McDonnell Douglas Helicopter Systems (MDHS) Explorer helicopters, grounded on 29 May after discovery of a broken drive-link, have been cleared to resume flying after the US Federal Aviation Administration approved a redesigned part.
MDHS originally discovered the problem on 8 May when a US operator found a broken adjustable collective drive-link, part of the rotor-head assembly, during a post-flight inspection (Flight International, 4-10 June). Subsequent tests then revealed more serious basic design faults with the part, causing the company to issue the grounding order. A redesign was authorised and the new link rapidly tested in the laboratory and on a flight test Explorer at MDHS' Mesa base in Arizona. The FAA approved the revised part allowing MDHS to lift the grounding order on 7 June.
MDHS, confident with its new design, also authorised the manufacturer of the part, Eason & Waller Manufacturing and Precision Grinding of nearby Phoenix, Arizona, to begin fabricating new assemblies before the laboratory and flight tests were complete.
The parts were then shipped to all operators to be fitted so that flying could restart immediately the ban was lifted.
The first MD 600N, a stretched derivative of the MD 520N light single-turbine helicopter, was handed over to Grand Canyon tour operator AirStar Helicopters on 6 June following its certification by the FAA. Design changes resulting from a series of flight-test incidents delayed certification and delivery by six months but the delay does not seem to have dented the orderbook, which has doubled to more than 40 since the helicopter was announced in 1995.
Design changes include a raised minimum rotor RPM, repositioned anti-torque thrust exit and control runs to prevent rotor-blade strikes as well as strengthened landing struts and an improved "crashworthy" fuel system. MDHS hopes to deliver up to 30 MD 600Ns by the end of 1997.
Source: Flight International
