1948 — Aug 29, Northwest 421 wing structural failure in thunderstorm/crash NW of Winona MN–37

Compiled by Wayne Blanchard Sep 21, 2023 for upload to: http://www.usdeadlyevents.com/

—  37  Aviation Safety Network. Northwest Orient Airlines Flight 421, 29 Aug 1948.

—  37  CAB. AIR. Northwest Airlines, Inc. – Winona, Minnesota – August 29, 1948.

Narrative Information

Civil Aeronautics Board Accident Investigation Report (SA-178; File No. 1-0117):

“The Accident

“Northwest Airlines Flight 421, aircraft…a Martin 202, en route from Chicago, Illinois, to Minneapolis, Minnesota, crashed near Winona, Minnesota, at approximately 1705, August 29, 1948. All of the 37 occupants were killed, and the aircraft was destroyed.

“History of the Flight

“Northwest Airlines Flight 421 departed from Chicago for Minneapolis at 1550, August 29, 1948. On board were 33 passengers, four crew members…

“Routine position reports were received from the flight as it progressed on course at its flight plan altitude of 8,000 feet. At 16555, one hour and five minutes after takeoff from Chicago, the flight reported over La Crosse, 125 miles southeast of Minneapolis, and was at that time cleared to start an en route descent. Four minutes later, at 159, the flight reported passing through the 7,000-foot altitude level. No indication of any trouble was contained in this transmission. This was the last word heard from the flight.

“Between 1645 and 1700 several persons in the vicinity of Winona, Minnesota, approximately 95 miles southeast of Minneapolis were watching a thunderstorm which was approaching from the northwest. As the storm moved over Winona, the wind increased in intensity and considerable lightning and thunder were observed. At this time, about 1705, 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…

“Investigation

….

“The last and major portion of the aircraft wreckage was found 0.75 of a mile from the inner portion of the left outer wing….All safety belts were broken, and no seats were found attached to the floor mountings.

“Portions of the wreckage were removed from the scene of the accident for tests and detailed examination….

“Several flights were in the vicinity of Winona for a period one hour before and one hour after the accident occurred. The pilots of these flights described the storm as vicious and violent in appearance. Several flights successfully circumnavigated the storm arriving at their destination without incident. Another air carrier flight over the Winona area at approximately 1750 proceeded into the thunderstorm activity at an altitude of approximately 4,000 feet The pilot stated that the turbulence was more severe than he had ever encountered before….

“In conjunction with the study of the structural parts of NC-93044, parts of another Martin 202, NC-93042 were examined. This second airplane had completed a flight from Chicago to Minneapolis shortly after the scheduled flight of NC-93044 during which the pilot experienced average turbulence with two rather severe jolts. Then a flight was made to Duluth and return to Minneapolis after which a mechanic noticed a discontinuity in the right wing. Upon inspection It was found that the front spar lower flange of the right wing had failed at a point corresponding to the initial failure found in the left wing of NC-93044….

Analysis

“The separation in the left front lower spar flange of NC-93044 was followed by failure of the lower rear spar flange, and then by failure of the top connections of the outer wing to the center section. Without the left wing the airplane necessarily rolled to the left. A piece of fuselage structure found in the leading edge of the left wing, and a piece of the trailing edge of the left wing flap found imbedded in the right horizontal stabilizer showed that the left wing was struck by the top of the fuselage and the right horizontal stabilizer. As a result of this impact the left wing panel was broken into two parts, and the tail section was severed from the rest of the airplane….

“A fatigue crack not only reduces the strength of a material, but serves as a nucleus for the very stress concentrations which cause the crack to extend and develop. Each application of load has its effect. and eventual failure is certain. Had the spar flange in NC-93044 not failed at Winona, it would have failed at a later date unless the defect from which the separation originated had been discovered. This was graphically illustrated by the failure of the spar flange in NC-93042 where the progression of fatigue over a relatively longer period of time than in NC-93044 was indicated. and where the severe jolts experienced during the flight from Chicago to Minneapolis were in all probability sufficient to cause complete separation….

“In view of the foregoing facts we are compelled to the conclusion that due to the high local stress concentrations of this particular design of the attachment fitting, fatigue cracks had developed in the attachment fitting which so weakened the structure as to cause failure of the complete outer wing panel under the stress of the severe turbulence encountered in the thunderstorm….

“Probable Cause

“The Board determines that the probable cause of this accident was 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. AIR. Northwest Airlines, Inc. – Winona, Minnesota – August 29, 1948.)

Aviation Safety Network, Flight Safety Foundation, Database, 1948: “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.” (ASN. Northwest Orient Flight 421, 29 Aug 1948,)

Sources

Aviation Safety Network. Accident Description. Northwest Orient Airlines Flight 421, 29 Aug 1948. Accessed 2/8/2009 at: http://aviation-safety.net/database/record.php?id=19480829-0

Civil Aeronautics Board.  Accident Investigation Report. Northwest Airlines, Inc. – Winona, Minnesota – August 29, 1948.Washington, DC: CAB, June 30, 1949. Accessed 9-21-2023 at:

https://rosap.ntl.bts.gov/view/dot/33343

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