On September 25, 1997, approximately 1015 mountain daylight time, a Canadian-registered Piper PA-23-250, C-GBUA, registered to Foto Flight Surveys Ltd. of Calgary, Alberta, collided with a truck on U.S. Highway 30 while on final approach to runway 16 at Allen H. Tigert Airport, Soda Springs, Idaho, during a precautionary landing attempt after the crew noted fuel leaking from around a fuel filler cap. The collision with the truck knocked off the airplane's left main landing gear wheel. Following this occurrence, the pilot aborted the landing attempt and returned to the flight's departure airport at Pocatello, Idaho, where the airplane landed on a grass infield area at about 1325 after circling the airport for approximately 3 hours to decrease fuel load. Postflight inspection of damage to the aircraft revealed it to be minor. The commercial pilot-in-command, who was a U.S. resident and held a U.S. FAA pilot certificate, and one crew member, who was on board the aircraft to operate its photo equipment, were not injured. The pilot reported that visual meteorological conditions (clear skies with unrestricted visibility) existed at Soda Springs at the time of the collision, and no flight plan was filed for the 14 CFR 91 aerial photographic survey flight. The pilot reported that the flight did not have a specific planned final destination, as the landing airport was to have been determined in flight based on flight conditions, circumstances, and requirements.

The pilot reported that while the flight was en route to one of its survey sites, the crew noted fuel leaking from the left inboard fuel filler cap and elected to land at Soda Springs to investigate the problem, as Soda Springs was the nearest suitable airport to the aircraft's position at that time. Soda Springs has one paved runway, 16/34, which is 3,500 feet long. The airport elevation is 5,839 feet above sea level. Runway 16 is equipped with a tri-color visual approach slope indicator (VASI) on the left side of the runway. The pilot reported that the airport Unicom operator reported the winds at the time as being from 120 to 160 degrees at 15 knots, gusting to 20 knots. The pilot reported that he used the U.S. Government Airport/Facility Directory (A/FD) and the Western States Flight Guide (Airguide Publications, Inc., Long Beach, California) to obtain landing information for Soda Springs while approaching the airport. The pilot reported that he noticed the runway had a displaced threshold due to a perpendicular road on short final, and stated he was on a normal glide path in a stabilized, power-on approach when the truck strike occurred just prior to the landing flare.

The Idaho Airport Facilities Directory, published by the Bureau of Aeronautics of the Idaho Transportation Department, advises that a state highway (U.S. Highway 30, according to the state's official highway map) crosses the primary runway surface 50 feet from the end of runway 16, and advises extreme caution for numerous high vehicles. The FAA's Airport Master Record for Soda Springs, on file with the agency's Northwest Mountain Region airports division in Renton, Washington, also contains the remark "RWY 16 +16' HWY CROSSES PRIM SFC 50' FM EORS NUMEROUS HIGH VEHICLES: NUMEROUS CLOSE IN OBSTNS ON APCH TO RY 16"; another remark in the FAA regional airport master record states that the runway 16 threshold is displaced 300 feet at night. The FAA regional airport master record states that the displaced portion of the runway is marked and unlighted. The entry for Tigert Airport in the Northwest U.S. volume of the U.S. Government A/FD states that the runway threshold is displaced 300 feet at night, and identifies obstructions for runway 16 only as "Road." The Tigert Airport A/FD entry does not give the specific distance from the crossing road to the runway 16 threshold, nor does it contain any cautionary remarks regarding numerous high vehicles, as do the state A/FD and FAA airport master record remarks for the airport.

The glide slope of the runway 16 tri-color VASI and the clearance of the VASI glide path over the crossing U.S. highway were examined during the incident investigation. The FAA's regional airport master record does not give the visual glide path angle of the tri-color VASI, nor does it give the threshold crossing height for the system. Inquiries were made to the FAA's Seattle, Washington, Airports District Office (ADO) and to the Idaho Transportation Department's Bureau of Aeronautics in an attempt to obtain this information. The Seattle ADO subsequently asked the director of city services for the City of Soda Springs, in a letter dated October 21, 1998, to check the glide path angle to which the runway 16 tri-color VASI was set, and to advise the ADO as to whether this angle provided 15 feet of clearance over the road (the minimum clearance above a public roadway other than an Interstate highway, as specified by 14 CFR 77.) In a reply letter to the ADO dated November 13, 1998, the director of city services for Soda Springs replied that the glide path angle "has been increased 1% [sic] to 4% [sic]", and that the new glide path angle gave approximately 25 feet clearance over the U.S. highway on approach to runway 16.

The Soda Springs Police Department, which responded to the incident, reported that no reports were ever received from the vehicle involved in the collision, and that the vehicle involved was not identified.

A previous aircraft/vehicle collision, involving a Learjet 24D (N78AE) on approach to the airport, occurred at Soda Springs on April 7, 1978. In that accident, according to the NTSB's brief of accident (file number 3-0546), the aircraft, being flown by an airline transport pilot, collided with a tractor/trailer truck while on final approach. The aircraft was substantially damaged and there were no serious injuries or deaths to the crew of two and one passenger on board. The NTSB determined the probable cause of that accident to be the pilot-in-command's failure to see and avoid objects or obstructions, with airport facilities and the vehicle driver being identified as factors. The NTSB accident brief noted that the highway was 70 feet from the runway, and that there was an airport warning sign 1 mile from the runway at the time of the April 7, 1978 accident.

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