Martin 202 accident at Winona, Minnesota, 29 Aug 1948

Accident narrative

On August 29, 1948, Northwest Airlines Flight 421, a Martin 202, crashed near Winona, Minnesota, after losing its left wing in a thunderstorm. All 37 occupants were killed, and the aircraft was destroyed.

### The flight

Northwest Airlines Flight 421 departed Chicago, Illinois, at 1550 for Minneapolis, Minnesota. On board were 33 passengers, four crew members, 800 gallons of fuel, and 1,088 pounds of baggage. The aircraft's total weight was 38,015 pounds, which was within certificated limits and properly loaded.

Captain Robert L. Johnson had 5,502 flying hours, including 311 in the Martin 202, and had flown the route continuously since 1944. Second Pilot David F. Brenner had 2,280 flying hours, with 155 in the Martin 202. Weather information available before departure indicated relatively good flying conditions, except for scattered rain showers and thunderstorms near La Crosse, Wisconsin, and Rochester, Minnesota.

### Sequence of events

The flight made routine position reports as it progressed on course at its flight plan altitude of 8,000 feet. At 1655, the flight reported over La Crosse, 125 miles southeast of Minneapolis, and was cleared to start an en route descent. Four minutes later, at 1659, the crew reported passing through the 7,000-foot altitude level. This transmission contained no indication of trouble and was the last word heard from the flight.

Between 1645 and 1700, several persons near Winona, approximately 95 miles southeast of Minneapolis, watched a thunderstorm approaching from the northwest. As the storm moved over Winona, the wind increased in intensity with considerable lightning and thunder. At about 1705, Flight 421 was seen flying below the overcast. As it passed over Winona, witnesses observed the aircraft appear to enter the roll cloud or leading edge of the thunderstorm, at which time it was lost from view. Seconds later, parts of the airplane were seen falling. The flight crashed on a bluff on the east side of the Mississippi River, approximately four miles northwest of Winona.

### What the investigation found

The wreckage consisted of four large sections and many small pieces, falling in a straight line bearing 335 degrees over a distance of 1.8 miles. The first section found along the flight path was the outer portion of the left outer wing. Progressing forward, investigators found the tail assembly and attached rear fuselage, then the inner portion of the left outer wing, and finally the main wreckage area containing the engines, propellers, right wing, center section of the left wing, and most of the fuselage.

Examination of the main wreckage showed the landing gear and flaps were retracted. All safety belts were broken, and no seats remained attached to the floor mountings. The engines and propellers showed no evidence of mechanical failure prior to impact, and propeller markings indicated only small rotational forces when they struck the ground. Radio and electrical equipment showed no malfunction or lightning damage.

The investigation focused on the wing root fittings, where a dihedral wedge connected the outer wing spar flanges to the center section. The attachment ends of the lower spar flanges and dihedral wedge featured four steps, or vertical increases in thickness. The fourth step inboard on the center section spar flange had a vertical increase of approximately 3/4 of an inch, with a fillet radius of approximately 1/8 of an inch.

Evidence indicated the left outer wing first separated in the fillet of this fourth step. The fracture revealed that separation started from a fatigue crack approximately 7/8 of an inch long and 3/32 of an inch deep, with the remaining cross section failing in tension. After the lower front spar flange separated, the lower rear spar flange and the top connections failed. Deprived of the left wing, the airplane rolled to the left. Paint and structural fragments showed the top of the fuselage and the right horizontal stabilizer struck the detached left wing in the air.

Tests at the National Bureau of Standards confirmed the aluminum alloy in the spar flanges met chemical and strength specifications. However, the Board found that the connection's design induced high local stress concentrations due to the small fillet radius and large sudden change in cross section, making it readily susceptible to fatigue.

Following the accident, another Northwest Martin 202 that flew through the same storm area shortly after Flight 421 was found to have a completely separated right lower front spar flange at the corresponding point, also originating from a fatigue crack. Inspections of the airline's fleet revealed fatigue cracks in the same location on three additional aircraft. Furthermore, microscopic examination of a test wing used during the aircraft's original certification cycling tests revealed that fatigue cracks had developed in its wing root fitting during those tests.

Meteorological analysis showed a vigorous squall line 70 to 75 miles in length in the area. Other flights in the vicinity described the storm as vicious and violent, though several successfully circumnavigated it. Data from the Thunderstorm Project and the National Advisory Committee for Aeronautics indicated that gust velocities in excess of 30 feet per second were extremely unlikely at the flight's altitude. The Martin 202 was designed to sustain a 53 foot-per-second gust at its ultimate load factor. Post-accident tests by the manufacturer indicated a gust of 38 to 48 feet per second would be required to fail the spar flange after a fatigue crack of the size found on Flight 421 had developed. The Board concluded the outer wing panel failed due to the joint action of fatigue cracks in the attachment fitting and the stress of severe turbulence encountered in the thunderstorm.

As a temporary corrective action approved by the Civil Aeronautics Administrator, operating speeds for the Martin 202 were reduced by ten percent, frequent inspections were instituted, and the spar flange was modified to include five steps with a larger 3/4-inch fillet radius.

### Probable cause

The Board determined 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."