In terms of cost, the TD is going to be the most expensive Skyhawk to buy-it's $15,000 more to purchase than the 172SP. But it will be the least expensive of the bunch to operate. The fuel savings are the big driver for this, as the airplane will burn less fuel and, potentially, less-expensive fuel. Here in the States, the price delta between 100LL and jet-A is still quite small (100ll.com shows the average price for the two fuels within about 50 cents a gallon). But in nearly every other part of the world, burning jet-A will give operators a big break. And it's hard to imagine the price difference not growing here in the States, as well.
On the downside, the diesel will probably have higher upkeep costs than the Lycoming. The Centurion has, as you might be aware, a TBR (time before replacement) instead of a TBO, and that cost is a substantial percentage of the full purchase price of the engine. Unlike in other airplanes, the TBR for the engine in the 172 is set at 2,400 hours at inception, and there's a major inspection mandated at 1,200 hours. The wood/composite MT prop also has a TBR of 2,400 hours. The upside is, the Thielert warranties seem to be very good.
Cessna's estimates of savings are based on operating costs of $101.81 per hour for the Skyhawk SP and $96.39 an hour for the TD. While $5 an hour might not seem like that much, when it's spread out over the life of the airplane and across a fleet, it adds up to a lot of money. And the difference in DOCs would be even greater, except for the fact that the engine and prop hourly reserves are both substantially higher for the diesel airplane.
The TD is expected to be ready for delivery around the middle of the year, and Cessna expects to build around 125 diesel-powered 172s in 2008, nearly all of them rolling down the production line in the second half of the year. It could sell more if it built more, and the demand has been strong from both flight schools and foreign operators.
I asked Doman what percentage of Skyhawks he thought might eventually be powered by diesel engines, but he wasn't willing to hazard a guess, in part because he, along with everybody else at Cessna, was surprised by just how many orders they got right off the bat for the TD.
Many of those orders, I'd venture to say, are from customers who took a long look and realized that the TD, despite its impressive capabilities and advanced technological features, remains faithful to what a 172 is, and has always been.