Flutter under scrutiny in crash of Grob SPn

 - March 7, 2007, 9:11 AM

Germany’s Federal Bureau of Air Accident Investigation (BFU) has provided some initial findings from its inquiry into the crash of Grob Aerospace’s second SPn jet prototype on November 29 last year. The early conclusions show that the investigators now have a fairly clear idea of how the accident happened, but they are not yet completely sure why it happened.

According to a February 7 statement from Grob, the preliminary BFU report said that parts had separated from the SPn test aircraft before the crash as it was approaching the manufacturer’s private Tussenhausen-Mattsies Airport in southern Germany, intending to make a high-speed pass with the landing gear retracted. Parts of
the elevator and horizontal stabilizer skin were found some 1,300 feet from the first impact site, which was almost a mile from the runway threshold.

BFU officials have not yet determined why the parts separated, but they are investigating the possibility of flutter. The investigation is now focusing specifically on the speed of the SPn before the crash, and how this might have deviated from the permitted envelope for the second prototype, which was not fully opened at the time of the accident. In fact, as a result of earlier flight tests on aircraft number one, Grob had enlarged the elevators and horizontal stabilizer of the second prototype.

As the BFU continues its investigation, test aircraft number one has been undergoing a 300-hour maintenance check, combined with thorough scrutiny of the whole aircraft. According to Grob, the BFU has said that it has no concerns about the initial tail design of the first SPn as its envelope has already been opened to high speeds in excess of Vmo up to Vd. Grob anticipates that Germany’s Federal Office of Civil Aviation will allow flight-testing of the SPn to resume once the examination of this airframe is complete, probably this month.

Meanwhile, Grob is accelerating production of test aircraft number three, which it hopes to have flying by next month. The company has confirmed that a fully conforming fourth prototype is due to join the flight-test program in September, along with the first production aircraft. The revised target date for certification is to get European Aviation Safety Agency approval in the first quarter of next year and FAA approval in the following quarter.

“We are grateful for the superb support from customers and have been greatly encouraged by the fact that we are continuously taking firm orders for the SPn,” commented Grob CEO Niall Olver. Speaking to AIN at the MEBA show in Dubai, he said that prospective customers in the Middle East have reacted favorably to the SPn’s relatively spacious cabin size and to its price/performance ratio (including unprepared runway capability).