October 1, 2007
By Thomas A. Horne
There's an old saying in aviation: There's no substitute for power. Want to go faster? Get a bigger engine. Want a sprightlier climb? Power is the answer. Want to fly a heavier airframe at a faster speed? Add more power. Want shorter takeoff runs better acceleration during go-arounds or missed approaches? Then bump up the power-to-weight ratio. It's rules like these that have motivated aircraft manufacturers to make predictable horsepower increases with each successive new model introduction.
For the past eight years, Blackhawk Modifications, Inc., of Waco, Texas, has been successfully promoting the "more power" mantra for a range of older turboprop twins. Blackhawk installs more powerful versions of the Pratt & Whitney PT6A engine family on Beechcraft King Airs, Cessna Conquest Is, and Piper Cheyennes. Those airplanes came off the assembly lines with PT6A engines of their own, but Blackhawk's upgrades boost takeoff, climb, and cruise performance far above the stock airplane's numbers. A Blackhawk modification also gives an airplane's resale value a healthy shot in the arm. To date, Blackhawk has retrofitted new engines to some 80 King Air C90s, seven King Air F90s, 31 Cessna Conquest Is, and 17 Cheyennes.
For the most popular Blackhawk conversion, the King Air 90 series, the centerpiece of the program is the Pratt & Whitney PT6A-135A engine. This powerplant has a maximum thermodynamic horsepower rating of 944 equivalent shaft horsepower (eshp — a measurement that combines both the horsepower exerted at the propeller shaft [shaft horsepower, or shp] and the thrust added by the engine's exhaust). The PT6A-135A engine is limited by a torque restriction to 750 shp, giving it greater durability and longer life, which makes sense because it's not working to its full potential. Blackhawk buys these engines new, straight from Pratt & Whitney Canada, and then derates them further, to 550 shp. This is the same horsepower rating used in stock 90-series King Airs — but they use PT6A-20 or -21 engines (except the F90s, but that's another story).
So if Blackhawk's -135A and the stock -20/-21s both put out the same maximum power at takeoff, what's the advantage of the conversion?
Answer: -20 or -21 engines can put out 550 shp all right, but they're working at their maximum torque and/or temperature limits to do so. After these engines reach about 14,000 feet, their power drops off. By this point, they reach their inter-turbine temperature (ITT) limits — redline on the ITT gauge — and the power levers must be managed with an eagle eye on the engine gauges. If the pilot pushes the power levers forward for more power, ITT redline is exceeded, and engine damage soon follows. That's why many pilots call the ITT gauge the "resume" gauge — bust the limit and you're looking for a new job.
Blackhawk's -135A engines can maintain their 550 shp all the way up to 18,000 feet. Derating these engines to 394 shp less than their full capacities means that they're essentially loafing — all the time. Their ITT limits — set for running at the full thermodynamic power rating — are 800 degrees Celsius. The stock -21 engines have 695-degree-Celsius ITT redlines. The -135A's higher ITT redline means that you won't "temp out" at altitude. Where the -21s begin reaching ITT redline, the pilot flying a -135A engine can keep on adding power, since extra torque and ITT margins still are available.
This higher ITT redline translates into all those power advantages mentioned earlier. Blackhawk-equipped airplanes use less runway for takeoff, climb quicker, maintain higher climb rates to altitude, and cruise faster than their stock-engine counterparts.
I flew a Blackhawk-converted 1990 King Air C90A and noticed quite a performance boost. Even after leveling off twice during a step-climb, the airplane reached 14,000 feet in nine minutes and maintained a 2,000-fpm rate of climb along the way. It took 12 minutes to reach 25,000 feet.
In cruise at 25,000 feet, the King Air XP (that's what Blackhawk calls its 90-series modification) turned in 265 KTAS (the stock airplane maxes out at 224 KTAS, Blackhawk says). That's at a weight of 8,500 pounds; maximum takeoff weight is 10,100 pounds. "For a ballpark number, the Blackhawk King Airs are about 45 knots faster than the stock airplanes," said demonstration pilot Glen Vanhouten. "And on an 800 nautical mile trip, you'll get to the destination anywhere from 45 minutes to an hour before the standard airplane." Aviation consulting firm Conklin and deDecker says the speed and time advantages are more like 20 knots and 15 minutes, respectively.
With the -135As, the C90A's single-engine climb rate is about 20 percent better than the standard airplane's 554 fpm (at maximum takeoff weight and under standard conditions), depending, of course, on weight and density altitude. Blackhawk says the King Air XP also has a single-engine service ceiling of up to 21,000 feet, depending again on weight and atmospheric conditions; the stock C90A pilot's operating handbook claims a 14,260-foot single-engine service ceiling at maximum gross takeoff weight.
Blackhawk's 90-series conversion costs approximately $595,000 (plus installation, which runs between $30,000 and $35,000) for both engines, which is an exchange price. You trade in your tired old -20s or -21s, which are prorated in value based on their time remaining before recommended overhaul. You're credited $35 per hour per engine, based on the hours to TBO. The conversion takes three weeks and includes new reindexed engine gauges. The work can be performed at one of Blackhawk's installation facilities and, in a new development, also at Hawker Beechcraft Corporation factory-owned service centers. This option amounts to something unusual: a manufacturer tacitly endorsing an aftermarket modification to its product — and a major modification at that.
But then again, if imitation is the sincerest form of flattery, Hawker Beechcraft has already paid a compliment. Its new King Air C90GT also sports the PT6A-135A engine, so in effect Hawker Beechcraft is now doing what Blackhawk has been doing for several years. It's more confirmation of the "power rule": yet another horsepower increase in yet another new model introduction.
E-mail the author at firstname.lastname@example.org.
For more information about Blackhawk and the rest of their offerings, visit the Web site, or contact them at Blackhawk Modifications, Inc., 7601 Karl May Drive, Waco, Texas 76708; telephone 254/755-6711; fax 254/754-1532.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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