An engine upgrade moves the MU-2 to new heights
Fly faster, higher, and more safely while lowering your maintenance bills! While it sounds like one of those too-good-to-be-true deals on late-night television (but wait, there's more), in this case, it's true — at least for owners of certain models of Mitsubishi MU-2s, Twin Commanders, and Cessna Conquest IIs flying upgraded Honeywell engines.
Adding some 30 to 40 knots to the indicated airspeed, the engine conversion is like dropping a Viagra pill into the fuel tanks of these old airplanes.
Even with their original Honeywell — formerly AlliedSignal and Garrett — TPE331-5 or -6 engines, depending on model, the various MU-2 models in particular have always been considered hot rods. A poor safety record a number of years ago caused MU-2 values to decrease. But typically, the culprit has proven to be the pilot, not the airplane. With proper and consistent training, the airplanes can be flown safely. For someone looking to upgrade from a piston twin, they can be an excellent value, but be prepared to pay high insurance premiums until you acquire a reasonable amount of turbine time.
While the line is no longer manufactured, Mitsubishi Heavy Industries, of Japan, does an admirable job supplying maintenance support, parts, and training. SimCom is now the authorized training center, and it is bringing two new training devices online soon, replacing a pair of antiquated motion simulators. Owners report few problems getting parts, with support reported to be at least as good as that provided by domestic manufacturers. Turbine Aircraft Services near Dallas is the primary parts distributor in the United States.
Considering that the newest models are nearly 20 years old and the oldest ones were built in the Johnson administration, the airplanes have held up well. Many have been upgraded with new interiors and the latest avionics. Garrett didn't neglect the engines either. The development of the TPE331-10 engine series solved some ongoing maintenance issues with the earlier engines and improved fuel specifics to the point that for owners facing an engine overhaul, conversion to the upgraded engines is a no-brainer. For those with midtime engines, the reduced maintenance costs and higher performance of the Dash 10 conversion, as it is called, offer an interesting study in economics.
The original -5 and -6 engines powering most MU-2 models are thermodynamically rated at 840 shaft horsepower, but derated to either 665 or 715 shp, depending on the airplane model. The -10 engines have a thermodynamic rating of 1,000 shp. Like normally aspirated piston engines, turbine engines lose power the higher they climb. With the Dash 10 conversion, MU-2s have a greater "reserve" of power, if you will, to draw on as they climb into the flight levels. The -10 engines are derated to the same shp as the originals, but the extra power allows the engine to make rated power at higher altitudes. It also improves operations at hot and high airports the same way that turbocharging assists piston airplanes in that environment.
A number of internal changes allow the engines to produce more power. The most significant change is the addition of air cooling to the -10 engine's first-stage turbine section blades. In the original engines, cooling air was forced through the turbine shaft. In the Dash 10-converted engines, cooling from the shaft is tapped to cool the blades in the first stage. The cooler temperatures allow the engine to run hotter without damaging the internal components. The cooling air also reduces blade erosion and increases reliability, according to Mark James, manager of business development and maintenance sales for Intercontinental Jet Corporation (IJC), which developed and owns the supplemental type certificate for the MU-2 engine conversions. In addition, the conversion includes a re-bladable first-stage turbine wheel. If a blade erodes beyond limits during the life of the engine, it can be replaced individually. On other models, if one blade goes bad the entire wheel must be junked — a roughly $15,000 part.
Another significant change involves how temperatures in the engine are measured and how power is set. The -5 and -6 engines use interstage turbine temperature (ITT) for power management. The -10 engines use exhaust gas temperature (EGT) for power management — a more accurate means of controlling an engine. As a result, part of the conversion involves swapping out the ship's ITT gauges for EGT gauges — the only obvious change that an airplane has been converted. But go fly -t, and you'll notice the difference quickly.
I had the opportunity to fly a P model MU-2 on several occasions both before and after the engine conversion and can report that the difference in performance is very real. The airplane is owned by Brian Curpier, president of the Orison B. Curpier Company. OBC provides advertising representation to AOPA's magazines and several other titles. A longtime pilot and aircraft owner, Curpier uses the airplane to traverse the country visiting clients.
Prior to the engine conversion in early 2002, I would routinely see cruise speeds of about 265 kt to 280 kt true airspeed, depending on conditions. While the airplane is certified to 30,000 feet, most operators of non-modified airplanes seldom fly above about 25,000 feet because by then the airplane is out of performance. Climbing higher is a slow ordeal and, with less power available, cruise speed begins to suffer.
During several flights since the conversion, true airspeeds have always been greater than 300 kt, with 310 kt a normal cruise speed. On one trip we ripped along at 318 kt. This all happens on about the same total fuel burn of around 80 gallons per hour. Curpier now routinely flies higher than 25,000 feet. On one leg we climbed to FL290 to take advantage of tailwinds. The airplane was still climbing at about 500 fpm when we leveled off. The true airspeed drops closer to 300 kt at those altitudes, but fuel burns also drop to about 70 gph, extending range.
"Every airplane is 30 to 40 kt faster," declares Bob Kidd, president of Tulsa-based IJC. Some airplanes with run-out engines gain even a greater amount with the conversion, he says. One company, Kidd explains, converted its MU-2 because of the desire to regularly meet with a new customer. The customer was a bit farther from headquarters than its other customers — enough farther that the MU-2 would need to make an extra stop en route, which the company didn't want to make. With the conversion, the MU-2 can fly higher with a lower fuel burn, extending the range by the needed 40 minutes to make it to the new location. The conversion, which costs about $450,000 including installation, saved the company from having to buy a new $2 million to $4 million aircraft to serve that client, Kidd says.
According to Kidd, the overhaul of a -5 or -6 engine costs about $186,000. For $200,000 you can overhaul it and convert it to the -10 configuration. In either case, you'll spend another $33,000 total in labor for removing and reinstalling the pair of engines and other items associated with an engine change. The Dash 10- conversion owner can easily recoup the additional expense. For one thing, according to price guides, a converted airplane is worth about $273,000 more in resale value — a realistic figure, reports James. In addition, the -5 and -6 engines have a 3,600-hour TBO, including a hot section inspection (HSI) at 1,800 hours. With a gearbox inspection at 3,600 hours, owners can opt to extend the TBO to 5,400 hours, but the gearbox inspection costs more than $80,000. The -10 has no gearbox inspection requirement if TBO is limited to 5,000 hours. An HSI at 2,500 hours is the only heavy maintenance required between overhauls. IJC estimates that a -5 or -6 engine costs about $119 an hour in heavy maintenance over the life of the engine. The -10 engine averages only $91 an hour.
All told, IJC estimates an owner who flies 100,000 miles a year will save about $24,000 annually in direct operating costs, partly because the faster converted airplane will fly about 48 fewer hours — 322 hours versus 370 hours for a -5 or -6 airplane. Not accounted for is less time away from home or office for the owner and guests or staff. You gain an extra workweek a year by flying faster.
The economics get a bit more complicated for those owners not yet facing an engine overhaul or heavy maintenance interval. In what is called the "Continued Time Conversion," owners can elect to upgrade to a -10 at any time during the engine's life. The upgrade price for a pair of engines, which includes hot section and gearbox inspections, is $335,000, plus the same $33,000 in parts and labor facing an overhaul customer.
With the conversion, owners can elect to stay on a 5,400-hour TBO including the 1,800-hour HSI and 3,600-hour hot section and gearbox inspections. Or they can switch to a 5,000-hour TBO with a 2,500-hour HSI. That decision and whether it makes sense for a continued time conversion depends on many factors. Intercontinental Jet sales representatives can help prospects work through their individual scenarios.
James says that most owners facing any sort of heavy maintenance opt to make the switch to the -10s. Greater safety, less maintenance, and the allure of breaking the 300-kt barrier is too great for most to resist. After all, going fast is what flying an MU-2 is all about.
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