SEA06LA004

The pilot was descending from 9,500 feet for weather, and while maneuvering around a cloud cell at 8,000 feet the airplane began to vibrate. He reduced power and rate of descent, and continued to his destination. After a normal landing, he discovered the "serious wrinkling" between the fuselage and the empennage. Postaccident evaluation of the airplane found no pre-existing cracking in the engine mounts; engine vibration analysis results were within limits. Ruddervator balance tests were also within limits. Radar data indicated that the airplane's maximum ground-track speed was 185 knots, or 213 mph. When this was corrected for altitude and winds aloft, the Investigator In Charge determined that the airplane's indicated airspeed should have been approximately 190 mph when it was descending through 8,000 feet. The airplane's Pilot's Operating Handbook (POH) states that this speed is in the airspeed indicator's yellow arc, or between 176 and 202 mph. The POH further states that maximum structural cruising speed (Vno) is 176 mph, and remarks, "Do not exceed this speed except in smooth air and then only with caution."

On October 7, 2005, at approximately 1400 Pacific daylight time, a Beech G35, N4647D, was substantially damaged when it experienced an in-flight airframe vibration during a descent near Longview, Washington. The non-instrument rated private pilot, the sole occupant in the airplane, was not injured. The pilot/owner was operating the airplane under Title 14 CFR Part 91. Visual meteorological conditions prevailed for the personal, cross-country flight that originated from Oak Harbor, Washington, at approximately 1255. The pilot was flying on a VFR flight plan to Eugene, Oregon; he landed at 1508.

The pilot said that he climbed to 9,500 feet, and was flying above clouds which gave him better visibility and radar contact. He said that he was approaching another band of clouds at his altitude and decided to descend through a "large break in the clouds." The pilot said there was no turbulence and it appeared to be a stable air mass below the clouds. He said that during his wings-level descent to the east, he reduced power to maneuver around a cloud cell. While descending through 8,000 feet at about 180 mph, he said that the airplane began to shake all over; there was no loss of flight control. The pilot said that he reduced power and rate of descent. The shaking stopped, and he continued his flight to Eugene. He said that after a normal landing, he discovered the "serious wrinkling" between the fuselage and the empennage.

Postaccident evaluation of the airplane found no pre-existing cracking in the engine mounts; engine vibration analysis results were within limits. Ruddervator balance tests were also within limits. Radar data indicated that the airplane's maximum ground-track speed was 185 knots, or 213 mph. When this was corrected for altitude and winds aloft, the Investigator In Charge determined that the airplane's indicated airspeed should have been approximately 190 mph when it was descending through 8,000 feet. The airplane's Pilot's Operating Handbook (POH) states that this speed is in the airspeed indicator's yellow arc, or between 176 and 202 mph. The POH further states that maximum structural cruising speed (Vno) is 176 mph, and remarks, "Do not exceed this speed except in smooth air and then only with caution."