Guy Norris/OLYMPIA
POWERPLANT-modification specialist Soloy plans to start testing a revised design of its 785-1000 Dual Pac twin-turbine powerplant in June, as part of an effort to meet more ambitious time-between-overhaul (TBO) targets.
The Dual Pac consists of two Pratt & Whitney Canada PT6D-117A turboprops and a Soloy combining- and propeller-reduction gearbox driving a single propeller. The Olympia, Washington-based company began endurance testing of the unit for certification on a modified Cessna 208B Grand Caravan, among other proposed applications, but uncovered greater-than-expected wear in some areas after a teardown following a series of throttle "jams".
"We could have finished as far as the [US] Federal Aviation Administration was concerned, but there were parts that neither we nor Pratt & Whitney Canada wanted," says company president Joe Soloy. "We were flying it on demonstration, without any engine problems, and passed through the 500h inspection period OK, but a 3,500h TBO gearbox is our target," he adds.
Improvements include a redesigned, strengthened spline on the propeller shaft, the addition of a "Z"-shaped steel lining between a heavily loaded bearing and the Dual Pac magnesium case, and the possible addition of cooling gills on the gearbox centre-section casing. Improvements will be incorporated into the oil pump and regulator.
Endurance runs of the revised Dual Pac are expected to be completed in September, with supplemental type certification under Federal Aviation Regulations Part 33 anticipated as early as November.
Installed in the stretched Caravan, the Dual Pac develops 990kW (1,330shp) at 1,700RPM. In the event of one of the engines shutting down, the powerplant automatically disengages the affected engine and its associated drive train from the propeller shaft.
Barring unforeseen problems, the Dual Pac-equipped Caravan is expected to be certificated to Part 23/25 standards in late 1997.
With a 1.83m fuselage stretch, the modified Caravan is capable of carrying up to 15 passengers at an eventual target maximum-gross-weight of 5,675kg, compared to 3,970kg for the current model. Although the Caravan is being used by Soloy primarily for proving and certificating the Dual Pac concept, Soloy believes that a market exists for "about 100 retrofits over the next ten to 15 years".
Source: Flight International
