Departure and Initial Climb
Flight NW6231 departed New York-JFK at 19:14 for a ferry flight to Buffalo. Departure control cleared the flight to climb to FL140, and at 19:21 the flight was cleared to climb to FL310. The aircraft began to climb at 2,500 feet per minute (fpm) with an airspeed of 305 knots.
Increasing Airspeed and Climb Rate
As the aircraft climbed through FL160, both the airspeed and the rate of climb began to increase. Upon reaching FL230, the airspeed had reached 405 knots and the rate of climb exceeded 6,500 fpm. The overspeed warning horn sounded shortly thereafter, followed 10 seconds later by a stick shaker stall warning.
Leveling and Loss of Control
The aircraft then leveled at 24,800 feet with a speed of 420 knots. It turned rapidly to the right 13 seconds later, after which the airplane started to descend out of control. During the descent, the vertical acceleration reached +5g. The aircraft struck the ground in a slightly nose-down and right wing-down attitude. The descent from 24,000 feet to 1,090 feet occurred in 83 seconds.
Probable Cause
The official findings state: "The loss of control of the aircraft because the flight crew failed to recognize and correct the aircraft's high-angle-of-attack, low-speed stall and its descending spiral. The stall was precipitated by the flight crew's improper reaction to erroneous airspeed and Mach indications which had resulted from a blockage of the pitot heads by atmospheric icing. Contrary to standard operational procedures, the flight crew had not activated the pitot head heaters."