Flutter cracks the tail boom of a Silent 2 Electro on final glide

The pilot of a Silent 2 Electro was unhurt, but the glider was substantially damaged when flutter cracked its tail boom on the final glide home near Wallis. Descending from 1,830 m MSL at 146 km/h, at about 1,220 m it began to shake severely; the pilot opened the spoilers and the shaking stopped at about 111 km/h, then returned as he accelerated again. He slowed once more and landed straight in without further trouble, and only after climbing out did he find the composite tail boom cracked in several places. Why the flutter began could not be determined.

1. Final glide home: After a cross-country flight the pilot set up his final glide at 146 km/h, descending from 1,830 m MSL toward the GHSA gliderport at Wallis.
2. Severe shaking at 1,220 m: In straight and level flight the glider began to vibrate and shake severely. The pilot reported no bird strike and no severe turbulence.
3. Spoilers out to slow down: Suspecting elevator flutter, the pilot opened the spoilers to reduce speed. The shaking stopped once the glider was down to about 111 km/h.
4. Shaking returns with speed: He checked that the glider still handled normally and slowly increased speed, at which point the vibration came back. He slowed again and held that speed to the field.
5. Tail boom found cracked: The straight-in landing was uneventful. Only after climbing out did the pilot see that the composite tail boom was cracked in several places, 51 cm forward of the fin.