Narrative:Northwest Airlines' Flight 421 departed from Chicago for Minneapolis at 15:50. At 16:55 the flight reported over La Crosse, 125 miles southeast of Minneapolis, and was at that time cleared to start an en route descent from its cruising altitude of 8,000 feet. Four minutes later the flight reported passing through the 7,000-foot altitude level. The flight proceeded in the direction of Winona where it encountered a thunderstorm. Flight 421 was seen flying below the overcast. As it passed over Winona, the aircraft appeared to enter the roll cloud or the leading edge of the thunderstorm at which time it was lost from view. Seconds later parts of the airplane were observed falling. It was later found that the flight had crashed approximately 4 miles northwest of Winona on a bluff on the east side of the Mississippi River.
PROBABLE CAUSE: "The loss of the outer panel of the left wing which separated from the aircraft as a result of a fatigue crack in the left front outer panel attachment fitting which had been induced by a faulty design of that fitting, the fatigue crack having been aggravated by severe turbulence encountered in the thunderstorm."
» CAB File No. 1-0117
Follow-up / safety actions
Several hours after the accident, fatigue cracks similar to those found in NC93044 were found in the wing root fittings of another Martin 2-0-2, NC93042. This airplane had flown through the same storm area about an hour after the accident. A complete separation had occurred in the front spar lower flange of the right wing of NC93042 at a point corresponding to the initial failure which was found in the left wing of NC93044. Two days after the accident, three other Martin 2-0-2 aircraft, upon inspection, were found to contain fatigue cracks in similar locations to those found in NC93044 and NC93042.