prepared me for the kind of performance I felt in the seat of my pants that day.
It was a terrible day to fly around Central Texas. Severe weather was the order of the day, with lines of storms charging west to east through the middle of the state. But somehow Bobby Bishop, president of Texas Turbines, had managed to make his way down to Austin from Celina, up near Dallas, to go flying with me in his Supervan 900 demonstrator. The airplane is a Cessna Grand Caravan with a Honeywell TPE331-12JR taking the place of the stock Pratt & Whitney PT6-114a and, in the process, bringing a surplus of power to the party.
Bishop, as mentioned in Lane Wallace's conversion overview ("Something Old, Something New,"), started as a jump-plane operator who was looking for the most efficient way possible to get the most jumpers to altitude. His solution, putting a Honeywell TPE331 in a single-engine Otter was a stroke of genius, and the marketplace responded with a lot of orders. Texas Turbines has today done 40-plus Otter conversions.
So a few years back, when Bishop started looking for a new application, he naturally turned his attention to the Cessna Caravan, an airplane with more than 1,600 candidates in the field.
It was a good call. By 2007, Texas Turbines had turned out its first TPE331 conversion, and by last year it had delivered a couple of float-equipped "short" Caravan 208 conversions, the most recent to Talon Air Service in Soldotna, Alaska. Certification for the Grand Caravan on floats is expected later this year.
As Bishop saw so clearly in that original Otter conversion, power does good things for airplanes. And when you're talking about an airplane primarily intended to haul loads, like the Caravan, there's a lot of good to be done. Plus, with 900 shp available, the Honeywell engine has power to spare. But to get operators to make the switch, Bishop knows he has to convince them that, while it costs more upfront, the switch to Honeywell power from the tried-and-true Pratt & Whitney PT6 makes sense in several ways, offering improved performance, economy and flexibility.
As I noted, instead of 600 to 675 horsepower in the Pratt PT6 in the Caravan, the TPE331 cranks out 900. The engine, for the record, is rated for 1,000 shp for takeoff and 975 continuously, but it is flat-rated to 900 shp in the Caravan. That figure still represents an increase of around a third, a huge boost for any airplane, and the performance improvement you get with that jolt of power is as good as you might expect. Though all published book speeds and restrictions remain in effect (a fact that makes the STC that much easier to perform), the benefits are still enormous.
The Honeywell Option
While perhaps not as well-known as the Pratt engine, the TPE331 has a remarkable pedigree of its own. Developed and first made by Garrett AiResearch, the TPE331 family dates back to the late '50s, when it was developed from scratch for military helicopters. Over the years there have been more than 12,000 TPE331 engines built, and they have amassed more than 70 million hours of operation on more than 75 applications, with 18 engine models and more than 100 configurations.
Shortly after its introduction, the engine was put to work powering a number of popular turboprop twins, including Swearingen Metros/Merlins, Aero Commanders and Piper Cheyennes, among many others. Over the years the engine has been constantly improved, and one of the latest versions provides the motive force for the Predator B unmanned aerial vehicles. In addition to its high power output, chief selling points of the engine include its quick responsiveness and high TBOs. Unfortunately, it's also an extremely loud engine on the ground. There's no mistaking when a Garrett-powered airplane taxies onto the ramp.
Flying the Supervan 900
We'd just lifted off VFR from Bergstrom and were dodging weather and climbing when we found clear air while heading south toward slightly better weather. So far, it was an eye-opener. I'd flown several stock Caravans before; they were nothing like this. With 900 shp upfront, Cessna's already potent turboprop single was transformed.
Our flight in the Supervan underscored all of the good things Bishop was saying about the conversion. The engine started easily and quickly. While the sequence is slightly different from that of the PT6, it's simple and straightforward, and the temps stay low, about 100 degrees lower than on the PT6 during the start sequence, as Bishop claims.
