Page 18 - Volume 19 Number 10

Twin Proficiency:14 12 10 8 6 4 2Updraft DowndraftCumulus SDissipatingtage Mature Stage StageFigure 1: Life cycle of a thunderstorm cell. In the Cumulus, or Updraft stage, vertical speeds of 2,000 feet per minute can occur. The mature stage, defined as when precipitation begins to fall (and therefore the first echoes appear on radar), has rising and falling air currents on the order of 5,000 to 6,000 fpm. In the Dissipating (Downdraft) stage, air descends at 6,000 fpm or more. It’s encountering the wind shear between still air and rising and falling columns of air that is the major hazard of thunderstorms. An individual cell can complete the entire life cycle in about 30 minutes, from beginning to end.Ispoke with a pilot at Oshkosh who had very recently survived a harrowing encounter with thunderstorm turbulence as he was flying IFR between cells. The pilot was working with Center to find a path between the growing cells. ATC suggested a deviation based on its real-time weather radar display; the pilot’s onboard NEXRAD upload agreed with the controller’s description of the cells’ locations. So, the pilot confidently accepted the new course.With no warning, in the clouds but outside the areas of heavy precipitation on both ATC radar and the NEXRAD uplink, the airplane16 • TWIN & TURBINEsuddenly hit strong turbulence. It was thrown into a steep bank that disengaged the autopilot and caused the airplane to enter a sprial. The pilot recovered after entering visual conditions beneath the cloud deck, at roughly 1,300 feet above the ground. He thinks he may have momentarily blacked out from the G-load of pulling out of the dive.ATC granted the pilot’s request to land at the nearest airport reporting VMC, which was uneventful. Shakenup, his brief visual inspection of the airplane revealed no apparent damage. Later, the pilot and his mechanic inspected the airplane more closely. The pilot had not looked under the fuselage, but the mechanic noticed the fuselage skin below the wing’s carry-through spars was wrinkled. Removing the cabin interior, he found wrinkling in the carry-through itself. Further, the wing attach fittings appeared to be bent. The pilot and his wife wereOCTOBER 2015Confident “Go”Don't Drive on the Greenskilometres