The second prototype of the Grob SPn business jet was destroyed in an accident near Mindelheim-Mattsies Airport, Germany. Chief test pilot Gerard Guillaumaud was killed in the accident. On the day of the accident, November 29, 2006, the airplane had conducted a 60-minute test flight from the factory airfield of manufacturer, Mindelheim-Mattsies Airport. Various flight maneuvers and system tests were completed and the airplane landed at 11:40. The aircraft was parked on the apron and prepared for the next flight. This flight would be a demonstration flight for a group of visitors with several fly-bys. The jet took off from runway 33 at 13:12 and the pilot flew a right hand circuit in and out of clouds. As it was lining up for a fly-by parts from the stabilizer separated. The pilot lost control and the airplane impacted a field. Given the weather circumstances, the flight should have been conducted using Reduced Flight Display specifications. These included a maximum speed of 200 knots. The probable speed of the accident airplane was between 240 and 270 knots. This speed was below the maximum allowed speed for flutter tests, 297 kts. The manufacturer had changed the design of the control surfaces on this second prototype due to anti-icing requirements and to provide more roll authority. PROBABLE CAUSE: The accident occurred because there was an in-flight separation of the tailplane due to flutter and the plane subsequently was no longer controllable. The circumstances that led to the flutter could not be determined clearly due to lack of flight data and limited investigation.