Cessna put themselves in the “Sweet Spot” with the Grand Caravan EX
Cessna has been looking for the right opportunity to increase the horsepower on the Caravan for several years but it was critical that the timing was perfect and that the right engine was available. Previously, the next more powerful engine in the Pratt lineup from the PT6A-114A was the PT6A-42 and for several reasons, Cessna recognized that the PT6A-42 was not the optimal engine for the Caravan and Pratt and Whitney was not moving forward to offer additional engine choices. The -42 had been the success of the King Air 200 with its heritage going back to 1975 when the first 200’s were delivered with the -41 version. This series of engine was designed for high altitude performance including providing bleed air for pressurization and did not fit the Caravan niche requirements.
Approximately three years ago Cessna made an inquiry to three engine manufactures to replace the engine on the Grand Caravan, they submitted an RFP (Request for Proposal) to Pratt and Whitney, Honeywell and GE (GE had purchased the Walter turboprop engine) to offer an engine for the Grand Caravan. Honeywell offered a version of the TPE331 specifically for Cessna that combined the -10 and -12 compressor to reduce the ground noise. The TPE331 has long been the power king of engines and over the years has made some huge advances in TBO’s and cost of operation. Conversely, GE offered the not yet certified H80 variant of the Walter M701. Pratt came back to Cessna with a new design, borrowing heavily from the turbo shaft family a new core to the turboprop engines combined with a new gear box design and using the accessory gear box from the tried and true -42A Pratt created the PT6A-140.
Previous to the competition Cessna conducted a blind survey of operators regarding potential engine manufacturers for the Caravan and the results made it clear that there was only one correct engine manufacturer selection for Cessna Caravan. Pratt’s offering of the -140 engine made it the quick and clear winner. Based on the criteria that were important to Cessna for the Caravan the -140 came the closest to meeting all of the three candidates. More specifically, it had to have a single exhaust port, one of the big things standing in the way of the -42 being a candidate. Cessna engineering held firmly that dual exhaust was not acceptable for Cessna based on a long held understanding the reason for the original -114 having single exhaust to keep fumes from entering the cabin. Ironically, exhaust penetration of the cabin through dual exhaust is proven by the issues the Kodiak suffers regarding exhaust fumes in cabin. Additionally, the 1900 RPM gear box was also important for fly over noise to keep the tip speeds down with a 106 inch diameter propeller. The large diameter propeller would provide the needed static thrust over short diameter four bladed propellers for water takeoffs that this Grand Caravan EX would now have the power for floats. While not the only reason for adoption, the -140 conveniently fit into the same engine mount and cowling as the 114 series.
The GE engine did not have the horsepower and the support system did not match Pratt and Whitney. The TPE331 had ample power, more than was needed, a new design firewall forward to make it work and the customers told Cessna it was too noisy. The -42A Pratt also required a new design firewall forward, dual exhaust was unacceptable, it had additional cycle limited parts and the 2000 RPM gearbox had higher tip speed than a 1900 RPM gearbox.
Referring back to our earlier article about Flat Rating* the -42A has more power at altitude than the -140 and at 20,000 feet it makes for a faster Caravan. However, the designers didn’t consider that a primary altitude for Caravan operations and at 10,000 feet both engines provide similar performance. Pratt and Whitney also published that the -140 engine at its maximum output has 5% better specific fuel consumption than the -114A it replaces and future engine development for the single power turbine PT6’s will be based on the -140 technology.