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On January 9, 1998, at 0713 mountain standard time, a Cameron R-420 Roziere gas balloon, N298AR, known as the "Global Hilton," was substantially damaged when the bottom of the main gas cell ruptured near Yeso, New Mexico. During their emergency parachute landings, the commercial pilot-in-command (PIC) received minor injuries and the commercial second-in-command (SIC) was seriously injured. The balloon was being operated by World Flight Limited Company of Albuquerque, New Mexico, under Title 14 CFR Part 91. Visual meteorological conditions prevailed, and a VFR flight plan had been filed. The flight originated from Albuquerque, New Mexico, at approximately 0618.
The project manager stated that the flight was intended to be an around-the-world flight in the jet stream. The jet stream is a boundary layer between the troposphere and the stratosphere, normally found between 25,000 and 40,000 feet mean sea level (msl). The project manager stated that weather conditions dictated a launch window between December 15, 1997, and March 15, 1998.
The ground crew calculated the amount of helium gas needed to be approximately 176,400 cubic feet, or 42 percent capacity, based on a gross weight of 11,505 pounds and the free lift desired by the ground crew. This calculation included enough helium to provide a normal/recommended lift factor. An additional lift factor was used due to the Sandia mountains located 6 nm east, and two temperature inversions located at 2,000 feet above ground level (agl) and 10,000 feet msl. The SIC stated that a 400 pound ballast had unintentionally been left on the ground. The launch master stated that the balloon's total free lift, at lift off, was approximately 1,200 pounds.
The balloon was launched, during the hours of darkness, from Albuquerque's Balloon Fiesta Park (elevation 5,280 feet). Radar data indicated that the balloon's initial rate of climb accelerated to approximately 1,500 feet per minute (fpm). The SIC, who was flying the balloon, reported that he recognized the accelerating rate of climb and immediately began "valving off" helium to slow the climb rate, which took approximately 3 to 4 minutes. At approximately 0633, the balloon's climb rate was stabilized between 200 and 300 fpm, which the SIC maintained until 25,500 feet msl.
The SIC stated that he slowed the rate of climb to approximately 100 fpm as they approached their initial float altitude of 27,500 feet msl (see attached radar data). He reported that the sun's rays began shinning on the balloon's envelope approximately 10 minutes before leveling off. He then he noticed helium flowing down one appendix, and hesitate at a pinched point (a buckle attachment point) approximately 15 to 20 feet above the bottom opening. The SIC said that helium continued down the appendix, and "bubbled" out the bottom opening. The PIC viewed the other appendix, and he observed that it had not fully inflated. He reported that he did not see helium gas exit the opening at the bottom of the appendix.
Both pilots reported that after level off all was quiet for approximately 10 to 12 minutes, and at 0713, they heard a loud "whoosh" and felt a push followed by a mild bounce. The balloon immediately began to rise. Both crew members looked up through a porthole and saw that the bottom of the main gas cell, or diaphragm, had ruptured. The PIC estimated that 62 percent of the diaphragm was open, and the pilot controlled gas valve in the top of the gas cell could be seen. Radar data indicated that the balloon rose to approximately 32,500 feet. As the PIC declared an emergency, the SIC began "valving off" helium gas to establish a 300 to 500 fpm rate of descent. Both crewmembers said they heard continual "pops, rips, and tearing" as the balloon descended. At approximately 18,000 feet msl, the crew heard a particularly loud noise, and the balloon began to rotate.
At approximately 12,000 feet msl, the crew depressurized the gondola and photographed the ruptured diaphragm. The crew said they considered a heavyweight landing, but the approximate 3,500 pounds of propane and 60 liters of liquid oxygen fastened around the gondola would have required ideal conditions for the landing. The ground crew advised the pilots that surface winds were 30 knots with occasional gusts to 35 knots, and that the terrain was rugged with canyons and rock formations. This report, along with the flight crew's belief that the structural integrity of the envelope had deteriorated to an unsafe level, facilitated the crew's decision to bail out.
The flight crew climbed out of the gondola/capsule with little difficulty, but had to assist each other in getting through the load tapes supporting the 18 propane bottles surrounding the gondola. They said that it took approximately 10 minutes to get into a position where they could jump from the gondola. The PIC stated that he manually tied open the helium control valve before following the SIC to the ground at 0922. Radar data indicated that the flight crew jumped from the balloon at approximately 9,000 feet msl (4,025 feet above ground level [agl]). The flight crew used 26 foot conical canopy parachutes with an 8 knot forward speed capability, and they were jumping into wind conditions of 30 to 35 knots.
Radar data indicated that after the crew's departure, it ascended briefly, then descended until impacting the ground approximately 20 minutes later. During the impact sequence, 11 propane bottles, 2 nitrogen tanks, and miscellaneous other equipment separated from the balloon. The ground crew estimated approximately 3,000 pounds of weight was lost.
The PIC observed, and radar data indicated, that the balloon immediately ascended, and at 1010 leveled off at approximately 35,000 feet msl. The unmanned balloon drifted eastward reaching ground speeds of 100 knots. The U.S. Air Force, the North American Air Defense Command, and the FAA Air Traffic Control Centers tracked the balloon as it traveled across New Mexico and Texas. Approximately 7 hours and an estimated 375 nautical miles (nm) later, the balloon touched down near Gainesville, Texas. The balloon impacted the ground 5 times, each time losing tanks or equipment, and subsequently rising again. On the 6th touch down, the balloon contacted transmission wires and a fire ensued which destroyed the gondola. The envelope separated from the burning gondola, became airborne, and came to rest approximately 5 nm away.
The balloon was airborne for a total of 10 hours and 22 minutes. The crew was onboard for approximately 3 hours, 4 minutes.
The PIC received his fixed wing aeronautical training in the U.S. Air Force. He was a commercial certificated balloon pilot, and he had accumulated approximately 85 hours of lighter-than-air flight experience of which less than 5 hours was in gas balloons. The SIC had approximately 10 years of flight experience in balloons, and was a commercially certificated pilot. He had approximately 700 hours of lighter-than-air flight experience, of which approximately 175 hours were in gas balloons.
The balloon (envelope with gondola) received a Special Airworthiness Certificate in the United States in the category of Experimental-Exhibition (See attached excerpts from FAA Orders 8130.2C and 8130.27). It had a maximum gross weight of 11,980 pounds. The envelope was a hybrid gas (helium) and hot air envelope, which was referred to as a "Roziere" system. Helium, being lighter than air, provided the main lifting force by displacing the heavier ambient air surrounding it.
The balloon envelope was one of 26 manufactured by Cameron Balloons, Ltd., in Bristol, England. The manufacturer had recommended a maximum rate of climb for the balloon of 1,000 fpm. The upper portion (main gas cell) of the envelope was approximately 420,000 cubic feet in size. The balloon's lift control was dependent upon maintaining the temperature and pressure of the helium. Solar heating, resulting in expansion of the helium, was controlled by the use of a Mylar reflective insulating film and an insulating tent which covered the topmost section of the envelope. The insulating tent provided a boundary layer (cavity) over the main cell, and was supported by its own 20,245 cubic foot helium filled gas cell. Eight solar powered electric fans were attached to the insulating tent to assist in daytime cooling.
The lower hot air portion of the envelope (cone) was approximately 140,000 cubic feet in size. Its primary purpose was to allow control of the balloon's lift during the cool nights. The jet stream temperature was between -30 to -60 degrees Fahrenheit. The air in the cone was heated by four propane fueled burners. The heat in the cone would in turn heat the helium in the main gas cell, permitting helium volume control or lift control. The balloon carried 18 aluminum propane bottles (45.9 gallon capacity), which weighed 243 pounds each when filled.
The upper portion of the main gas cell was made from a 200 denier nylon fabric with a urethane coating (one denier equals .05 grams per 450 meters of thread). The bottom of the main gas cell, which separated the helium from the hot air, was made from a 70 denier nylon fabric with a urethane coating. The urethane coating was applied to the nylon fabric to provide a helium gas barrier. According to the launch master, the coating process reduced the structural strength properties of the fabric by a significant amount.
The location on the envelope, where the bottom (the 70 denier fabric) of the main gas cell met the top (the 200 denier fabric), was known as the waterline or equator (see attached diagram). The waterline was also the attachment point for the hot air portion of the envelope (cone), to the main gas cell. The cone was made from Cameron Duraflight hot air balloon rip-stop fabric followed by Mylar coated aluminum foil at the base of the cone.
The balloon was equipped with two appendixes, approximately 115 feet in length, attached to the main gas cell just above the waterline and directly opposite each other (estimated weight, 50 pounds each). They were used for filling and venting the main gas cell. During the launch of the Global Hilton, approximately 42% of the main gas cell was filled with helium through its appendixes. As the balloon rose in the atmosphere, the helium expanded in response to reduced external atmospheric pressure. Upon reaching its maximum altitude (float altitude/ceiling) the helium gas would have expanded to fill the main gas cell. Surplus helium gas may be vented off by the pilot (to control the rate of climb), or passively evacuated through the appendixes after the helium in the main gas cell expanded to the cell's full capacity.
The capsule (gondola) was manufactured by World Flight, Mojave, Californa, and was a sphere 93 inches in diameter constructed of carbon fiber. Crew access was gained through a hatch in the top, which was approximately 24 inches in diameter. The gondola was designed for a crew of two. The flight crew's breathing gas came from two 30 liter liquid oxygen dewars, one high pressure oxygen tank, and two nitrogen tanks.
The entire balloon (gondola, hot air cone, main gas cell, and upper tent) was approximately 180 feet tall. The balloon had never been flight tested/flown before the January 9, 1998, flight.
At 0955, the weather conditions at Cannon Air Force Base, Clovis, New Mexico (elevation 4,295 feet), 085 degrees for 78 nm from the accident site, were as follows: wind 280 degrees at 19 knots; visibility 7 statute miles; scattered clouds at 20,000 feet; temperature 36 degrees F., dew point 27 degrees F.; altimeter 29.75 inches of mercury. The ground crew reported that near the bail out location, the wind was 30 knots, gusting to 35 knots. The sunrise at the flight crew's landing location was at 0706.
Radar data indicated that the balloon was traveling east-southeast at 35 knots when the pilots parachuted from it.
WRECKAGE AND IMPACT INFORMATION
The envelope and all associated equipment were recovered on January 10, 1998, and transported to Albuquerque, New Mexico, for postaccident evaluation. The ground recovery team reported that because of the muddy terrain and the trees, a farm tractor was used to retrieve the envelope, and an unknown amount of damage was inflicted during the recovery process.
The upper portion of the main gas cell had several rips and tears in it, and it could not be determined if these were created while the balloon was airborne, or during the recovery sequence. None of the 80 upper main cell load tapes were broken. One appendix was found damaged, with signatures that suggested that it was damaged on the ground. The second appendix was undamaged.
The lower diaphragm (bottom of the main gas cell) had several tears, the largest portion of intact fabric extending from the waterline between load tapes 5 to 43. This portion contained most of the base circle which included the majority of the circumferential tape and its main cell access zipper. The other portions of the diaphragm split radially from the base circle outwards, and between the main reinforcing tapes. None of the 40 radial main reinforcing tapes were found broken (see attached failure mode study).
The base circle, the center of the diaphragm (see photographs), was approximately 41 inches in diameter and was dominated by a black sealable zipper, which permitted access to the inside of the main gas cell (see attached drawing). The non-elastic zipper was oriented along the bias direction of the fabric circle, which was the most flexible direction of the circle. The fabric was found failed along the warp and fill lines. These tears extended from the point where the zipper intersected the circumferential tape, at 90 degrees to each other, across a segment of the circle to the circumferential tape.
TESTS AND RESEARCH
FAR Part 31.49(c) states: "Each balloon using a captive gas as the lifting means must have an automatic valve or appendix that is able to release gas automatically at the rate of at least 3% of the total volume per minute when the balloon is at its maximum operating pressure." The manufacturer was not required to meet this standard due to the balloon's Experimental category certification, but the manufacturer did state that the balloon's envelope conformed to the quality standards of other Roziere balloons which had received Certificates of Airworthiness from the United Kingdom Civil Aviation Authority. The manufacturer designed two openings in the main gas cell for appendixes attachments, each measuring 886 millimeters in diameter, or 34.88 inches. The manufacturer stated that this provided a total of 13.273 square feet for helium gas to exit.
The two openings in the main gas cell, with their associated elbows and appendixes, were located at load tapes 2 and 42. The Investigator-In-Charge (IIC) measured the appendixes while they were laying flat on the hangar floor, and the narrowest point between seams was 45 inches across. Their inner circumference would have been 90 inches, or a diameter of 28.66 inches. The total area of the two appendixes would have been 8.96 square feet, or 32.5% less than the manufacturer's design specifications.
The launch master reported that balloon fabric is manufactured in 48, 54, 60, and 72 inch widths. He further stated that a French Felld sewing seam (which was used in the construction of the appendixes) would use between 1 1/4 to 1 1/2 inches of fabric to complete.
When Cameron Balloons manufactured N298AR, they made another almost identical Roziere 420 balloon. The other Roziere 420 was registered in England as G-JNEE, and differed from N298AR in that the fabric utilized in envelope construction was different, and there were some other minor variations. These two balloons will be referred to as R-420-1997s. G-JNEE was launched December 31, 1997, at 2045 central standard time. As the balloon approached ceiling, its diaphragm failed, and the pilot landed as soon as possible.
The manufacturer modified N298AR's appendixes from G-JNEE's appendixes by attaching four plastic buckles down the length of the envelope, to secure them to the cone. The manufacturer stated that "these were to prevent twisting of the long appendices by air currents during the flight." Photographs of the balloon, taken during its initial accession, showed the lowest buckle made a 26 degree angle (kink) in the appendixes. An engineer with Winzen Engineering, Inc., a consultant who designs helium balloons for the military, estimated that this would reduce the effective geometric area of the appendixes by 56% (see attached document). Using the Winzen data with the IIC's calculations, the total area of the Global Hilton balloon appendixes could have been as small as 3.94 feet, or a 70.3% reduction from the manufacturer's design specifications.
The manufacturer of the envelope reported that the main gas cell was originally designed to be a natural shape, or zero pressure envelope. The design dictated that the lift forces be primarily carried by the vertical load tapes, and not by the envelope fabric. To achieve this, the manufacturer cut the vertical gores to have surplus fabric. The manufacturer stated that as the envelope became fully inflated, it would take on a bulbous appearance. He further stated that the design of N298AR was modified slightly by flattening the main gas cell bottom (the vertical radius was approximately 48 feet, and the horizontal radius was approximately 52 feet). He said that the fully inflated main gas cell would have been a flattened sphere, or oblate spheroid.
In a letter to the IIC, the pilot of G-JNEE stated that "the internal pressure forces in a pressurized cell will be at their least potential energy when the cell shape has the least surface area, i.e., is spherical in shape." He further stated that in the case of the R420-1997s, if the main gas cell became pressurized (from possible vent restriction), the cell will attempt to become more spherical, and circumferentially narrower. To become taller, the vertical load tapes and fabric would have to stretch, and subsequently reduce the balloon's circumference.
A representative of the envelope manufacturer, the pilot of G-JNEE, the Chairman of the Safety Committee of the Balloon Federation of America (BFA), and the past licensed Cameron Balloon manufacturer in America had a conference on the possible failure mechanism of both R-420-1997s. They stated that they believe that the main gas cells of both R-420-1997s, when pressurized and stretched vertically, would permit the surplus gore material to wrinkle inwardly. They further stated that this process of inward wrinkling has been observed in other Roziere balloons, and could provide sufficient fabric to restrict or block the openings to the appendixes.
The Chairman of the BFA's Safety Committee reported that he observed one around-the-world attempted Roziere flight, and as the balloon approached its ceiling, there were large internal fabric wrinkles in the vicinity of the appendixes.
Radar data showed that N298AR initially ascended at approximately 1,500 fpm. Radar data suggests that the SIC stabilized the rate of climb after 15 minutes of flight. At this point in the flight, the balloon was at approximately 16,000 feet msl, and the main gas cell would have been an estimated 70% inflated. During the next 35 minutes of climb, N298AR averaged 300 fpm, which was 30% of the manufacturer's recommended maximum rate of climb. Radar data indicated that the last 5 minutes of climb was accomplished at approximately 100 fpm, or at 10% of the manufacturer's recommended maximum rate of climb.
The President of Winzen Engineering stated that "using the Winzen design procedure, the total duct area required for this balloon is 12.53 square feet which would provide adequate venting for an ascent rate of 2,000 feet per minute for this size balloon."
The manufacturer designed the bottom of the gas cell to have a centrally located base circle of 52.4 inches in diameter. The postaccident evaluation of the base circle revealed that it was 41 inches in diameter, or 21.75% smaller than original design specifications.
The pilots reported that the manufacturer telephoned them just prior to their launch, warned them that the rip panel, used for deflating the envelope during landing, was probably inoperative. The manufacturer stated that the rip panel on G-JNEE had failed to open, even with five fireman pulling on the line.
The balloon was released to the Global Hilton project manager on January 28, 1998. Don Cameron (the manufacturer), Dick Rutan (the PIC), and Richard Abruzzo (the original PIC and originator of the project), each declined to sign the NTSB's party member form.