Piper's Archer DX Diesel
Piper chose the Aero show two weeks ago to unveil itís new diesel-powered Archer DX for good reason. The work on the conversion was done at Continental/Technifyís skunk works at the Altenburg airport, a former East German military base where Thielert did all of the original test flying for its initial certification of the Centurion line. Further, as Piperís Simon Caldecott said in this video interview, Piper, rightly, sees the DX as more world airplane than a U.S. airplane.
After the close of Aero, I took a brief flight demo in the DX with Piperís Dave Athay to collect some performance data. As with all of the diesel conversions, the DX performed about as I expected or maybe a little better in cruise.
The DX has the 155-HP Centurion 2.0s; same basic engine as the 2.0, but with a tweaked ECU to deliver the higher power. For now, it has a 1200-hour TBR and a 600-hour gearbox replacement requirement. Although it concedes that the TBR needs to be raised, Continental insists the economics work at 1200 hours. More on that later.
According to the DXís draft POH and what performance observations I could make, the DX requires a little more runway for takeoffó918 feet (280 m) against about 800 (244 m) for the Lycoming-powered version. Thatís at gross weight, sea level on a standard day with 25-degrees flaps, which the DX POH recommends. The diesel climbs a little slower; the POH says 737 feet initial, compared to 775 for the gasoline version. But Iíd like to see a little more flight data on that.
But the comparison between the two is not straightforward and flightschools considering one will want to put a sharp pencil on their own numbers from a gasoline trainer against the diesel numbers. For example, although the DX initially climbs slower than the Lycoming Archer, it catches up at higher altitudes because the diesel is turbocharged. In time/fuel/distance to climb, the two are close. The DX gets to 8000 feet in 11.2 minutes and flies 16.3 miles on 1.8 gallons. The gasoline Archer gets there a half minute sooner at about the same downrange distance, but requires 2.2 gallons.
Cruise comparisons are muddier.†Do you compare them as equivalent horsepower? Percentage power? Or money burned? Using equivalent horsepower, the relevant metric is speed and fuel burn. Seventy percent power in the DX is 108 HP. But for the gasoline versionóremember, it has 180 HPó108 HP is 60 percent power. Comparing the two at those power settings, the DX will deliver 112 knots on 5.8 GPH, the gasoline Archer 114 knots on about 8.4 GPH. At higher power settings, the fuel consumption difference can be greater than 4 GPH. The higher you go, the more advantage the DX has, in speed, if not fuel burn delta, since the Lycoming necessarily burns less the higher it goes because itís making less power. I saw 126 knots TAS at about 80 percent power at 6000 feet, which was as high as we could get, due to cloud cover. That was on about 6.2 GPH. The Lycoming Archer POH says it will do that †speed at 85 percent power on 10.5 GPH.
I wonder if flightschools might change their training doctrine a little to accommodate the fact that diesels climb slower at pattern altitudes. I noticed this in the Redhawk conversion and itís true in the DX as well. If I were grinding out landings in the pattern, I might prefer the gasoline model or, in the diesel, simply not climb to 1000 AGL for the downwind. Thereís no particular need to do that. (In the Cub, Iím thrilled to make 400 feet.)
Proving once again that flying is hours of sheer terror interrupted by moments of profound stupidity, I found the means to test the carefully considered cert protocol for the Centurion. The Centurions use common-rail injection, returning fuel to the tanks from the pressure side. Dave Athay noted that the G1000 tracks tank temperature and sure enough, the left tank we were running on was warmer than the right. So Athay asked me to switch. Somehow, I managed to get the fuel switch to off briefly, before getting it on the right tank and feeding again. The engine sputtered and resumed running, just like a Lycoming would.
Well, not exactly. Two warning lights came on announcing both ECUs had failed. Obviously, they had not, otherwise weíd have been gliding. What we were seeing was evidence of the Centurionís automotive DNAóthe equivalent of a check-engine light. In the car, the ECU might very happily shut the engine down to protect the pumps and injectors from a low fuel-flow condition, but that logic would never get through aircraft engine certification. So the ECUs stay alive, but the fail lights serve as a fault indicator to check the high pressure pump and reset the codes, which the Tecnify techs did. (CORRECTION 4/30/14: The fault lights can be cleared inflight by holding down the FADEC button for three seconds.)
While the fuel valve is a little different than the gasoline Archer because the fuel system is also different, I donít think itís a bad design. My error was just due to simple carelessness, even if I caught it immediately. (The switch has an interlock pin to off; I donít recall pulling it, but I sure as hell must have.) Interestingly, the software has two layers of warning for low fuel. The first is a yellow low-tank warning light, the second a low rail-pressure warning light. Both of these assume a tank just running dry, not actually switching the fuel off, because no one would ever do that. The POH probably needs a limitation restricting journalists operating three video cameras and a notebook from touching the controls.
Besides paying closer attention to tank switching, this also serves as a reminder that diesel enginesóat least the Centurionódeliver impressive economy at the price of complexity, expense and if not the same robustness as a gasoline engine, a different kind of robustness. Thereís no free ride here. In any device run by software, there are always surprises, as Diamond found out when both Centurions on a DA42 quit because of an unforeseen power interruption to the ECUs caused by a flat main battery.
My initial analysis of the DXís operating costsóindependent of capital costs--show that itís not a player for the U.S. market in lower operating costs, which Piper concedes. But in Europe, where avgas costs $10 to $12 and Jet A is around $7, the DX looks attractive, even at a 1200-hour TBO. On paper, it appears capable of direct, engine-only operating costs of about $88 an hour, versus $112 for the gasoline Archer. At higher TBRs, the cost difference increases to nearly $50 an hour.
But here again, the Centurionís automotive DNA intercedes. Back in the beginning, Thielert struck a deal with regulators to enhance safety by replacing gearboxes at low hourly intervals. It made sense then. But 12 years later, the restriction lives on. On an hourly basis, the 600-hour, $6948 gearbox replacement package is, of itself, more expensive than the hourly cost of an entire Lycoming O-360 overhaul. I canít help but wonder if what was prudent conservatism in the beginning is now just regulatory inertia. Might those gearboxes really be good for twice as many hours or maybe even the life of the engine? If thatís true, the Centurionís economics appear incrementally better.†
Whether Continental got a good deal on its purchase of the former Thielert Aircraft Engine assets is, at this point, an unknown. It really depends on how rapidly they convince OEMs to adopt the Centurion line and how lively the conversion market becomes. Just as a datapoint, Thielert did about 270 conversions of Cessna 172s and Piper Warriors, plus some Robins, between the time the engine was available for STCs (about 2003) and 2008, when the company became insolvent. Call that about 50 aircraft a year, just to round it off. In some ways, the early STC work by Thielert was the smartest thing the company did.
During roughly the same period, but including the years after the bankruptcy, Diamond built about 600 DA42 twins and another 400 DA40 singles. Given current market conditions, thatís none too shabby. But the market has evolved since then, so demand is an unknown.
Of late, Piper has been on a tear with the Archer. It sold 48 Archer IIIs in 2013, making the PA-28-181 the hottest selling airframe in the Piper line, a position it hasnít occupied for years. Caldecott believes that rather than cannibalizing the Lycoming-variant, the diesel model will open new markets where Piper couldnít gain a foothold before. That will be Europe, Asia and probably Africa at some point, largely for a training market where avgas wonít be available, if it isnít already.
The airplane will live or die on operating costs; not payload, not performance and certainly not as a cheaper mousetrap, although Piper says the Archer DX, at $400,000, will sell for just under what the Cessna 172 SP does now. I havenít crunched the full number set yet, but the flight data I collected confirms that the airplane does burn about 3 to 4 GPH less than the gasoline version in typical operations or about a third less in block-to-block fuel burn.†
As with the DA42 and DA40, the real measure of the DX will be after a half dozen are out in the training world for a year. Weíll then see how robust the 2.0s is and how the long-term numbers play out. Piper has taken a measured risk on this project. Itís not a new cert project and I donít think it was a particularly expensive STC to develop. Itís nothing like the risk Diamond took with the DA42. Either way, risk doesnít define success; sales do. General aviation has been known to both reward and punish boldness.