On March 15, 1998, at 1845 hours Pacific standard time, a Piper PA34-200, N4442T, landed and porpoised down runway 16R at the Van Nuys, California, airport. The landing gear collapsed and the aircraft subsequently slid off the runway. The airplane sustained substantial damage and the commercial rated pilot and two passengers were not injured. A third passenger received minor injuries. Visual meteorological conditions existed for the local personal flight and no flight plan was filed. In an interview with a Federal Aviation Administration (FAA) inspector from the Van Nuys Flight Standards District Office, the pilot stated that he landed hard on the first touchdown and heard a loud noise that he believed was a "tire popping." The aircraft ballooned up, the pilot reduced the power, and the aircraft impacted the runway. FAA operations and airworthiness inspectors noted that the main landing gear tires were inflated and showed no evidence of having deflated on impact. Due to the damage of the nose landing gear, the FAA inspectors could not verify the condition of the tire. An FAA witness to the accident reported that after touchdown the aircraft porpoised down the runway. He stated that the porpoise got progressively worse until the landing gear collapsed and the aircraft veered off the runway. FAA operations and airworthiness inspectors inspected the aircraft on-scene. Flight control continuity was established and no further anomalies were found. The on-scene investigation revealed that no airworthiness certificate, aircraft registration, or flight manuals were discovered onboard the aircraft.