Walter-powered King Airs, a better idea?

Aviation International News » September 2003
August 8, 2008, 9:54 AM

Dan Sigl, president of Seagull Aviation in Clintonville, Wis., has invested a significant portion of his career in Walter 601-powered derivatives of existing airframes, specifically the Beech King Air series. The story of converting C90s to Walter power has its share of twists and turns, and the saga is far from over.

Sigl and his Wisconsin shop are associated with Performance Conversions of Incline Village, Nev., holder of the STC under which the aircraft in the accompanying pilot report was converted to Walter power. Robert Turnage is the managing director of Performance Conversions. Sigl’s shop is also at work on a new STC for the King Air 100 and also has a de Havilland Twin Otter project currently on the back burner. Another unrelated company, Lone Star Propjets of Waco, Texas, also has a separate STC for converting King Air 90s to Walter power.

Unfortunately for both Lone Star and Performance Conversions, Walter-powered King Airs got off to a rocky start. Since the fall of the Soviet Union, the Czech engine has become attractive to retrofitters for its low cost and availability. The engine has been hung on a variety of airframes, including aerial applicators. Its low cost led one company to develop an STC (SA01366AT) in the early 1990s for replacing the smallest King Air’s Pratt & Whitney Canada PT6A-20s with the former Soviet-bloc powerplants. That company completed two aircraft, and both suffered major engine failures. One was uncontained, which led to the seizure of the second engine. Fortunately, both aircraft landed safely, but the incidents led to a February 2000 FAA Airworthiness Directive (AD 2000-02-15). In unusually plain English, the AD outlined the fates of the two aircraft that had the conversions and included the following succinct language: “Basically, this AD prevents future installation of the configuration specified in STC SA01366AT.”

Sigl told AIN, “I still wonder how they got that STC in the first place.” As outlined in the AD, the configuration of STC SA01366AT did not include engine control electronic limiters to prevent overtemping; propeller de-icing; a feathering pump; nor a cabin supercharger for pressurization. In the elegantly blunt understatement of the FAA’s final rule, “These discrepancies, if not corrected, could lead to engine failure and the inability to feather the propeller. This could result in an uncontained engine failure [as indeed, it already had in one case] with consequent loss of control of the airplane,” which fortunately it did not.

Though the economics of re-engining C90s with W601s was tempting, the configuration certainly faced an uphill battle for acceptance by operators. One reason for the attraction is the cost of overhauling the original PT6A-20/21s relative to airframe worth as that early version of the P&WC turboprop becomes increasingly obsolete over time. According to Performance Conversions’ promotional material, the typical cost of overhauling a pair of Pratts and their three-blade Hartzell props is $347,000. Compare that with an estimated net investment of $325,000 ($495,000 minus estimated resale of P&W cores and props at $170,000) for the Performance Conversions upgrade. (Lone Star lists its price as $575,000 with engine exchange, $625,000 without exchange.) Besides the $22,000 difference, the Performance Conversions STC yields a 500-pound increase in mtow; 40-knot increase in cruise speed; improved time to climb and 70-percent-power cruise level (to FL210 from FL180); and better single-engine performance. The brochure also touts the Walter M601E-11’s manually backed up fuel control unit and the Avia VJ8-510 propellers’ electronically backed up feathering mechanism. Finally, the combination of the Walter engines and the five-blade Avia props is said to be significantly quieter (inside and outside) and have less vibration than the original configuration. Maintenance costs of the M601E-11/Avia setup are also said to be markedly lower, on the order of $49 per hour compared with $131 per hour for the PT6A-20/21/Hartzell combination.

But that’s all getting ahead of the plot. Going back to the original STC with all its warts, Sigl picked up the story. He said in the mid-1990s, a Sanford, Fla. businessman named Ron Rosenburg agreed to buy STC SA1366AT from the company that developed it, but later tried to back out of the deal. Still believing the Walter was a good idea for King Air operators, Rosenburg contacted Doug Carlson of Turbine Design of Deland, Fla., who had experience with Walter conversions on other airframes. It was determined that the original STC was not salvageable, so the men decided to start from scratch with a new program designated STC SA 01994AT–the current Performance Conversions configuration. Sigl told AIN that he got involved at this point based on his relationship with Carlson and became a 40-percent partner as they worked together on the engineering for the King Air. In 1999 the STC was approved, and in April 2001 Performance Conversions–formed by Sigl, Turnage and another man (since bought out)–acquired the rights to the conversion configuration.

Based in Incline Village, Performance Conversions markets the STC and recently received approval from the Brazilian aviation authority for the conversion. At the Latin American Business Aviation Conference and Exposition (LABACE) in March, Turnage announced a partnership with São Paulo, Brazil air-taxi operator Taxi Aero to market the conversion in Latin America and allow Constructora National de Avionies (CONAL) to perform the work. He views Australia and Europe as other potential markets.

For his part, Sigl and Seagull Aviation are a designated installation facility for Performance Conversions (along with Dakota Aero of Devil’s Lake, N.D.) and continue to work on a separate STC for the King Air 100. First flight of the prototype is expected this month, and Sigl said he expects the flight-test program to consist of about 25 hours of flying time. “It’s not rocket science,” he said. “The 100 and the 90 have the same wing, engine mount and nacelle.” He has high hopes for the King Air 100 conversion market, expecting the cost to be in the $580,000 to $620,000 range. Sigl said the PT6A-28s on the larger King Airs are worth a lot more in resale than the -20/21s of the 90 series–about $340,000 for the pair of -28s. Another plus for Walter conversions is the recently announced total engine maintenance program from the Czech manufacturer. The program is listed as $25 per hour and $5 per cycle and covers “all but routine basic maintenance,” said Sigl.

In the past, bad initial press and tangled corporate involvement have torpedoed some of the best technical ideas in aviation. The story behind the Walter-powered King Air certainly has had its share of both. Whether or not Walter-powered King Airs become commonplace depends in no small measure on whether the market attains the critical mass needed to bring existing and future programs financial success. Sigl believes the performance and cost-saving numbers add up. It remains to be seen if enough operators agree with his math.

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