NTSB Factual Report

On June 23, 2009, at 1249 central daylight time, an Urban Air SRO UFM-13 Lambada, N17UA, registered to and operated by San Antonio Light Sport Aircraft, Inc., was substantially damaged when the fuselage failed in-flight over Boerne Stage, Texas. Visual meteorological conditions prevailed at the time of accident. The personal flight was conducted under provisions of 14 Code of Federal Regulations (CFR) Part 91 without a flight plan. The private pilot was not injured. The local flight originated at Boerne Stage Field (5C1), at 1202.

According to the pilot, this was his first flight in the motor glider. During the early portion of the flight, he noticed a "light fluttering of the rudder pedals" that stopped when he applied "gentle" foot pressure. At the time of the breakup, the aircraft was in level cruise flight at 3,500 feet when it suddenly began to “shake violently.” The pilot was thrown back and forth in the cockpit against his restraints. The pilot said that the airspeed was about 80 KIAS (knots indicated airspeed). However, available data indicates that the aircraft was flying approximately 110 KCAS (knots calibrated airspeed). He immediately reduced power and the nose of the motor glider pitched down to a near-vertical attitude. He deployed the emergency ballistic parachute, and the aircraft landed "gently" on top of small trees, sustaining additional damage. The pilot said that from the time the shaking began until deployment of the parachute, 2 to 2-1/2 seconds had elapsed. Examination of the motor glider revealed the empennage had separated from the fuselage, but remained attached by control cables and tubing. Another pilot who had flown this same motor glider on a previous flight said he had also experienced “rudder flutter.” The pilot later examined the tail structure of the aircraft and said it appeared that there were no structural supports in the tail, that “it is just a hollow fiberglass tube.” He attributed the lack of structural components to the requirement for light sport aircraft to be below a specific weight in order to qualify for the “light sport” category.

A GPS (Global Positioning System) was installed in the airplane for navigation. It recorded time, position, altitude, ground speed and ground track at one-second intervals. A National Transportation Safety Board (NTSB) performance engineer used this data along with temperature and wind data to determine the flight parameters for the Lambada Motor Glider. The data show that the climb was normal at about 1000 fpm, gradually slowing to 500 fpm. Airspeed varied between 60 and 80 KCAS. The airplane leveled off at 6,000 feet and briefly accelerated to about 90 KCAS. For the next 15 minutes, the airplane traveled in an outbound direction at about 6,000 feet and 80 KCAS. The maneuvers appeared gentle. The airplane made several turns over the San Antonio area and then turned to an inbound course to Borne Stage Field (northeast of San Antonio, TX). It accelerated to about 100 KCAS and descended to 5,000 feet. The airplane flew relatively straight at 5,000 feet and 100 KCAS for about 7-1/2 minutes. At that point, the airplane appeared to start a descent and the airspeed had gradually increased to about 110 KCAS. That is when the in-flight breakup occurred.

The failed portion of the fuselage was sent to NTSB’s Materials Laboratory for examination. The following are excerpts from the report: “In the area of the fracture, the fuselage was nearly circular. The fractures were oriented nearly perpendicular to the fuselage axis, and the fracture surfaces had a flattened or crushed appearance consistent with fracture under primarily compressive loads. In addition, the composite material showed evidence of in-plane delaminations adjacent to the fractures, consistent with fracture under primarily compressive loads. The fractures were generally at more of an angle to the fuselage axis, and the fracture surfaces had a more fibrous appearance, consistent with fracture under tension or tearing from local bending. The disbond fracture along the seam revealed the surfaces of the glass fabric making up the composite, indicating that fracture occurred in the matrix material adjacent to the fabric rather than in the adhesive. Although the fracture through the fuselage ran across the lap joint seam at the top of the fuselage, that seam showed no evidence of any significant disbonding. There was no evidence of any manufacturing errors or pre-existing damage such as fatigue cracking.”

After a similar accident in the Czech Republic, the never-exceed speed (Vne or redline) was lowered from 119 KCAS to 81 KCAS. N17UA’s airspeed indicator had two redlines displayed. One was at 119 knots and the other was taped onto the instrument glass at 81 knots and was in the middle of the green arc. The pilot said he had received the memorandum containing the “temporary restriction on airspeed not to exceed 81 knots in any configuration,” but he “did not think the 81 knots was a Vne speed,” and noted that at ¾ throttle, the motor glider was “way past 81 knots.” He believed the change was a “temporary flight recommendation” from the manufacturer pending completion of another accident investigation involving an in-flight break-up that occurred in Czechoslovakia. In that accident, the aft portion of the fuselage structure failed in-flight as the result of aerodynamic flutter in the empennage. The accident report was issued by the Air Accident Investigations Institute, Republic of Czechoslovakia (Ref. No. CZ-09-046, FINAL REPORT, Investigation of Microlight Aircraft Accident, Type UFM 13 Lambada, Registration Mark OK-NUA 09, on 21st March 2009). According to this report, “the most likely cause of the accident was “flutter”, an aeroelasticity phenomenon, which occurred on the horizontal tail surface bringing about a loss of integrity of the fuselage and in-flight separation of its rear part.”

As a result of the investigation, the manufacturer of the UFM-13, Urban Air, issued Mandatory Service Bulletin UFM-13-1/2009, dated March 24, 2009, that required the inspection and reinforcement, if necessary, of the elevator hinges in the horizontal stabilizer. The bulletin also required that Vne be decreased to 81 KCAS. Seven months prior, on August 17, 2009, the company had issued Mandatory Service Bulletin UFM-13-2/2009, following reports of damaged elevator hinges as a result of the inspection program, and provided instructions for reinforcing the hinges.

Because of these bulletins, the horizontal stabilizer and elevator were also sent to NTSB’s Materials Laboratory for examination. According to that report, “The Urban Air service bulletins require a visual inspection of the Y-shaped hinge fittings on the horizontal stabilizer for damage or delamination or inordinate deflection while applying upward and downward forces of 1 to 2 pounds (5 to 10 Newtons) at the position of the hinge pin. Carrying out the procedure in the service bulletins revealed no damage or excessive deflection of any of the five hinge fittings on the horizontal stabilizer. The five hinge pins on the elevator itself were also inspected, and no damage was found at any of the hinge pins.”

NTSB Probable Cause

The pilot’s exceedance of the aircraft’s placarded never-exceed speed (Vne), resulting in aerodynamic flutter in the empennage and the subsequent in-flight structural failure of the aft portion of the fuselage.