The National Transportation Safety Board is investigating two runway incursions that occurred at Teterboro (N.J.) Airport (TEB) over the past three weeks. Led by NTSB ATC investigator Daniel Bartlett, the probe will focus on a June 25 incident in which a Learjet 45 landed on a closed runway and a July 9 ground incursion involving a NetJets Dassault Falcon 2000 and Cessna 172. At 5:34 p.m. on June 25 the Learjet, operating as Windrider Flight 988, landed on Runway 19, which was closed a half hour earlier and properly marked with a lighted X at the departure end. According to the NTSB, the TEB tower controller not only failed to notify approach control of the closure but also approved the airplane for the approach and eventually cleared the pilot to land on the closed runway. Two weeks later, a Cessna 172 crossed the hold-short line of Runway 24 when the NetJets Falcon was cleared for takeoff. The tower controller, who had failed to issue Runway 24 hold-short instructions to the Cessna pilot, cancelled the Falcon’s clearance in time to prevent any potential collision. The FAA reported the two airplanes were 1,200 feet apart.