German composite aircraft maker Burkhart Grob has modified the rudders on aerobatic G115s to overcome restrictions imposed after a fatal crash caused by an unauthorised repaint.

The US Federal Aviation Administration banned G115C and D pilots from performing aerobatics, following the loss of a G115D-1 in Florida in August 1996. The aircraft broke up in flight, following control-surface flutter. Both occupants were killed.

According to Grob, investigators found that the aircraft had been repainted in an unauthorised paint shop. None of the control surfaces was removed during the work, which, in addition, was not signed off by an FAA inspector.

The ailerons, elevators, rudder and flaps were not reweighed and rebalanced, despite this being mandatory in the operating manual. The control surfaces' residual movement was not checked against approved limits, and the empty-weight centre of gravity was not recalculated. Experts found that the increased mass and residual movement of the repainted rudder could lead to flutter, which was "almost certainly" the crash cause.

Grob has introduced a modifica-tion involving the installation of a small additional mass balance in the rudder and elevators to provide an extra safety margin to the critical flutter speed.

Source: Flight International