Complex sentence: The NTSB used flight simulators to test the possibility that the plane could have returned safely to LaGuardia or diverted to Teterboro; only eight of the fifteen runs succeeded, although all four attempts to reach the nearest LaGuardia runway, Runway 22, were successful.
Split into simpler sentences: The NTSB used flight simulators to test the possibility that the plane could have returned safely to LaGuardia or diverted to Teterboro; only eight of the fifteen runs succeeded. However, all four attempts to reach the nearest LaGuardia runway, Runway 22, were successful.