FTW97LA219

During the initial takeoff climb, the agricultural airplane's engine lost power, and the pilot executed a forced landing in a corn field. There was insufficient space available to bring the airplane to a complete stop before it reached the edge of the field and collided with trees. Examination of the airplane revealed that a foreign object had completely blocked the fuel line leading from the forward left wing tank outlet to the fuselage header tank. Chemical analysis indicated that the object was composed of silicone rubber. The 1996 model airplane had accumulated 696 hours total time. Visual inspection of the left and right wing fuel tanks disclosed no evidence that silicone rubber had been used as a tank sealant or as gasket material during manufacture of the airplane. No mention was made in the maintenance records of any maintenance, repairs or modifications to the airplane's fuel system. The source of the silicone rubber contamination could not be determined.

On June 13, 1997, at 1140 central daylight time, an Ayres S2R-G10 agricultural airplane, N6133X, was destroyed when it impacted trees during a forced landing following a loss of engine power near Biggers, Arkansas. The commercial pilot, sole occupant of the airplane, was not injured. The airplane was registered to and operated by C & C Flying Service of Pocahontas, Arkansas. A flight plan was not filed, and visual meteorological conditions prevailed for the Title 14 CFR Part 137 local aerial application flight which was originating when the accident occurred.

During a personal interview conducted by the NTSB investigator-in-charge and in a written statement, the pilot reported that the engine "quit" as he completed rolling out of a left turn after takeoff from the private airstrip. He executed an "emergency landing in a small corn field;" however, the "field was not big enough," and during the landing roll, the airplane collided with trees at the edge of the field. Both wings separated from the fuselage, and the fuselage and tail section sustained structural damage.

On July 10, 1997, the engine, a Garrett TPE331-10-511K, S/N P-38032C, was disassembled at the facilities of Allied Signal in Phoenix, Arizona, under the supervision of a FAA inspector. According to the FAA inspector, no discrepancies were found that would have contributed to the power loss.

On August 19, 1997, the airframe was examined at Arkansas Airframe in Clinton, Arkansas, under the supervision of the NTSB investigator-in-charge. The fuel line leading from the forward left wing tank outlet fitting to the fuselage header tank was found completely blocked by a foreign object. Approximately 80 psi of air pressure was required to dislodge the object from the line. The object was irregularly shaped, measured about 1 inch by 1/2 inch by 1/2 inch, and appeared to be composed of black silicone rubber. Several other similar objects were found in the fuselage header tank.

Both the left and right wing tanks were inspected by removing tank access panels, and no foreign objects were found in either tank. Visual examination of the inner surface of each tank revealed no evidence that black silicone rubber had been used as a tank sealant or as gasket material during manufacture of the airplane. The lack of chipped paint on the tank access panels and the presence of paint on the heads of the screws holding the access panels in place suggested that the panels had not been removed subsequent to the factory paint being applied.

Review of the maintenance records by the NTSB investigator-in-charge indicated that the airplane's date of manufacture was February 1996. The most recent annual inspection was completed on February 20, 1997, at an airframe total time of 390.3 hours. According to the hour meter in the airplane, the airframe had accumulated 695.7 hours at the time of the accident. No mention was made in the records of any maintenance, repairs or modifications to the fuel system.

The object which was found blocking the fuel line was forwarded to Chemier/Polytech Laboratories, Inc., of Maryland Heights, Missouri, for chemical analysis. In their report dated September 23, 1997, Chemir/Polytech stated that the object "appears to be composed primarily of a poly(dimethylsiloxane) material, chemically similar to materials which are typically referred to as silicone rubber."

The Safety Board has investigated one other mishap in which silicone rubber was found blocking fuel lines of an Ayres S2R-G10. For details of this incident, refer to NTSB Factual Report FTW97IA362, Munday, Texas, May 8, 1997, Ayres S2R-G10, N3298Y.

In October 1997, in response to the findings of this accident investigation and the investigation into the May 8, 1997, incident, personnel from the FAA's Aircraft Certification Office in Atlanta, Georgia, and the Manufacturing Inspection Satellite Office in Mobile, Alabama, inspected the production line at the Ayres Corporation's factory in Albany, Georgia. No evidence was found that the company was using silicone rubber to seal fuel tanks.

On October 30, 1997, the Ayres Corporation issued Service Bulletin No. SB-AG-40, entitled "Fuel Tank Inspection," applicable to "all turbine powered S2R aircraft including conversions." The bulletin stated that "as a result of contamination, several aircraft have experienced fuel flow problems between the wing fuel tanks and the header tanks." It provided instructions for a one-time inspection of the wing fuel tanks, the header tank, and the fuel lines from the wing tanks to the header tank for contamination and obstruction.

On December 16, 1997, the FAA issued Special Airworthiness Information Bulletin (SAIB) No. ACE-98-17 to advise owners/operators of turbine powered Ayres S2R aircraft of the safety information contained in Ayres Service Bulletin No. SB-AG-40. The SAIB stated, in part:

Issuance of SB No. SB-AG-40 was prompted by reports of silicone rubber compound clogging fuel lines. There were two reports of accidents involving model S2R-G10 airplanes; one resulted in the destruction of the airplane while the other resulted in only minor damage to the airplane. ... An investigation revealed that it is highly unlikely for the contamination to have been introduced into the fuel system at the factory. It is therefore concluded that these silicone rubber compounds were introduced into the system after manufacture. Silicone rubber is not compatible with jet fuel; it enlarges several times the original size when placed in contact with or immersed in jet fuel. ... The source of the contamination has not been determined. The airplanes noted above were located in different parts of the country and do not share a common history. A possible scenario is that silicone rubber is being used to seal refueling hoses/nozzles, tank fittings, etc. and excess material at joints could break loose and either fall or be swept into the airplane fuel tank.