On October 4, 1998, at 1804 eastern daylight time, a Piper PA-34, N1025U, was substantially damaged after a loss of control during landing at the Teterboro Airport (TEB), Teterboro, New Jersey. The certificated private pilot and the airline transport rated flight instructor were not injured. Visual meteorological conditions prevailed for the local instructional flight that originated at 1745. No flight plan had been filed for the flight conducted under 14 CFR Part 91. In a written statement, the flight instructor said: "...after a training flight we were heading back to TEB. We completed all the check lists prior to landing. Everything was normal. On short final, we made a final check; props full forward, gear down, three green and one in the mirror. Touchdown was smooth and on centerline. About 6 to 10 seconds after landing while on rollout the nose gear collapsed and [I] tried to maintain directional control...I did for a few seconds then the aircraft drifted to [the] left. As a result of this, the left wing struck taxiway signs. The impact slowed us down and [the] aircraft came to a full stop in the grass." The wreckage was examined by a Federal Aviation Administration (FAA) Aviation Safety Inspector. In a telephone interview, the Inspector stated the gear collapsed because it was not fully locked in the down position. He said the gear would not fully lock because of a defect in the gear mechanism. Examination of the nose landing gear revealed pre-existing damage to the bungee assembly of the down lock link assembly. The Inspector said the damage would be difficult to detect during preflight and the pilot would still "...get three green lights..." when he positioned the gear selector to the down position. In a written statement, the Inspector said: "Bungee [assembly]...failed at rod end...not allowing overcenter link to lock gear down. This was due to possible hard landing prior to this flight bending bungee rod [assembly]."