The Convair was operating on a flight from Providence to Tulsa. The last intermediate stop was at Joplin, were it departed at 23:23 CST. At 23:47 the flight reported crossing the south leg of Chanute low frequency range and was cleared direct to Owasso by Tulsa Approach Control. After hearing the Tulsa weather (1,75 mile visibility) the crew chose to make the Owasso (straight in, runway 17) approach instead of an ILS approach. At 24:00 the flight reported over Owasso inbound and was cleared to land on runway 17L. At that moment the approach checklist had been completed and the aircraft was at an altitude of 1200-1300 feet. The captain’s altimeter setting was ground level, the 1st officer’s setting was mean sea level. Reading 1200-1300 feet from the 1st officer’s altimeter, the captain may have thought that they had some 800-900 feet to descend before reaching minima, while his own altimeter read about 700 feet. This may have prompted the captain to permit the descent to continue without realizing the close proximity of the ground. Some 3,6 miles north of the approach end (and 613 feet msl), the Convair plane struck trees and hit the ground. The aircraft slid along the ground to the top of an upslope and then jumped a deep and wide ditch, finally coming to rest some 540 feet from the point of initial touchdown. PROBABLE CAUSE: “The captain’s lack of alertness in allowing the 1st officer to continue an instrument descent to an altitude too low to permit terrain clearance.”