NYC94FA176

After about 40 minutes of flight, the airship crew noted the envelope pressure began to drop. They turned back to the airport & dropped 300 liters of water ballast 'per the flight manual.' At about 700'AGL & 2 miles from the airport, with the pressure less than 15MM, the crew realized they would not reach the airport. The airship went into a nose low attitude & would no longer maintain altitude. The crew shut down the engines & electric system & landed in a street, but the gondola & forward part of the envelope were damaged. Investigation revealed the center damper valve was open. There was evidence that during flight, the open valve allowed the fore & aft ballonets to empty & the center ballonet to fill. Apparent, over-filling of the center ballonet resulted in crushing of the collapsible tube (trunking) that was used to fill the fore & aft ballonets. There was evidence that as the airship descended, the trunking remained crushed, the envelope pressure dropped, & the fore & aft ballonets would not control the trim of the airship. A postaccident check of the center damper valve revealed that it was operable; this valve was for maintenance use only & should have been closed for flight. The crew reported helium contamination during several days preceding the accident. Purification of the helium had not been attained & more had been added to keep the total amount high, which resulted in less space for the fore & aft ballonets to expand.

History of flight

On September 11, 1994, at 1111 eastern daylight time, a WDL, 1B (Airship), N317TW, registered to Airship International and piloted by Hans-Friedrich Gunther and Russell Mills, sustained substantial damage during a forced landing 1 mile south of Republic Airport (FRG), Farmingdale, New York. The two pilots and one passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was operated under 14 CFR Part 91.

At 1100 feet above ground level (AGL), and about 40 minutes into the flight the crew noted they were losing air pressure. They elected to return to FRG. At 700 feet AGL the nose of the airship went nose low about 45 degrees, and efforts by the crew to bring the nose up were not successful. The crew elected to land on a road, in a residential area.

According to the pilot's statement on the NTSB Form 6120.1/2, their intentions were to fly down the shore for flight over the United States Open Tennis Tournament, for television coverage. The pilot wrote that the static weight of the airship at takeoff was "80Kg" (kilo grams). At 1000 feet AGL, the pilot wrote that he checked the pressure of the envelope, pressure of the ballonet, all the engine instruments, the trim of the airship, and according to him, "all was well." The pilot wrote that, "...at 1100 [feet] we became aware that the pressure of the envelope was dropping very slowly."

The pilot made some checks, "...by opening [the] dampers to check the rate of increase in pressure," and confirmed that there was "a small pressure problem." The pilot wrote:

At this point we made 180 degree turn heading back to KFRG [Farmingdale]. Informing our [ground] crew and also the tower. Still the pressure dropping (900 feet, 22mm)...carried out the regular emergency procedures for a pressure problem and also dropped 300 liters water ballast as per the flight manual...a ground speed of 31 knots we made an effort to land at KFRG...approximately 2 miles west of KFRG at 700 feet...pressure of less than 15mm we became aware that we would not make the airport...due to the drop in pressure we lost some of our aerodynamic center. At this point the airship became nose heavy and the ship was going down. At an angle of 45 degrees quickly...tried to use engine power to raise nose [without success]...at 30 feet above ground we shut down engine and electric system and landed.

The accident occurred during the hours of daylight approximately 40 degrees, 43 minutes north, and 73 degrees, 24 minutes west.

Personnel information

Mr. Hans-Friedrich Gunther was born on December 10, 1963. He held a Commercial Pilot Certificate (Special Purpose), No. 2517640, with Airship rating. Mr. Gunther was issued a Second Class Airman Medical Certificate on November 1, 1993, with no limitations.

Mr. Gunther's records showing his total flight hours indicated that at the time of the accident he had a total of 3200 flight hours, 1643.3 flight hours in this make and model airship.

Mr. Russell Mills was born on February 2, 1964. He held a Commercial Pilot Certificate (Special Purpose), No. 589116304, with Airship rating. Mr. Mills was issued a Second Class Airman Medical Certificate on September 17, 1993, with no limitations.

Mr. Mills' records showing his total flight hours indicated that at the time of the accident he had a total of 1010 flight hours, 419.5 flight hours in this make and model airship.

Aircraft information

According to the airship's airframe log book, a 500 hour inspection was due, and completed on September 6, 1994, at a total time in service of 3,965.3 hours. The log book entry read, "...inspection I.A.W. WDL 1B, M/M [maintenance manual], Chapter 5-20, pages 2-11. A/C returned to service at this time."

On September 7, 1994, total time in service 3,972.9, the log book entry read:

Due to volume and purity loss, inspection of envelope and ballonets required-inspected entire envelope for suspected holes- none noted...inspected front and rear ballonets and found excessive chaffing from ballonet folding in the center to the envelope floor. Materials are on order to halt the influx of air to helium from the ballonets...

Wreckage and impact information

The wreckage was examined at Farmingdale Airport on September 12, 1994, and again at a storage facility near the airport on September 16, 1994. The airship impacted between some homes located on Albers Road. The gondola was damaged, and the front of the envelope was crushed. The tail of the airship came to rest up in the air.

The airship was removed from the accident site and examined after it was removed. The envelope was laid out and visually inspected for tears or cuts. None were observed and it was decided to have the airship taken to the manufactures facilities for further examination.

Test and research

At the request of the NTSB and under the supervision of the German, Flugunfalluntersuchungsstelle (FUS), the airship was examined on March 7, 1995, at the manufactures facility's in Germany. The FUS examined the envelope, controls, fabric and components of the airship.

A detailed examination of the airship by the FUS revealed, that the center damper (valve) was found in the open position, and is usually in the closed position during flight operations. The center damper works to control the center ballonet which is also not used during flight operations.

According to the NTSB airworthiness Group Chairman's report:

...according to the operator, [the center ballonet] is only used for maintenance and, very occasionally, during flight if the fore and aft ballonets are full." The pilots reported that they could not close the center ballonet damper during the accident flight. The control for the damper is in the rear of the gondola.

The envelope was examined, no rips or tears were notated, and no discrepancies were found.

The forward and aft ballonets were examined and no discrepancies were found.

Examination of both engines and propellers revealed no discrepancies.

Examination of the flight control surfaces, cables, and gondola controls, revealed no discrepancies. Continuity was established to all the flight deck air controls.

Examination of the helium valve and controls revealed, that the control cable was corroded. The valve operated without any discrepancies.

The NTSB Airworthiness Group Chairman's Factual Report explained that:

...if the center ballonet damper was left in or failed in the open position then the center ballonet would fill. According to the FUS if the crew obtained the pressure height [height at which all ballonets are empty], then the fore and aft ballonets would be empty and the center ballonets pressure would crush the collapsible tube that fills the fore and aft ballonets. As the airship descends the tube stays crushed, the pressure drops, and the fore and aft ballonets can not control the trim of the airship.

According to the FUS written report:

...according to the declaration of the pilot, the center ballonet had been open when the incident occurred and since this ballonet cannot be controlled from the cockpit it must be assumed that this condition had already existed prior to take-off. The statement that the pilot tried to close the ballonet, which was impossible, could not be reconstructed within the investigation. At the time of the examination the damper valve was in perfect condition. The gas spring used for closing the damper valve was unremarkable and should have closed the damper in any case if the control cable had not locked on the appropriate hook...it is most probably