On January 11, 1994, at 1535 hours mountain standard time (MST), a Piper PA-18-150, N82166, registered to/operated by Minuteman Aviation, Inc., and being flown by Mark G. Mamuzich, a certificated commercial pilot, sustained substantial damage when it collided with a taxiway direction sign during landing rollout, following a loss of control on landing at the Missoula International Airport, Missoula, Montana. The pilot and a commercially certificated rear seat pilot were uninjured. Visual meteorological conditions existed at the time and no flight plan had been filed. The flight, which was local and personal in nature, was to have been operated in accordance with the requirements set forth in 14CFR91 and originated at Missoula at approximately 1530 hours. Pilot Mamuzich, the front seat pilot, reported that after he had executed several touch and go landings he allowed pilot Kenneth G. Fielding to execute a touch and go. During the second of Fielding's approaches, which was to have been a full stop on runway 25, the aircraft began to diverge from the centerline toward the left side. Pilot Mamuzich reported that he then "got on the controls to help save the aircraft from doing a complete ground loop." During his recovery the aircraft impacted a frangible taxiway direction sign (refer to photographs 1 & 2) sign in the northwest corner of the intersection of taxiway "D" and runway 25 (refer to DIAGRAM I). Pilot Mamuzich reported that the aircraft had almost stopped when the left side of the empennage just forward of the horizontal stabilizer impacted the sign.