Accident Overview
On June 23, 2009, at 1249 central daylight time, an Urban Air SRO UFM-13 Lambada, registration N17UA, sustained substantial damage when the fuselage failed in-flight over Boerne Stage, Texas. The aircraft was operated by San Antonio Light Sport Aircraft, Inc. under 14 CFR Part 91 as a personal flight without a flight plan. Visual meteorological conditions prevailed. The private pilot, on his first flight in the motor glider, was not injured. The flight originated at Boerne Stage Field (5C1) at 1202.
Flight Sequence
According to the pilot, early in the flight he noticed a light fluttering of the rudder pedals that ceased when gentle foot pressure was applied. While in level cruise at 3,500 feet, the aircraft suddenly began shaking violently. The pilot estimated airspeed at 80 KIAS, but GPS data indicated approximately 110 KCAS. He reduced power, the nose pitched down to near-vertical, and he deployed the emergency ballistic parachute. The aircraft landed gently atop small trees, sustaining additional damage. The pilot stated that about 2 to 2.5 seconds elapsed from the start of shaking to parachute deployment.
Another pilot who had flown the same motor glider previously also reported experiencing rudder flutter. The accident pilot later examined the tail structure and noted an apparent lack of structural supports, describing the tail as a hollow fiberglass tube. He attributed this to the weight restrictions of the light sport aircraft category.
GPS data recorded at one-second intervals showed normal climb at 1,000 fpm, gradually slowing to 500 fpm, with airspeeds between 60 and 80 KCAS. The aircraft leveled at 6,000 feet, accelerated briefly to 90 KCAS, then flew for 15 minutes at about 80 KCAS. It turned over San Antonio, then headed inbound, accelerating to 100 KCAS and descending to 5,000 feet. After about 7.5 minutes straight flight at 100 KCAS, it began descending and airspeed increased to 110 KCAS, at which point the breakup occurred.
Examination Findings
Examination of the motor glider revealed that the empennage had separated from the fuselage but remained attached by control cables and tubing. The failed fuselage section was sent to the NTSB Materials Laboratory. The fracture surfaces had a flattened, crushed appearance consistent with compressive loads, with adjacent delaminations. Some fractures were more angled with a fibrous appearance, consistent with tension or local bending. A disbond along the seam revealed fracture in the matrix material rather than adhesive. The lap joint seam at the top of the fuselage showed no significant disbonding. No manufacturing errors or pre-existing damage such as fatigue cracking were found.
Related Service Bulletins
After a similar accident in the Czech Republic involving an UFM 13 Lambada, the never-exceed speed (Vne) was lowered from 119 KCAS to 81 KCAS. N17UA's airspeed indicator displayed two redlines: one at 119 knots and a taped marking at 81 knots within the green arc. The pilot acknowledged receiving a memorandum about the temporary airspeed restriction not to exceed 81 knots but did not consider it a Vne; he noted that at three-quarter throttle the aircraft exceeded 81 knots. He believed the change was a temporary recommendation pending investigation of the Czech accident, where the aft fuselage failed due to aerodynamic flutter of the empennage.
Following those events, the manufacturer, Urban Air, issued Mandatory Service Bulletin UFM-13-1/2009 (March 24, 2009) requiring inspection and reinforcement of elevator hinges on the horizontal stabilizer, along with a Vne reduction to 81 KCAS. A subsequent bulletin (UFM-13-2/2009, August 17, 2009) provided instructions for reinforcing damaged hinges found during inspections.
Examination of the horizontal stabilizer and elevator from N17UA, performed in accordance with the service bulletins, revealed no damage or excessive deflection at any of the five hinge fittings or elevator hinge pins.