Texas Turbine upgrades Caravan

 - December 28, 2009, 7:06 AM

After Bobby Bishop’s Texas Turbine Conversions converted the single-engine de Havilland Canada DHC-3 Otter from a PT6 to a TPE331 engine, the company needed another project. The Cessna Caravan looked like a good prospect, so Bishop bought a 1995 Cessna Grand Caravan 208B that had been used for hauling newspapers and had logged 9,000 hours. Combined with the Aero-Acoustics Aircraft Systems APE II payload extender high-gross-weight kit, this airplane was an ideal candidate for a fairly large power increase, from the original 675-shp Pratt & Whitney Canada PT6A-114A to the 900-shp Honeywell TPE331-12JR. Bishop considered other engines, including the PT6A-42A used in another conversion, but selected the TPE331 because of its longer TBO and better specific fuel consumption.

Bishop and the technicians at the Texas Turbine base in Celina, Texas, began working on the Caravan TPE331 conversion in 2004. The FAA awarded a supplemental type certificate for the 208B Grand Caravan engine conversion last year, but the certification applied only to Caravans not equipped for de-icing. Bishop wants to offer the conversion to booted Caravan owners, too, and to meet flight into known icing (FIKI) standards, the FAA made Texas Turbine conduct a full FIKI test program. Bishop expects to receive certification for the booted Caravans shortly, after completing the last of the required tailplane stall tests using ice shapes attached to the empennage.

One of Bishop’s goals with the conversion was to leave the airframe alone as much as possible, so that the engine mod involved only the area forward of the Caravan’s firewall. Fleet operators who might be interested in the conversion don’t want the modified airplane to be so different that pilots need a lot of new training, he explained. Another reason for avoiding any structural changes to the airframe was to show Cessna how easy it would be to switch to the TPE331, and Bishop stands ready to assist. Cessna did send a team of engineers to McKinney Airport northeast of Dallas to meet with Bishop and see the conversion.

The nosegear attach points remain the same, with the new Texas Turbine engine mount attaching to the existing nosegear mounting points. Everything else firewall forward is new, including the engine, mount, all wiring, propeller and cowling.

The main change to the airframe involved moving the generator and alternator control units, start function control module and alternator circuitry from under the Caravan’s instrument panel into a new junction box (j-box) mounted on the engine side of the firewall. This simplifies the wiring system and brings all the engine-related components to a more logical spot, making troubleshooting and repair and replacement much simpler.

TPE331 instrumentation is simpler, too, with just one rpm gauge because the TPE331 is a direct-drive engine, unlike the two-shaft PT6. Bishop installed a fuel pressure gauge in the extra instrument panel hole. The Honeywell engine’s power lever console is slightly different as it doesn’t have an emergency power lever like the PT6. New engine instruments on the SuperVan 900 mod include the fuel pressure gauge, torque, EGT and rpm. Another airframe change was removing the motive-flow fuel pump, which is not needed for the TPE331 installation.

The new cowl is made of aluminum-honeycomb composite. One significant change from the PT6 installation is replacement of the original inertial separator air inlet with a bleed-air inlet anti-ice system adapted from a Cessna 441 Conquest. Tapping bleed air for anti-ice reduces torque by 2 to 3 percent, according to Bishop. A 300-amp starter/generator is backed up by a new 95-amp standby generator.

The Texas Turbine conversion includes a new Hartzell four-blade propeller system with 110-inch blades, four inches longer than the blades on the Grand Caravan’s three-blade McCauley prop. The TPE331 turns the prop at a maximum of 1591 rpm, compared with 1900 rpm on the PT6A-114A Caravan.

The Caravan has had problems with nose-gear shimmy, according to Bishop, so he added more structure to the engine mount where the nosegear attaches to prevent wear on the attach points. Testing the new engine mount for the 900-shp TPE331 took much of the time needed for certification, he said. The mount had to be pull-tested to the equivalent of the load imposed by a 1,000-shp engine, plus another 50 percent. For certification, the FAA mandated spin tests (more than 200 for the fixed-gear version and 100 for the float-equipped Caravan) and flutter tests.

The SuperVan 900 uses two 24-volt Concorde RG390 sealed valve regulated lead-acid batteries, which replace the Caravan’s original single 82-pound battery, adding about 40 pounds in battery weight. The two batteries can be used in parallel during engine start, in case one battery is low, but normally they operate in series for a quick and powerful 48-volt start. Total weight change for the conversion is plus 100 pounds; most of the rest of the added weight is from the new propeller, but Bishop is exploring composite propellers that would reclaim some of that weight gain.

The SuperVan 900 conversion costs $495,000, including the new TPE331-12JR engine, propeller, all firewall-forward changes, engine instruments, single redline/ torque- and temperature-limiter control system and installation at a Texas Turbine service center. Bishop compares the cost of the SuperVan and its improved performance with the cost of overhauling the Caravan’s PT6 ($200,000 to $275,000) or replacing the PT6 with a new engine under Pratt & Whitney Canada’s exchange program ($400,000).

Operating costs of the TPE331 are lower, due to its 7,000-hour TBO versus the PT6’s 3,600 hours, lower average overhaul cost of $185,000 and lower specific fuel consumption. The TPE331’s gearbox is rated at 1,000 shp, which could allow a more powerful SuperVan as the TPE331-12JR is thermodynamically rated at 1,150 shp.

Performance improvements include a greater rate of climb, more than 20 knots faster max cruise speed, shorter takeoff ground roll, greater endurance and more range.

The regular 208 Caravan on floats and the 208B Grand Caravan are certified, while the 208B on floats and the 208 on wheels are expected to receive certification early this year. FIKI certification will apply to both the 208 and 208B. The $495,000 SuperVan price is the same for both models.

Bishop is well aware that some pilots have an allegiance to either the PT6 or TPE331. “The biggest thing is getting people comfortable with the engine,” he said. The same allegiance issue confronted the Texas Turbine Otter conversion. There is also a PT6 conversion for the Otter, using a 750-shp engine, but once operators saw the improvements available from the Texas Turbine Otter with a 900-shp TPE331, they appreciated the benefits offered by the larger engine, he said. Texas Turbine has converted 40 Otters. “Home-brand favoritism will go by the wayside,” he predicted.

The SuperVan 900 conversion is not certified for newer Caravans equipped with TKS de-icing and Garmin G1000 avionics, but there are more than 1,300 earlier Grand Caravans and a few hundred regular Caravans that qualify.

Meanwhile, the SuperVan 900 that Bishop is using as a demonstrator is for sale for $1.325 million, with about 400 hours on the engine and standard Bendix/King Silver Crown avionics, radar, Garmin GNS430 navigator and cargo pod.   

Flying the SuperVan 900

Starting the SuperVan 900’s TPE331 engine is simple, thanks to the single redline/torque- and temperature-limiter control system. Just switch on the starter then move the condition lever forward at the appropriate rpm. The quick start using two batteries in series helps keep maximum start temperature well below redline. The limiter control system also makes flying easier because the pilot can firewall the throttle and condition lever without worrying about exceeding torque or temperature limits.

We flew the SuperVan 900 out of Denton Municipal Airport north of Dallas on a 24 degree C day carrying about 1,000 pounds of fuel and at a takeoff weight of about 7,400 pounds, much lighter than the SuperVan’s 9,062-pound mtow (with the AeroAcoustics APE II kit). One factor that SuperVan pilots will have to get used to is that the TPE331 spins counterclockwise, so during takeoff and at high power settings the airplane yaws to the right. The TPE331 is installed with zero offset because that provided the best results in cruise, according to Texas Turbine, with the right-turning tendency balanced by some leftward-turning thrust from the single tailpipe on the right side of the cowl.

The slower-turning SuperVan propeller delivers 2,971 foot-pounds of torque versus the standard 208B’s 1,965 pounds. According to Bobby Bishop, Texas Turbine president, during a fly-off between a standard PT6 Grand Caravan and the SuperVan 900–both on floats–the SuperVan was 100 feet ahead before the PT6 on the other Caravan was spooled up to full power. “It was a pretty decisive whipping,” he said. Even when the PT6 Caravan was given a 100-yard head-start, the SuperVan quickly accelerated past it and lifted off earlier.

Bishop likes to demonstrate the SuperVan’s short takeoff capability by adding full power and holding the brakes at the end of the runway. Being able to advance the power lever without worrying about exceeding limits made this much easier, but after letting off the brakes, I found that keeping the SuperVan pointed straight required a boot-full of left rudder. The modified Caravan leaped off the runway after about 600 feet of ground roll, and climb rate settled at more than 2,000 fpm. At 10,000 feet, we still climbed at 1,800 fpm at 90-percent torque.

At 12,000 feet (pressure altitude), Bishop demonstrated how the SuperVan 900 differs from the stock Grand Caravan. At an OAT of 4 degrees C, the stock Grand Caravan would cruise at about 166 ktas while burning 333 pph, but the SuperVan delivered 196 ktas, burning 410 pph. Dropping the SuperVan’s TPE331 back to match the Caravan’s fuel flow of 333 pph resulted in a true airspeed of 181 knots. So at the same fuel flow the SuperVan flies 15 knots faster, hence its greater range. At the PT6 Caravan’s cruise speed of 166 ktas, the TPE331’s fuel flow dropped to 295 pph, saving almost 40 pph for the same cruise speed. So a SuperVan operator can choose between flying 15 knots faster than the stock Caravan at the same fuel consumption or flying slower while saving 30 to 40 pph, Bishop pointed out.

Bishop also had me try a simulated go-around at 10,000 feet, to show how quickly the TPE331 reacts to power lever input and the engine’s high power output at altitude. Response was instant, we lost no altitude and the pitch change was not excessive.

On approach and landing, the SuperVan 900 again demonstrates the TPE331’s rapid response to power input. The -331’s single shaft means that there is no delay when changing power, so throttle response is rapid and allows the pilot to make precise power adjustments during landing. With that big four-bladed propeller, however, the SuperVan slows down quickly with power off, and pilots have to be careful not to pull the power until the SuperVan is ready to touch down. Once on the ground, pulling the condition lever into reverse stops the SuperVan promptly.

The slower-turning TPE331 propeller helps make the SuperVan quieter than the PT6 installation, according to Bishop. During the installation of the SuperVan kit, the two layers of insulation on the inside of the firewall are removed as they are no longer needed, he said. The cockpit noise level was comfortable with passive headsets, and it was possible to converse without headsets.