More performance for less cost
Beechcraft's 90-series King Airs — the so-called "baby" King Airs, because they are the smallest of the series — have been in production for 40 years now. Some of the earlier versions are getting very long in the tooth and face expensive engine overhauls. Traditionally this meant overhauling the 550-shaft-horsepower Pratt & Whitney PT6A-21s using Pratt & Whitney parts and Pratt & Whitney service facilities — to the tune of some $120,000 to $200,000 per engine, depending on the work necessary.
Seagull Aviation, of Clintonville, Wisconsin, has come up with a supplemental-type-certificated (STCed) modification that gives owners of aging King Airs an alternative. Its Premier Walter Conversion replaces those old Pratts with new Czech-built Walter M601-E11A engines of 705 thermodynamic horsepower (the same engines used in the Lancair PropJet kitplane and the Soviet-era Let 410 commuter airliner), flat-rated to 550 shaft horsepower to conform to King Air type certificate specifications. Think of that 705-horsepower rating as giving the Walter engines a power reserve at altitude. By de-rating the engine to 550 horsepower, the engine's power doesn't fall off in the 16,000-to-20,000-foot range, like the standard-issue Pratts. In effect, it's loafing under most hot-and-high altitude situations.
The resultant performance boost pushes Walter-equipped early-model King Air C90s' true airspeeds to the 230-knot range at 25,000 feet — the modified airplane's maximum operating altitude (late-model, production C90s have maximum operating altitudes as high as 30,000 feet). "Pratt-powered older C90s true out at 215 to 220 knots, max," says Seagull President Dan Sigl. "There's also a performance boost when it comes to runway requirements and initial climb rates at higher density altitudes. And the engines won't temp out until you reach 19,000 to 21,000 feet, depending on the outside air temperature." (As a turboprop engine climbs, its torque drops off, but its interturbine temperatures rise in the thinner air, until temperature limits are eventually reached. This becomes the power-limiting factor in high-altitude cruise.)
But perhaps the main attraction of the Walter conversion is its price. It goes for $595,000, and includes both engines; five-blade, aluminum-construction MT Avia V510 propellers (for better takeoff acceleration); installation hardware; new cowlings; autostart with temperature limiters for protection against hot starts; and a 450-pound maximum gross takeoff weight increase. The engines have a recommended time between overhauls (TBO) of 3,000 hours, and those overhauls should cost about $65,000 — about half the cost of a PT6's.
As for combustion-chamber hot section inspections (HSIs), none are required for the M601-Es. That's because hot spots and dirty fuel nozzles — the main causes of combustion-chamber problems, Sigl says — are eliminated with the Walter design. Except for a single nozzle used for starting, there are no fuel nozzles. Instead of conventional spray nozzles, the Walter engines use a rotating diffuser ring to atomize fuel spraying into the combustion chamber. The diffuser ring is stone simple: It has a series of holes around its circumference, and fuel is slung from them as the gas generator rotates. Because the holes aren't subject to plugging, Sigl says, the engines don't require expensive nozzle maintenance or replacement. However, there is a downside: The Walters burn about 4 more gallons per hour than the PT6As.
Other technical features of note include autostart sequencing, electronic temperature limiting, a feathering switch, and a mechanical backup system should the electronic fuel controllers fail. Autostart — not provided on even the newest King Airs — is a significant enhancement.
Push its start button and the Walter begins turning, and fuel is automatically introduced when the proper rotation speed is reached. If a hot start causes combustion-chamber temperatures to rise past limits, fuel flow is automatically shut off. Then the starter motors the engine — turns it without fuel or ignition — to purge fuel and draw cooling air into the hot section. Conventional overspeed limiters are also part of the engine's standard equipment.
There is no autofeather system, so you have to manually feather the propellers should an engine need to be shut down. Should you somehow forget this step before oil pressure is lost, there are toggle switches that activate the feathering process via a dedicated electrical subsystem. Two other switches let the pilot isolate any malfunctioning fuel control units. Once isolated, fuel control is manual, via the condition lever(s).
The conversion takes between 12 and 14 weeks, and some of the cost can be recouped by selling the original Pratts and their propellers. The Walters use the same engine mounts, but because the engines are four inches longer and use different, squarish-shape exhaust stacks, new cowlings and adapters for the stacks are provided with the conversion kit.
A demonstration flight in Sigl's 1971 King Air C90 showed some numbers that edge out the standard airplane's. At 1,300 pounds under the airplane's maximum takeoff weight (10,100 pounds) initial climb rate under near-standard conditions was 1,800 fpm, and climbing through 20,000 feet the airplane was still showing a 1,300-fpm climb rate. At Flight Level 210, minus 12 degrees C (15 degrees warmer than standard), propellers at 1,800 rpm, interturbine temperatures temped out at 720 degrees Celsius, and burning 278 pph (41 gph) per side, the true airspeed was 234 knots. That's about 15 knots faster than a stock C90 of the same vintage, under the same conditions.
Maintenance intervals on the M601-Es come every 300 hours, include simple filter, fluid, and gasket changes, and take 10 to 12 hours per engine to complete, according to Sigl. Seagull will train customers' mechanics in Walter maintenance procedures, and provide CD-ROMs containing maintenance, operations, and installation manuals. Special tools are also provided. Walter's West Helena, Arkansas, facility can handle major maintenance issues, but currently the engines must be shipped to Walter's factory in the Czech Republic for overhaul. As the market base expands, Walter plans to create an overhaul facility in the United States.
The M601-E will carry 1,100-hour/two-year warranties, and a total engine maintenance program (TEMP) covers parts and labor at a cost of $25 per hour and $5 per engine cycle.
To date, six airplanes have had the Walter conversion and two have racked up 900 hours in service with no reported problems, Sigl says. He sees an 1,800-airplane potential market for the Walter conversion. Currently the STC covers King Airs C90, E90, 100, and A100.
Contact: Seagull Aviation, 420 East Seventh Street, Clintonville, Wisconsin 59929; 800/842-6585; fax 715/823-8127; www.seagullaviation.com.
E-mail the author at [email protected].
