Cessna 172TD: Skyhawk With a Bang

We fly the Cessna 172TD and find out how the most popular airplane of all time translates into diesel.

Cessna 172TD diesel-powered SkyhawkRobert Goyer

The fuel filler caps on the new Cessna Skyhawk 172TD aren't your garden-variety flip-lid 100LL caps like I'd pried (or twisted) open countless times before. These babies are industrial, overbuilt Teutonic-looking things, placarded in bold letters to make it clear what kind of fuel to use: "Jet-A Only." They seem emblematic of this new breed of Skyhawk, and I was looking forward to climbing aboard and seeing how one of my all-time favorite pair of wings would do with a new approach to power.

Cessna's test pilots were still wringing the airplane out under its Experimental type certificate when I made my visit to Cessna's Pawnee engineering facility in Wichita in November to sniff a little kerosene, but it was already sitting pretty, with what could pass for a production interior and panel and paint scheme and all. The airplane isn't scheduled for certification until the middle of this year, but it sure looked ready for a customer to climb in and fly away.

Despite its business-only fuel caps, it looked pretty much like any other Skyhawk, so much so, in fact, that an inexperienced eye might not notice the three-bladed prop and walk right on by.

But make no mistake: This is no ordinary Skyhawk, not by a long shot. In some ways it represents a whole new path for the light airplane industry, a path that might lead to places that have been very lightly trod before. It's noteworthy that the latest word on small airplane engineering would find a home in the most prolific airplane design in aviation history. It tells you all you need to know about today's Cessna.

Now, diesel technology comes to the drafting table with its own set of issues-heavier fuel, additional engine weight, a tendency to be a bit shaky, all while carrying with it a reputation for being blue collar to a fault, no doubt due to their long association with trucks and trains and black smoke.

But that was back in the bad old days, and today's diesel engines are technological marvels. The strengths that modern diesel engines embody-turbocharging, easy start-up, full authority digital engine control (fadec) and one more biggie-the fact that they burn the most widely available fuels on the planet-are so compelling that nearly every light airplane manufacturer has taken notice. The question was never, "Is diesel technology desirable?" but, rather, "Is the technology ready yet?" The answer finally seems to be, "yes."

That's largely thanks to Frank Thielert, the visionary designer who decided that what the aviation world really needed was a diesel engine done right and then decided to be the one to do it.

The engine that his company has arrived at today, the Thielert Centurion 2.0, is the culmination of that quest, at least the current culmination. Derived from a production Mercedes automotive engine, the 2.0 is a 155 hp, 2.0 liter, fadec-controlled turbodiesel swinging a three-blade composite MT prop.

Is it ready for prime time? That's a tough call for even an experienced observer to make, and the future of auto conversion engines is a complex subject, to say the least. But it does speak volumes that Cessna is sold on the engine.

The company is not alone in its interest. Piper, Socata, Diamond and others have already worked on, or certified, Thielert-powered airplanes. But Cessna is far from a latecomer. It has been following the development of Thielert engines for nearly 10 years, and while it was intrigued by the previous model engine, the 135 hp 1.7 liter model, it wasn't until the launch of the 2.0 (a development necessitated by a model change by Mercedes) that the engine developed enough power to get Cessna's attention. The company began exploring the 2.0 in early 2007, and by early November it made the launch announcement of the diesel-powered 172.

The availability of the 2.0 was critical, in part because it allowed Cessna to get 155 horsepower out of the engine, as compared to 135 for the 1.7. And because of its larger displacement, the 2.0 has to work less hard to achieve 155 horses than the 1.7 did to develop 135.

It's important to point out that the primary weights and the flight envelope for the 172 have not changed with the addition of the Thielert engine. The engine does weigh slightly more than the Lycoming engines in the 180 hp 172SP and the 160 hp 172R, but the max takeoff weight (often referred to as "gross weight") is the same, at 2,552 pounds.

There are two key improvements in the Thielert diesel over conventional gas-piston engines: turbocharging and automatic engine control, referred to as fadec. Fadec does nearly all of the pilot's power management work for him, and it does it much better than he ever could. It controls both the mixture and the prop pitch-the TD does indeed have a constant speed prop, albeit one you cannot directly control. Behind the scenes, it also handles a number of other parameters, such as fuel injection timing, that only a computer could perform.

So the 172TD does indeed have the proverbial fadec "single power lever," except in this case it's not a lever. As with other Skyhawks, the power control (formerly the throttle) is a push rod with a knob on it. Cessna refers to it as the "power control." I suggested "single knob control," but they said they'd considered the term already and had, sensibly, passed on it.

Because there's only one thing you can do to control the power the engine is developing (other than shutting off the fuel supply, that is), the power is addressed not in terms of rpm (the red line is a slow, slow 2300) or manifold pressure but in percentage of power. Depending on the phase of flight, you simply adjust your power based on percentages.

Flying the 172TD is strangely different than flying the gas-powered model. And I say "strange," because the experience-the sights, sounds, procedures, even the smell of the fuel-is all brand-new stuff in an airplane that I've flown a lot and around which I have a very definite set of expectations. Such simple things as the new sound of the engine and the noticeable lack of a mixture control, despite my looking for it repeatedly, made flying a Skyhawk seem like a trip down memory lane in some slightly alternate universe.

As I'd seen when I got checked out in the Diamond DA42 Twin Star last year, the Thielert diesel is a joy to start. One flick and it's going, and the run-up consists essentially of making sure the gauges on the G1000 are all in the green and the fadec circuitry is all functioning properly. (The 172TD has a dual electrical system, with a user-friendly fallback if there's a failure. With the push of a switch, you can shed load and have time left to find a good place to land with more than 30 minutes of flat panel functionality at your disposal.)

As we-my flying buddy was Cessna flight test pilot Dale Bleakney-lined up on the narrow little Cessna factory runway tucked in among the huge lanes of concrete at McConnell Air Force Base, I was momentarily taken by the event. Here I was, ready to head out in a 172, the same legendary airplane that has been rolling out of Kansas factories for more than half a century now, and with nary a hiccup along the way. In fact, it's done nothing but get better in that time. And now, we're burning jet fuel. I smiled, checked the mixture, couldn't find it, smiled again, and advanced the throttle to go flying.

****The experience of the diesel, as I said, is different, but it's also an easy transition, especially since it's integrated so beautifully with the Garmin G1000 flat-panel avionics suite. (Duane Wallace, the visionary who dreamt up the present-day lineup of piston singles, surely never saw this coming.) The new engine sounds deeper, throatier, than the Lycomings, and the vibration is different, though not at all unpleasant. I'm sure that a lot of that has to do with the impressively smooth engine mounting system that Thielert has developed for the powerplant. There's no doubt that while it's different, the TD will be easy for new and old Skyhawk pilots to learn to fly. It's just not that complicated.

The "T" part of the TD moniker is for "turbo." In this case the system is set up to produce sea level manifold pressure even as you climb, an approach that some in the industry refer to as "turbonormalizing." On takeoff, you simply push all the levers, er, "knobs," forward. All one knob, that is. And the performance is fine at sea level, which probably would not have been the case had Cessna rolled out a diesel model with just 135 horses. The pilots at Cessna who've been flying the TD compare it with the 160 hp R model at sea level and with the 180 hp S model up higher, though it actually produces more power than that airplane, too, beyond around 8,500 feet.

And that's one of the big benefits of the new engine. As you climb, the engine power remains constant up to the altitude at which it can continue to develop its rated power. So at some point this least-powerful but turbocharged Skyhawk becomes the power champ. This is a great thing if you regularly fly at higher altitudes on cross-country trips, for instance, or when you're flying around higher terrain. In fact, on our flight, the TD continued to produce nearly 100 percent power all the way up to 10,000 feet, our ceiling that day.

**** And it's also a great thing for just tooling around down low, because the Thielert sips at the fuel, burning only around 6 gph at 75 percent power, compared with between 8 and around 10, respectively, for the R and SP model Skyhawks. This fact has generated a great deal of interest from fleet operators, because the prospect of collectively lowering fuel consumption and, hence, costs is an appealing prospect to flight schools and other volume buyers.

While it's called the "turbodiesel" model, Cessna chose not to certify it to burn diesel fuel, even though the engine in other applications is certified to use a certain grade of diesel fuel. But the lack of availability of that fuel in North America along with the ever-changing spec of automotive diesel here in the States convinced Cessna that the safer route would be to stick with old reliable jet-A, which is available worldwide and will be for decades.

One big goal for Cessna in certifying the TD was to avoid airframe-related changes that would require additional flight testing, and bear in mind that jet fuel weighs almost a pound more per gallon than 100LL.

To keep weights consistent between models, Cessna chose to limit the amount of fuel in the two wing tanks compared to the other Skyhawks. This makes it sound as though the TD customer is getting an airplane with less range, but not so. Because the Thielert engine burns less fuel, the TD's range/endurance envelope is actually very close to that of its gas-powered Skyhawk siblings.

One big concern is that aircraft fuelers will accidentally put 100LL in the TD's tanks, but Cessna has taken a number of steps to ensure that this doesn't happen. In addition to the hard-to-miss placards, the tanks are keyed (specially shaped, that is) so that only jet-A-and not 100LL-nozzles will fit. On top of that, there's a spring-loaded door that you need to press down on with the jet-A nozzle in order to get fuel into the airplane. So even if a boneheadedly persistent fueler opened the cap and tried to run fuel from an ill-fitting nozzle into the tank, the door to potential disaster would be closed.

I mentioned that the Skyhawk TD was easy to fly-well, as easy as any airplane is-but I definitely don't mean to imply that it's not a capable airplane, and in terms of avionics, it's remarkably so. The G1000 panel is integrated with the remarkable GFC 700 autopilot, which does a fabulous job of flying the airplane. This is, I should point out, the same autopilot that's in the Citation Mustang. (Remember if you will, that the first 172s entered production only a few years after electrical systems were standard fare in light airplanes.)

I won't give you much of a flight report on the 172TD, other than to say that it flies very much like any Skyhawk, smoothly, predictably, with a light and pleasing touch. Nothing about that equation changes with the TD.

I asked John Doman, head of Cessna's propeller aircraft sales, if prospective training organization customers were worried about how easy it might be to teach its clients how to fly the single power lever Skyhawk. He admitted that while there had been some questions, it wasn't a major concern, and he imagined that students would get add-on instruction on using conventional mixture and prop controls. The situation, he suggested, is a lot like what has happened with the move to PFDs, with add-on training offered to tell pilots about the bad-old days of steam gauges.

Back to the cockpit: As Dale and I climbed the Skyhawk up toward 10,000 feet to crunch the numbers, the most striking figure on the display was the percentage of power setting, which stayed at or very near 100 percent up through that altitude. Now, the Skyhawk is still no speed demon, nor is it meant to be. But the extra performance you get at altitude is a definite bonus.

The climb performance we saw was just as I guessed it would be, adequate down low and very respectable as we climbed. On our climb from Pawnee's 1,378 msl elevation up to 7,000 feet, we averaged right around 600 fpm on an almost exactly standard temperature day.

In cruise, the same trend held true. The TD did well at 5,000 feet and 100 percent power, with a cruise speed of 125 knots true on a respectable 8.5 gph, and at that same altitude at 75 percent power, it trued out at 112 knots on a miserly 5.8 gph. At 10,000 feet, an altitude where I would fly the airplane on many cross-country trips, the TD clocked a true airspeed of 130 knots at 100 percent power (well, 99 percent) and on 7.8 gallons of jet-A an hour. Pulling the power back to 75 percent power, we still saw 122 knots true on just 5.8 gph. The fuel savings at any altitude are impressive, and the increased performance at altitude was easy to see.

** The airplane flown for this report was the first conforming 172TD. It is equipped with the standard Garmin G1000 WAAS avionics package with two 10.4-inch displays, the standard-equipment Garmin GFC 700 two-axis digital autopilot, Traffic Information Services (TIS), Garmin?s IFR approach charting and airport awareness utilities, electronic engine monitoring and more.**

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.