NTSB Factual Report

HISTORT OF FLIGHT

On November 2, 1996, at 1344 mountain standard time, a Schweizer SGS 1-36, N3616Z, operated by The Cloud Base, was destroyed when it broke up in flight and impacted terrain in Boulder, Colorado. The commercial pilot was fatally injured. Visual meteorological conditions prevailed at the time, and no flight plan was filed for the personal flight conducted under Title 14 CFR Part 91. The flight originated at Boulder, Colorado, on November 2, 1996, approximately 1300.

According to the tow plane pilot, the pilot of N3616Z asked him for a tow and said he would like to leave the traffic pattern on the north side and maneuver so as to reach 8,500 (feet) msl before crossing the first ridge west of the airport. The tow plane pilot said that during the tow, he found the air to be smooth with little lift in the area. The pilot of N3616Z released from the tow at 11,300 feet msl in the vicinity of the water treatment plant by Boulder Canyon.

Numerous witnesses were interviewed and seventeen witnesses submitted written statements (attached), giving their locations and what they observed. Each witness statement was numbered and their locations were plotted on a witness location chart (attached). Twelve witnesses said they heard a loud noise. The noise was variously described as a loud boom (3, 10), snap or crack (8), clap (14), or pop (15), followed by a loud shrill (2), strong whoosh (7), or rushing sound (14). Looking up, they observed the aircraft spiraling or spinning to the ground (3, 5, 8, 9, 11, 12, 13, 15). Others said it was in a steep descent (7) or went straight down (2, 6, 10, 14). One witness (14) said it was not spinning. Witness no. 5 wrote: "When I first saw it, it was slowly spinning (1,000 feet above the ground)...At about 500 feet, it stabilized and stopped rotating, then began rotating in the other direction." Witness no 9 said it was spinning "at a rapid pace," but witness no. 12 said "it was not spiraling fast, only really slow." Witness no. 11 agreed, saying "it made a slow spiral...one rotation per second."

All of the witnesses agreed that one or both wings had failed. Two witnesses (7, 9) saw the wings strike each other. Some said the wings had folded back along the fuselage (2, 5, 10, 16). One witness no. 3 said the "starboard wing practically vertical...its port (left) wing wavering and flailing somewhat aft, (then) the right wing also appeared to loose fixity to the fuselage and began also to flail and trail aft." Witness no. 14 said "the left wing was pointing upwards and the right wing (was) folded up against the left."

One witness (1), who was almost directly underneath the glider, said it fell at a high rate of speed. Another witness (12) said "it seemed to have reached terminal velocity." Based on witness locations and the direction they were looking, the glider came across the Flatirons and was heading towards the Boulder Municipal Airport. Its altitude was estimated to be between 100 and 1,300 feet. Some witnesses observed the glider from the Flatirons and said its altitude was between 6,000 and 8,500 feet above sea level. Boulder Municipal Airport is 5,288 feet above mean sea level.

Of the 17 witnesses, only one saw the glider before the wings failed. Witness no. 16, who was hiking with two friends, saw the glider come from behind the mountains traveling in a northeast direction. She said that "it appeared to fly in an inverted position. Banking to the right (or southeast), the plane went upside down. As the glider righted itself, it was now traveling in an easterly direction and losing altitude. The glider then began to barrel roll in a clockwise direction...it continued to barrel roll...the plane was out of control...suddenly, both wings snapped into a vertical position and the glider went into a nose dive."

This witness was later interviewed in person. Using a model airplane for illustration, she said that when she first saw the glider, it was in a wings level attitude. She then observed the glider to do a complete 360 degree roll to the right about its longitudinal axis. The glider rolled out on a more easterly heading, then began losing altitude in an increasingly nose low attitude and entered either a spin or spiral. She then observed both wings fail and fold aft along the fuselage.

CREW INFORMATION

The 48 year old pilot was an engineer. He held Commercial Pilot Certificate No.523643480, dated November 28, 1995, with airplane single/multiengine land and instrument ratings, and private pilot privileges for glider aero tow only; a flight instructor certificate, dated June 11, 1996, with airplane single/multiengine and instrument ratings; and a ground instructor certificate, dated November 25, 1992, with an advanced rating. His second class airman medical certificate, dated September 25, 1996, contained the limitation: "Holder shall wear lenses that correct for distant vision and possess glasses that correct for near vision." The pilot was also issued a Statement of Demonstrated Ability No. 10D09325, dated November 3, 1993, for "defective distant vision."

All of the pilot's flight logbooks were examined. Beginning on September 19, 1986, and ending on October 29, 1996, these records indicated the pilot had accumulated 1,721.4 hours total flight time, of which 64.4 hours were in gliders and 29.7 hours were in the Schweizer SGS 1-36.

The pilot took his first glider flight lesson on September 4, 1989, soloed on October 21, 1989, and received his private pilot certificate for gliders on January 31, 1990. He received a company checkout in the Schweizer SGS 1-36 on March 3, 1990.

Between August 30, 1993, and October 3, 1993, the pilot took a total of 4 aerobatic lessons in a Pitts S-2B. The lessons totaled 3.4 hours.

AIRCRAFT INFORMATION

According to the SCHWEIZER SGS 1-36 PILOT'S OPERATING HANDBOOK, never exceed speed (Vne) and maximum dive brake speed (Vdb) is 121 mph (105 knots), and maneuvering speed is 64 mph (56 knots). The glider was designed for a maximum maneuver or gust limit of 5.33 and a negative limit of 2.67 g, and the ultimate load is +8.0 g and negative -4.0g." Pertinent portions of the HANDBOOK are attached to this report.

N3616Z (s/n 9) received a Certificate of Airworthiness in January 1981. On January 20, 1992, an oxygen regulator was repositioned, and a barograph holder and a radio were installed. According to the operator, the wings were attached to the glider and were not removed thereafter. The last annual inspection was performed on August 5, 1996, at 1,698.6 hours total time in service. The glider flew 17.8 hours, 11.9 hours, and 13.9 hours in August, September, and October, respectively. The accident flight, which originated approximately 1300 hours, was the first flight for November.

METEOROLOGICAL INFORMATION

Official weather data is contained on page 4 of this report.

Witness no. 14, a retired U.S. Department of Commerce physicist, said "the weather was clear, calm, and warm." Climbing to the summit of Green Mountain (8,144 feet msl, or about 2,500 feet above the crash site), the witness said the temperature "was somewhat cooler than it was at the foot of the mountain, but the difference was consistent with the normal lapse rate. Another glider flying near Green Mountain did not appear to be making much progress upwards."

One powered airplane pilot submitted an unsolicited statement, saying he had landed at Boulder just before N3616Z was towed aloft. "The weather was very stable and benign. There was a stable layer of smog southeast of Boulder, and I saw no evidence of mountain wave of thermal activity in the mountains to the west," he wrote. "The only glider pilot that I have talked to who did fly that day. . .reported finding only weak thermal activity over the mountains."

WRECKAGE AND IMPACT INFORMATION

The glider was located in the backyard of a private residence at 707 8th Street, Boulder, Colorado.

The forward portion of the wreckage lay on its left side against a cluster of bushes on a magnetic heading approximately 260 degrees. Surrounding trees appeared to be undamaged except for one. There was torsional twisting of the fuselage about F.S. 146.0, and the empennage was inverted. It was aligned on a magnetic heading approximately 305 degrees. The right wing lay atop the left wing. The left wing dive brake was deployed; the right wing dive brake was stowed. Both wing (particularly the right) leading edges exhibited extensive leading edge accordion-type crush damage. Canopy plexiglas was recovered about 8 blocks from the accident site. The area between F.S. 0.0 and F.S. 69.0 (the nose and cockpit) was demolished.

Examination of the wing structure revealed a "V" type spar fracture outboard of the right wing attachment point. The wing-to-fuselage attachment pins were all in place except for one. That pin was recovered. It was slightly bent and the mating hole was elongated. The fractured spar ends were cut from the wreckage and sent to NTSB's metallurgical laboratory for examination (see "Tests and Research").

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy (96A-138) was performed by Dr. John E. Meyer of the Boulder County Coroner's Office on November 3, 1996.

Toxicological protocol was performed by FAA's Civil Aeromedical Institute (CAMI)in Oklahoma City, Oklahoma. According to CAMI's report, no evidence of drugs was detected in blood, and no evidence of ethanol was detected in either blood or muscle fluid.

TESTS AND RESEARCH

The fractured spar was sent to NTSB's metallurgical laboratory in Washington, DC, for examination. According to the metallurgist's factual report, "all fractures on both wing sections were typical of overstress separations. No evidence of fatigue or other types of preexisting cracking was found."

The left and right wing-to-fuselage attachment pins were removed and examined. The left wing attachment pin was bent; the right wing attachment pin was straight. Additionally, the left pin hole was elongated.

On March 1, 1997, the vice president of Schweizer Aircraft Corporation examined the wreckage at Beegles Aircraft Service in Greeley, Colorado. According to his report (attached), both the left hand and right hand wing spar carry-through "failed inside the fuselage area from predominantly very high wing bending loads. These bending loads were coupled with large forward chord bending and nose down wing torsion loads. This caused the right hand rear spar attachment to tear loose which helped to finish overloading the right hand main spar carry-through plates, thus allowing the right hand wing to fold up and impact the left wing along its leading edge in flight. The flight condition which provides the largest combination of these loads is Condition PLAA (positive low angle of attack with aileron).

"Further review of wing structure showed pulled rivets and top surface skin inter-rivet buckling along the main spar. This phenomena only occurs if the wing has experienced much greater than limit wing bending loads."

ADDITIONAL INFORMATION

The wreckage was released to the owner's representative on March 1, 1997.

Parties to the investigation included the Federal Aviation Administration, Schweizer Aircraft Corporation, and the Cloud Base, Inc.

NTSB Probable Cause

The pilot exceeding the design stress limits of the aircraft by performing an aerobatic maneuver, resulting in total failure of the wing spar.