The Sikorsky S-97 Raider technology demonstrator compound helicopter that crashed in August 2017 had substantial damage that included collapsed landing gear, structural cabin damage, propulsor blade damage, and tip damage on all eight main rotor blades of its coaxial system, according to an NTSB factual report issued last week. Other damage included vertical stabilizers and rudders fractured at the attach points on the horizontal stabilizer, the tail wheel fractured at the strut mount, and collapse of the main landing gear—all damage as the result of ground contact, the report said.
During the 4.4-second accident sequence, the pilot flying was ground taxiing when the aircraft developed a slight left roll. The pilot then lifted into a five-foot hover in an attempt to stabilize the aircraft. As collective was applied to get airborne, the helicopter rolled quickly left and right with increasing attitudes that exceeded 60 degrees of bank angle. During one of these roll reversals, the pilot applied full down collective to a landing.
Control law changes introduced in 2015 on the S-97’s fly-by-wire (FBW) flight control system, designed to improve ground-to-air transitions, had the unintended consequence of increasing cyclic stick sensitivity by 2.5 times “while operating in the in-transition path,” the NTSB said. The FBW system on the Raider relies on weight on wheels switches to signal which of three FBW paths are used—in air, on ground, and in transition. Sikorsky said that it modified the FBW system on the Raider after the accident.
The rotorcraft manufacturer has proposed a variant of the Raider for the Army's Future Attack and Reconnaissance Aircraft (FARA) competition.