All turbine-powered airplanes must have an FAA-approved maintenance inspection plan. It is possible to obtain approval for a plan of your own design -- and that is common for an airline -- but realistically for the individual operator, the FAA approves the plan developed by the airplane manufacturer and you may add to it, but won't be able to subtract items. The maintenance requirements have been historically more intensive for transport airplanes and jets than for turboprops. For example, some jet maintenance plans require that the wing skin be periodically removed to inspect the spars and internal structure. In the Learjet 20 and 30 series the wing must actually be removed from the fuselage to inspect critical areas that cannot be seen any other way. The degree of disassembly required for major inspection phases in jets would shock owners of propeller-driven airplanes.
The military has taken much the same attitude toward the turboprops it operates. At set intervals of calendar or flying time, the military has private maintenance facilities with expertise in the model of airplane -- such as Stevens Aviation -- conduct an intense inspection that requires massive disassembly of the airframe. That would not be required if the airplane were operated by civilians, but the military, like the FAA, has learned that time can create unexpected wear.
So, I think the Conquest SID program is actually a glimpse into the future for other turbine-powered airplanes as the industry learns how to address aging airframes. The emphasis is on turbine airplanes because they are heavier and operate over a much wider range of speeds and altitudes than piston airplanes. And corrosion is always a threat in a turbine airplane because moisture intrusion is unavoidable. When the airplane becomes cold soaked in the frigid air at cruise altitude, and then descends quickly into the relatively moist air near the surface, condensation soaks the airframe. Condensation promotes corrosion, and corrosion weakens metal and creates the starting point for a crack.
Yingling has about six Conquests in the SID process at any time, and an amazing level of special tools, fixtures and personal expertise is required. As you can imagine, most of the attention is focused on the wings and tail, and certain phases require removal of the entire vertical and horizontal tail. On the 425 the wings must also be separated so the spar attachment fittings can be examined. Yingling has developed special fixtures so the wing isn't moved further from the wing root than necessary to simplify rejoining the two. Another phase calls for the engine nacelle to be removed from the upper surface of the wing. Yingling has developed a technique that allows them to separate the two for inspection, without removing the entire nacelle completely, saving a great deal of time, and thus money.
Other key areas to be inspected are less obvious. For example, the pressure bulkheads fore and aft require eddy current inspection so just about everything from the cabin and instrument panel must be removed. Windshields, cabin windows and the window frames all require special inspection. Even cabin door hinges have their own inspection requirement.
Most of the inspection phases are required to be repeated no more frequently than 2,500 hours or five years, and some of the complicated and expensive phases such as the wing spar and fittings on the 425 are good for 5,000 hours or 10 years.
The base inspection for the 425 at Yingling, including a new access panel to allow future inspection of the stub wing fitting, and new deice boots, costs $110,000. With all repairs made the out-the-door cost for the 425 is averaging between $160,000 and $190,000. The 441 does not have stub wing fittings so no wing separation is required, and no new access panels, so its base inspection is a little less at just under $100,000. With all repairs made the complete price at Yingling for the 441 is ranging from $170,000 to $200,000.
