Flutter in the frame as cause of SPn crash

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Elevator flutter leading to the separation of parts of the control surface and of the horizontal stabiliser are under investigation as a possible cause of the 29 November 2006 crash of a prototype Grob SPn light business jet, according to an initial factual statement from the German accident investigation agency BFU.

Grob's chief test pilot Gerard Guillaumaud was flying the manufacturer's second prototype SPn light jet (D-CGSP) when it crashed while he was lining up for a high-speed low pass over the company's Tussenhausen-Mattsies airfield in Germany during a demonstration flight for customers, according to Grob chief executive Niall Olver. Guillaumaud died in the accident.

Following the BFU revelation Grob says that "parts of the elevators and parts of the horizontal stabiliser sheeting were found 400m [1,310ft] away from the first impact site, about 1,500m from the runway threshold. The reason for the separation of these parts is not clarified at present, but one of the hypotheses is that it could have been caused by flutter. The investigation is therefore focusing on the speed of the aircraft [relative to] the allowed speed envelope of [the second prototype]."

Shortly after the accident the manufacturer explained that it had modified the second prototype for better performance in icing conditions, fitting larger ailerons and longer stabilisers, but when the accident took place these modifications had not yet been tested throughout the flight envelope. The company has now confirmed that this initial concern is the focus of the investigation.

Grob says the first protoype SPn has been undergoing a 300h programme of inspection and maintenance and that the BFU has no concerns about its empennage design, which has been tested throughout the speed envelope. It expects the aircraft to begin test flying again in the next few weeks, and says the third aircraft will join the test programme during the second quarter of this year. European type certification is due in the first quarter of 2008 with US certification following about three months later, says the manufacturer.