What happened
On the evening of the accident, flight NW6231 departed New York-JFK at 19:14, bound for Buffalo on a ferry mission. After receiving clearance from departure control to reach FL140, the aircraft was subsequently cleared to climb to FL310. During the ascent, which began at 305 knots and a rate of 2,500 fpm, the aircraft's performance began to deviate as it passed through FL160. Both the airspeed and the vertical climb rate accelerated significantly.
By the time the aircraft reached FL230, the airspeed had increased to 405 knots while the climb rate surpassed 6,500 fpm. Shortly after this, the crew received an overspeed warning, followed ten seconds later by a stick shaker stall warning. The aircraft eventually stabilized at 24,800 feet traveling at 420 knots, but thirteen seconds later, it entered a rapid right-hand turn. This initiated an uncontrolled descent where the aircraft experienced vertical acceleration of +5g. In just 83 seconds, the plane dropped from 24,000 feet to 1,090 feet before impacting the ground in a nose-down and right wing-down attitude.
Findings
The investigation determined that the flight crew did not identify or rectify the high-angle-of-attack stall and the resulting descending spiral. This loss of control was triggered by the crew's incorrect response to inaccurate airspeed and Mach data. These erroneous readings were caused by atmospheric icing blocking the pitot heads, a condition that occurred because the crew had failed to engage the pitot head heaters, deviating from established operational procedures.