Hartford, WI, USA
G-SUNA
GEFA-FLUG GMBH AS-105-GD
The commercial pilot stated that, before the 2nd aerial advertising flight of the day in the airship, he checked the wind conditions and determined that, although the wind was still within flight parameters, it would be increasing, which would make "flying more challenging." After 15 minutes of flight time, the pilot returned to land because the wind was stronger than forecasted. As the airship approached the runway the pilot released the drop line and attempted to open the Plexiglass panel that is used for venting and descent control. However, the panel jammed closed. Shortly thereafter, the airship encountered a thermal that caused the airship to rise. The airship leveled around 500 ft above ground level, at which time the pilot heard a "loud air explosion," which violently shook the airship. He looked up at the envelope and noted a panel of fabric located aft of his sitting position and near the top of the envelope was missing. Within seconds he heard up to four more air explosions. The pilot stated that the envelope began to sag, and the forward speed of the airship resulted in the nose of the airship collapsing in and around the burners, which ignited the fabric. The pilot shut off the fuel to the burners, secured his five-point harness, and braced for impact as the airship descended. After impact, the pilot was able to get out of the gondola and crawl a short distance before the ground crew arrived and pulled him away from the wreckage. The maximum wind for takeoff listed in the flight manual as 12 knots; the wind velocity reported 15 miles from the accident site at the time of the accident was 10 knots gusting to 15 knots; 1 hour earlier the wind velocity was 7 knots. The flight manual also stated that the airship must not be operated when thermals are present. Given the increase in wind velocity and the thermal activity, it is likely that a localized area of high pressure developed in the crown of the envelope that exceeded the strength capability of the envelope, resulting an overpressurization and the subsequent explosive failure of the balloon fabric. It is also likely that the jammed Plexiglass panel used for venting prevented the pilot from relieving pressure in the envelope as the airship climbed.
HISTORY OF FLIGHTOn June 15, 2017, at 1115 central daylight time, a Gefa-Flug AS-105-GD thermal airship, United Kingdom registration G-SUNA, collided with the terrain in Hartford, Wisconsin, following an inflight envelope panel failure. The commercial pilot was seriously injured, and the thermal airship was destroyed. The airship was registered to and operated by AirSign LTD, as a 14 Code of Federal Regulations Part 91 business flight. Visual meteorological conditions existed near the accident site at the time of the accident, and a flight plan had not been filed. The local flight departed from the Erin Aero Airport (WN75) at 1100 and was returning to the airport when the accident occurred. The purpose of the flight was to provide aerial advertising for the U.S. Open Championship golf tournament. The accident occurred on the second flight of the day. The first flight of the day originated about 0830 and lasted 2 hours. After the first flight, the airship was fueled in preparation for the second flight. Three of the four propane tanks onboard were empty, so they were off-loaded and replaced with two full propane tanks. The pilot reported he checked the wind condition again and determined that although it was still within his flight parameters the wind would be increasing, which would make "flying more challenging." He had a goal to fly three hours that day, so he departed for the second flight, climbing to 1,000 ft. He stated the airship's no-wind forward speed is about 15 knots, and he didn't like flying in wind higher than 10 knots because it limited the airship's forward speed too much. The forward speed during the 15-minute flight was 4 to 6 knots, so he decided to land. The pilot radioed the ground crew that he was returning because the wind was stronger than forecasted. He descended and released the 100 ft-long drop-line when he was about 150 ft above the ground (agl). He reported the line hung up on itself and did not extend the full length. The pilot reported that the Plexiglass slide panel in the gondola jammed, which had happened before, so he was not able to vent hot air from the envelope. The airship then encountered a thermal and it began to rise. The pilot thought that the increase in altitude would give him time to untangle the drop-line and unjam the Plexiglas panel. The airship leveled around 500 ft agl, at which time the pilot heard a "loud air explosion" which violently shook the airship. He looked up through the Plexiglass panel and a panel of fabric, located aft of his sitting position and the top of the envelope, was missing. Within seconds he heard up to four more air explosions but was unable to see any other missing or damaged panels as they were out of his sight range. The pilot reported the envelope began to sag and the forward speed of the airship resulted in the nose of the airship collapsing in and around the burners, which ignited the fabric. The pilot shut off the fuel to the burners and he secured his 5-point harness and tried to protect himself as best he could since he had no control over the airship as it descended on fire. After contacting the terrain, the pilot was able to get out of the gondola and crawled a short distance before the ground crew arrived and pulled him away from the wreckage. Numerous witnesses reported seeing the airship descending for the landing, then rising before the envelope collapsed, and the airship then descending rapidly while on fire. PERSONNEL INFORMATIONThe pilot held a Federal Aviation Administration (FAA) issued commercial pilot certificate with single-engine land, multi-engine land, instrument and lighter-than-air ratings. He held a FAA issued second-class airman medical certificated dated August 17, 2016. There are no FAA pilot certificates for thermal airships. In addition, the pilot held a commercial pilot license issued by the United Kingdom (UK) Civil Aviation Authority. The license included ratings for hot air balloons, Gp A hot air airships pressurized (up to 160,000 CuFt/4550Cu/m volume), free balloons hot air filled, and free balloons hot air Group A. The pilot reported having made 117 airship flights for a total flight time of 316.7 hours. AIRCRAFT INFORMATIONThe airship and gondola were manufactured by the Gefa Flug GMBH in Germany, in 2004, and Gefa Flug GMBH is no longer in business. The current type certificate holder and manufacturer is Cameron Balloons Ltd. in Bristol, UK. The airship had a UK Certificate of Airworthiness issued on July 30, 2015. The operator stated that they maintained the UK registration on the airship, because the United States does not have a certification category for thermal airships. If they had registered it in the United States, it would have been issued an experimental airworthiness certificate and they would not have been able to use it for hire. The most recent UK Airworthiness Review Certificate was issued on September 21, 2017. The airship was maintained by a FAA Designated Airworthiness Representative (DAR). Work done on the envelope would be reviewed and signed off by a repairman on behalf of the British Balloon and Airship Club. The repairman reported he either had to see the repair or have proof that the repair was correct before signing off on the repair. The last entry in the envelope and engine/gondola logbooks was an annual inspection on September 22, 2016. The owner of the airship stated there was a tear in the bottom of the envelope earlier in the year which was repaired by the DAR. There was no record of the repair in the logbook and the repairman in the UK stated he was not aware of the tear or repair. This area of the envelope was destroyed in the fire. The envelope was constructed from a coated nylon fabric with rip-stop load tapes along the entire length of the envelope. The envelope is an enclosed structure except for the keel opening above the pilot's seat which can be opened and closed using the sliding Plexiglas panel on the roof of the gondola and the pressure relief valves. The envelope forms an empennage with both vertical and horizontal fins. The vertical fin (rudder) can be deflected up to 45° in each direction. The airship was equipped with a rudder assist system. The gondola was connected to the envelope by four carabineer clips, one on each corner of the gondola, and snap hooks that were sewn into the envelope and hook. The envelope contained a manually operated rip panel in the empennage and two pressure relief valves at each end of the gondola. The pressure relief valves have elastic cords that stretched under pressure to allow air to escape if the envelope pressure is too great. The gondola had a stainless-steel tubular frame. The front of the gondola was covered with a Plexiglass windscreen. The gondola had a 4-place tandem seating configuration. The pilot flies the airship from the front right seat. In the case of the accident airship, the two rear seats could not be occupied because the pilot normally carried four propane tanks, two of which restricted the rear seats. The roof of the gondola contained the Plexiglass panel that the pilot can slide to control the temperature within the envelope. The double burner was mounted on top of the frame above the sliding Plexiglass panel. The pusher engine was mounted on the rear of the gondola frame. Engine power was provided by a liquid cooled Rotax 582 engine linked to a high-thrust, low-noise carbon fiber propeller. A fabric scoop behind the propeller directed airflow to pressurize the envelope and to provide oxygen to the burners. The burner system was a V-configuration consisting of two burners which were fueled by liquid propane. METEOROLOGICAL INFORMATIONThe pilot reported that he checked the weather conditions on the morning of the accident using multiple websites including Intellicast, BlastValve, and Ryan Carlton; and by releasing weather balloons. He determined that the wind condition was not favorable for flight, but the wind speed was forecasted to decrease, so he delayed the flight. About 0800, he determined that the wind condition was favorable. He confirmed the decreased wind by releasing another weather balloon, using the websites previously mentioned, and ForeFlight. The Waukesha County Airport (UES) was the closest official weather station, located 15 miles southeast of the accident site, and was equipped with an Automated Weather Observing Station (AWOS). At 104515, UES AWOS reported wind from 270° at 7 knots. At 1145, UES AWOS reported wind from 300° at 10 knots gusting to 15 knots. The airship flight manual states: The max. wind speed on takeoff site shall not exceed 12 knots for an experienced pilot and, as recommended, 8 knots for an inexperienced pilot. WARNING: Flights in thermal or thundery conditions are not permitted. AIRPORT INFORMATIONThe airship and gondola were manufactured by the Gefa Flug GMBH in Germany, in 2004, and Gefa Flug GMBH is no longer in business. The current type certificate holder and manufacturer is Cameron Balloons Ltd. in Bristol, UK. The airship had a UK Certificate of Airworthiness issued on July 30, 2015. The operator stated that they maintained the UK registration on the airship, because the United States does not have a certification category for thermal airships. If they had registered it in the United States, it would have been issued an experimental airworthiness certificate and they would not have been able to use it for hire. The most recent UK Airworthiness Review Certificate was issued on September 21, 2017. The airship was maintained by a FAA Designated Airworthiness Representative (DAR). Work done on the envelope would be reviewed and signed off by a repairman on behalf of the British Balloon and Airship Club. The repairman reported he either had to see the repair or have proof that the repair was correct before signing off on the repair. The last entry in the envelope and engine/gondola logbooks was an annual inspection on September 22, 2016. The owner of the airship stated there was a tear in the bottom of the envelope earlier in the year which was repaired by the DAR. There was no record of the repair in the logbook and the repairman in the UK stated he was not aware of the tear or repair. This area of the envelope was destroyed in the fire. The envelope was constructed from a coated nylon fabric with rip-stop load tapes along the entire length of the envelope. The envelope is an enclosed structure except for the keel opening above the pilot's seat which can be opened and closed using the sliding Plexiglas panel on the roof of the gondola and the pressure relief valves. The envelope forms an empennage with both vertical and horizontal fins. The vertical fin (rudder) can be deflected up to 45° in each direction. The airship was equipped with a rudder assist system. The gondola was connected to the envelope by four carabineer clips, one on each corner of the gondola, and snap hooks that were sewn into the envelope and hook. The envelope contained a manually operated rip panel in the empennage and two pressure relief valves at each end of the gondola. The pressure relief valves have elastic cords that stretched under pressure to allow air to escape if the envelope pressure is too great. The gondola had a stainless-steel tubular frame. The front of the gondola was covered with a Plexiglass windscreen. The gondola had a 4-place tandem seating configuration. The pilot flies the airship from the front right seat. In the case of the accident airship, the two rear seats could not be occupied because the pilot normally carried four propane tanks, two of which restricted the rear seats. The roof of the gondola contained the Plexiglass panel that the pilot can slide to control the temperature within the envelope. The double burner was mounted on top of the frame above the sliding Plexiglass panel. The pusher engine was mounted on the rear of the gondola frame. Engine power was provided by a liquid cooled Rotax 582 engine linked to a high-thrust, low-noise carbon fiber propeller. A fabric scoop behind the propeller directed airflow to pressurize the envelope and to provide oxygen to the burners. The burner system was a V-configuration consisting of two burners which were fueled by liquid propane. WRECKAGE AND IMPACT INFORMATIONThe gondola came to rest on its left side in a hay field. The tubular steel structure, firewall, burners, engine, and propeller were intact, but burned. The Plexiglass and aluminum structures were destroyed by the fire. The steel cables that attach the gondola to the envelope remained attached at the gondola. All three of the propane tanks exploded in the postimpact fire. The aft end of the envelope remained intact with the forward end of the envelope having been destroyed by fire. The remaining envelope was laid out for examination. The rudder lines were located attached to the rudder surface. The forward portion of both rudder lines were destroyed. There were several tears in the envelope along hoop tapes (LB) 9, 10, and 11, and between gores 29 and 3 and 6. The location of the tears were near the crown of the envelope just forward of the gondola. Most of the tears were along seam lines except for one which ran down the gore 30 fabric between LB 9 and 11 The area surrounding the torn sections of fabric was destroyed by the fire. TESTS AND RESEARCHThe torn section of the envelope was examined at Cameron Balloons USA on July 5, 2017. A 24-inch square of the envelope from gore 1 between LB 10 and LB 11 was cut out for grab and tear tests. This piece of fabric was then cut into 12 pieces. The test results were above the minimum allowable for the fabric. It is unknown how the exposure to the fire affected the integrity of the fabric. The stitching along the hoops tapes and patches appeared to have been done in an acceptable manner.
The overpressurization failure of the airship envelope as a result of the airship's encounter with gusty wind conditions and thermal activity. Contributing to the accident was the jammed Plexiglass venting panel.
Source: NTSB Aviation Accident Database
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