SEA95LA146

The pilot stated that at 65 mph on takeoff, a sudden crosswind gust hit the airplane, lifting it into the air, and that he could not subsequently maintain altitude at full power. However, a witness at the airport stated that he noticed no unusual wind gusts during the event. Witnesses stated that the airplane went right wing down, left wing down, right wing down again, then left wing down, contacting the ground; and that the airplane then crashed into a field. A nearby weather observation facility reported the wind as southerly at 9 knots. Density altitude was 6,944 feet. The aircraft owner's manual indicates a rotation speed of 65 to 75 mph and a flaps 15 degree/gear down stall speed of 63 mph.

On July 12, 1995, approximately 0830 mountain daylight time, a Mooney M20G, N3936N, received substantial damage in a collision with terrain following a loss of control on takeoff from Rigby, Idaho. Neither the private pilot/co-owner nor his passenger were injured. Visual meteorolgical conditions prevailed and no flight plan had been filed for the 14 CFR 91 flight, which was to have proceeded to Burley, Idaho.

The pilot stated the following in his narrative of the accident: "check[ed] wind sock...no wind observed entire time of preflight...[did] run-up...flaps in takeoff position...roll[ed] on [runway] 19 [a 3,500 by 50 foot asphalt runway at an elevation of 4,845 feet above mean sea level (MSL)]...at 65 MPH a strong crosswind [hit] plane, lift[ed] it about 5 to 10 feet above [the] runway, and to the [right of the runway]...I correct[ed] back to [the] runway and [got] a strong [cross]wind gust in the other direction which [blew] me to the other side of [the] runway...with full power I continue[d] but I [could not] maintain altitude...I contact[ed] the ground and hit [the] uneven surface of a field...I skid[ded] to [a] stop...".

In a written account of the accident, the passenger stated: "wind calm-very light if any wind...we took off and about 10 to 15 feet off the ground...we [drifted] a little to the right, and then to the left, and then [into] the dirt."

Two witnesses at the airport observed the attempted takeoff and the accident. One described the wind as a steady 15 to 20 MPH wind. The other characterized it as being out of the south and "not strong for Idaho", and stated that he noticed no unusual gusts of wind at the time of the accident. This witness stated that the airplane's nose-up attitude looked like a normal takeoff and did not appear excessively steep. The two witnesses agreed that the airplane became airborne about halfway down the runway (one witness also indicated that light airplanes normally begin their takeoff roll about one-quarter of the way down the runway at Rigby); that the airplane never got more than 5 feet off the ground; and that the airplane's motion consisted of a right-wing-down motion, followed by left wing down, followed by right wing down again, followed by left wing going down and contacting the ground, upon which the airplane came down in the field. Both witnesses stated that there had been no other takeoffs or landings at the airport for some time (30 minutes according to one of them) before the accident.

The 0847 surface observation at Idaho Falls (10 miles southwest of the accident site, at 4,740 feet above MSL) reported winds from 180 degrees magnetic at 9 knots, temperature of 74 degrees F, and altimeter setting of 29.94 inches Hg. Based on this observation, density altitude at Idaho Falls at the time of the observation was 6,944 feet.

Extracts from the M20G owner's manual, supplied by Mooney Aircraft Company, indicate that back pressure on the control wheel should be applied during takeoff roll at "about 65 to 75 MPH". The manual gives the flaps 15 degrees/gear down stall speed as 63 MPH for an aircraft gross weight of 2,525 pounds, and the no-wind takeoff run as 985 feet under conditions of 5,000 feet above MSL, temperature 80 degrees F, takeoff weight 2,200 pounds, wing flaps in takeoff position, and hard-surface runway.