When will a diesel be available for my aircraft?
See how many times we've been asked (organized by Aircraft Model).

News, facts, and comments on the coming revolution for piston-engine aircraft.

News of March 23, 2008

Considering the Cessna 172 Thielert after the Flying Magazine report (April 08 issue.)

Well, Cessna now sells a diesel airplane. And it is the famous, eternal 172, the most ubiquitous flying machine in history, a machine which will certainly still be flying in huge quantities 50 years from now. And Flying Magazine has published a very comprehensive report on the product. A major event, considering that only 7 years ago, when I first published the DieselAir Newsletter, the prevailing opinion was that the aero diesel engine would never get to the US market. And the report is very favorable, while underlining objectively all the pros and cons of this product. Bravo to Roger Goyer who wrote the report.

The pros: The 172 Thielert, called 172TD, is fast: 130 knots at 10,000 feet, despite a take-off power of only 155 HP versus 180 for the SP model (180 HP). This is of course because it is a turbo with a constant speed prop, and can fly higher. It is fuel efficient as one could expect: At 130 kts and 10,000 ft it burns 7.8 gallons per hour; but at 5,000 ft and 112 knots, only 5.8 gph. The test did not cover fuel flow at best glide speed, a test which would have verified our most important point: Between and a bit above best glide speed (1.3 to 1.5 times stalling speed), it is so low that one can consider staying in the air for ever, addressing any realistic need or factor preventing any landing anywhere such as unexpected fog, or obligation to fly back because of icing. (Yes, of course, it would never happen to you because you are such an experienced pilot. But have you ever thought that if GA is going to grow again, there will be again lots of inexperienced pilots flying?) I hope someone will complete that test some day. All these combined confirm our other point: A 172 diesel is a practical IFR machine, something which is somewhat questionable for a gasoline 172 because of its limited fuel capacity and relatively short range with full IFR reserves.

However, the no-wind range at 85% power (118 kts at 8,000 ft) is 588 NM. A respectable figure for sure, but relatively modest for a diesel. How come? This is because Cessna decided to reduce fuel capacity to 44 gallons instead of 50 on a 172SP. A 6 gallons difference with a fuel flow of 5.8 gph can be important. Why did Cessna do that? To understand them, we must discuss what the accessible market for this product is during the next ten years.

In the US, interest for diesel airplanes is slowly growing, but not that fast. Avgas is still very accessible except perhaps in Alaska. It is still priced much lower than anywhere else. Engine prices from Lycoming and Continental for new or remanufactured engines are low because these engines are still produced in mass quantities with fully amortized tooling at costs in a very low dollar. In front of them, Thielert, and even more SMA or Wilksch diesels, are produced in tiny quantities at costs in sky-high Euros or Pounds. It will take several years – at least 5, more probably 10 years - before their manufacturers can organize to outsource at lower cost and produce in such quantities. Thielert is the leader, but it manufactures in costly Germany the most complex of all aero diesels: liquid cooled, gear transmission with clutch and flywheel. It reflects in engine prices. (Besides, have you ever noticed that a BMW is more pricy than a Ford?) In the US, a private pilot considering buying his first new plane will think twice before buying a 172TD which, incidentally, is priced at $300,000 which is 15,000 more than a 172SP and some 50,000 less than a 182. If he is going to fly 150 hours a year, his choice is not obvious.

But this is completely different for a US flight academy planning to fly its 172s at 1,000 hours per year. In this case, Cessna demonstrates that the operating cost, even including very high TBO costs for the Thielert, will be $5 per hour less than with the Avgas model. That means $5,000 per year. And this difference will grow together with the price differential between Avgas and JetA, which can only go up. However, note that a flight academy does not need long range planes… Therefore the 44 gallons, allowing a higher payload.

It is also completely different, and more so, for foreign flight academies who have to pay a much higher fuel price differential, and who face the growing problem of vanishing Avgas.

And then you have the countries where Avgas has disappeared, or is available only in reusable tanks of dubious cleanliness, notably Africa. And the countries who never had Avgas at all, such as China. Overseas, diesel airplanes are in demand right now. The very small but growing US production of refurbished Cessna 182s re-engined with the SMA 230HP diesel is mostly for export. When the Maule MX9 diesel will finally obtain its long waited STC, it will first sell overseas. The overseas market needs diesels, period. Whether they can fly non-stop at 200 kts for 1,000 NM with 2 hours reserves will be nice, but they can wait. It will be the overseas market which will pull aero diesel production figures up to where costs will eventually go down.

In the mean time, Flying Magazine underlines the cons: Right now, a Thielert engine is very expensive. Also its TBO. It needs preventive maintenance at key times such as 300 hours, and a major inspection at 1,200 hours. No doubt this will improve with time. Also other diesels will compete with Thielert proposing simpler designs: Air-cooled non geared, O-engines such as the SMA, and 2-strokes engines such as the Wilksch, the Gemini and the DeltaHawk. And do keep in mind that, in the world of aviation, the only things that go fast is the airplane itself when it does fly, which is not very often. All the rest moves much, much slower than in any other industry…

Meanwhile, Paul Bertorelli (Aviation Consumer) is quite right to question if diesel are more economical. Right now, in the US, they are not, unless you fly 1,000 hours per year. Overseas, they are coming, period. And GA growth is happening overseas. So, the diesel will come. And when it will, finally, come full swing in the US, yes, by then, with Avgas at maybe 10 dollars a gallon, it will be economical. And in the mean time, Cessna is here now, and will be here by then. Bravo to Cessna: there still are US manufacturers who understand corporate strategy.

Andre Teissier-duCros

posted at 6:56 PM

Previous Posts

FAA issues emergency AD for Thielert Engines

News from the Ecofly diesel light aircraft (German...

What is the future of Avgas?

Continental will finally develop its own diesel en...

Jordan Aerospace, new OEM customer for Thielert?

News from the Beech Duke, Thielert conversion.

It seems that Delta Hawk may now be finally gearin...

Vulcan Air has abandoned its project to equip a P6...

A diesel for the Mooney?

Will aero diesel remain European? No.