Although avgas is expensive, there is no shortage in the U.S., and oil companies continue to support its production and distribution. Nonetheless, Cessna Aircraft has decided to outfit its best-selling 172 Skyhawk with a diesel engine. Starting in the middle of next year, Cessna dealers will sell the Skyhawk TD powered by a 155-hp Thielert turbocharged diesel engine installed under an FAA supplemental type certificate (STC).
Cessna officials had expected to see more demand for the Skyhawk TD outside the U.S., where avgas prices are much higher than in the U.S., according to John Doman, vice president for worldwide propeller aircraft sales. But since it announced the jet-A-powered Skyhawk TD at the AOPA Expo early last month, he said, “domestically the dealer organizations are ordering them in greater numbers than we expected.” (Cessna made the announcement to its dealers earlier in the year. See AIN, September, page 104.)
Cessna was able to bring the Skyhawk TD to market quickly because it is using Thielert’s 172 STC to install the diesel engine at the factory. If it had to go through the process to revise the Skyhawk’s type certificate for the new engine, the process would have taken longer and diverted valuable engineering resources at a time when the company is busy with many other projects and a massive order backlog. “We need to make the best utilization of engineering talent,” Doman explained. “And Frank Thielert has done a good job of getting the STC completed.”
Cessna is incorporating some changes to the Skyhawk TD earlier in the assembly process instead of building the entire airplane then bolting on the engine per the STC. For example, it is using a cowling that is better adapted to fit the engine instead of modifying the original cowling, for example. And the Skyhawk TD’s Garmin G1000 avionics system fully integrates the engine instrumentation for the diesel engine, instead of requiring that the company add supplementary gauges and indicators required on a modified Skyhawk.
The Skyhawk TD sells for $15,000 more than the 180-hp 172S, with the same avionics package that beginning next year will include Garmin’s integrated GFC700 autopilot. The Skyhawk TD weighs about 100 pounds more than the 172S due to the heavier engine, reduction gearbox and liquid cooling system. The three-blade MT propeller turns just 2,300 rpm at maximum power, so the airplane is much quieter than the original, Doman said. Pilots appreciate the dual-channel fadec and single-lever power control; there is no mixture control to fuss with, and the Skyhawk TD burns less fuel than the 172S. “It starts just like your car,” he said. “After a few blades, the engine is running.”
Until it reaches 3,000 feet, the Skyhawk TD doesn’t outclimb the 172S, Doman said, but after that the benefits of turbocharging move the TD up faster than the 172S all the way to the diesel’s 16,000-foot maximum altitude. At 8,000 feet, the TD flies about six knots faster than the 172S; at 10,000 feet it flies 10 knots faster. The TD’s maximum range is almost 700 nm, compared with 520 nm for the 172S. These speed and range figures are based on a modified 172; Cessna has not yet released final performance numbers for the Skyhawk TD.
The OEM hasn’t revealed other plans for diesel-powered airplanes, but it appears that the company might eventually discontinue the 160-hp 172R. Next year Cessna will offer the R and S models, but if the response to the TD remains strong, it might stop producing the 172R, according to Doman, and offer the 172S and TD in 2009. “Our production people would like as few variants as possible to maximize efficiency,” he said. The 172S outsells the 172R “by a healthy margin,” he added.