The student pilot told investigators that the experimental Titan Tornado S had experienced uncommanded yaw on previous flights, which he was trying to eliminate.
After some research and a conversation with the airplane kit manufacturer, he installed vortex generators.
The vortex generators did not eliminate the yaw, so he installed makeshift winglets on the stabilator, which were made from ½-inch plywood and aircraft speed tape. He did not ask the airplane kit manufacturer about the use of winglets on the stabilator, nor was he required to.
The student pilot conducted a test flight with the makeshift winglets and the uncommanded yaw was eliminated. Since the winglets proved successful, he created new winglets out of composite material, then attached them to the stabilator with glue and rivets.
He completed three to four test flights with the new winglets and remained in the traffic pattern at the airport in Haskell, Oklahoma, for all of the test flights. The airplane maintenance logbooks did not contain any entry for the winglets because the pilot was still conducting test flights and was not ready to have a mechanic sign off on the installation.
The student pilot stated that on the morning of the accident, he intended to complete the first winglet test flight away from the airport and at a higher altitude than the other test flights. He departed from 2K9 and climbed about 600 feet per minute to 2,000 feet where he intended to level off. He stated that the airplane flew really well with no anomalies noted at that point.
As he decreased the pitch attitude, the airplane suddenly vibrated aggressively and it “felt like the tail was thumping.” He decreased the engine power to slow the airplane down and descend in altitude, but the thumping and vibrating continued. He pitched the airplane down for an off-field emergency landing. Before he was able to land, the airplane rolled inverted and descended into the trees. The pilot was seriously injured in the crash.
The responding FAA inspector noted that the airplane was found in a densely wooded area with the right wing separated and significant impact damage to the entire airplane. The stabilator control horn was found fractured and disconnected from the push-pull tube.
A post-accident examination of the stabilator control horn revealed that it fractured due to overstress from gross mechanical deformation. The part also exhibited impact damage from the stabilator being actuated from control stop to control stop during the flight.
The airplane kit manufacturer engineer stated that winglets added to the stabilator would change the balance of the control surface and cause flutter. The stabilator is installed with a counterweight calibrated specifically for the stock stabilator. He had never seen anyone add winglets to the stabilator before and he hoped that a builder would contact him before adding winglets. He would tell a builder not to add winglets because it would alter the balance on the control surface.
Probable Cause: The student pilot/builder’s addition of winglets to the stabilator, which resulted in flutter and overstress failure of the stabilator control horn.
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This May 2020 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Flutter can KILL YOU. Control surface imbalance, loose linkage, poor design and even poor maintenance can all be contributors to flutter. That “Buzzing” you hear / feel when in a steep descent may be a subtle warning of an impending DISASTER and definitely should not be ignored.
As a pilot, you need to know how to react. Answer? REDUCE SPEED IMMEDIATELY! Chop throttle, ease back on elevator – Do anything you can to slow the aircraft in a hurry NOW and PRAY the buzzing / vibration stops.
Flutter can very quickly cause destruction of control surfaces, their attachment points, linkage or whatever surface the control is attached to.
Get on the ground to troubleshoot, but don’t be in so big a hurry as to let your speed build up enough again to re-excite and worsen the problem.
Many early ultralights had problems with control surface flutter because of their lightweight construction and flexibility. Fortunately, it did not take long for them to improve designs with adjustable mass balance weights which cured most problems. BUT, even a thin layer of new paint on a control surface can affect the static balance enough to cause flutter. There is a definite reason why aircraft manufacturers require control surface re-balancing after aircraft painting.
Aviation Safety Magazine published a good article on fighting aerodynamic flutter.
https://www.aviationsafetymagazine.com/features/fighting-flutter/
Jerry King