Investigation Summary
The crash of Flight 421 was the first hull loss of the Martin 2-0-2 and exposed a critical vulnerability in the aircraft's design. Investigators found a 7/8-inch fatigue crack in the left front outer wing attachment fitting. This fitting was designed with high local stress concentrations that invited material fatigue. During the flight, the aircraft entered a roll cloud at the leading edge of a thunderstorm. The combination of existing fatigue and the aerodynamic loads from severe turbulence caused the outer left wing panel to separate completely. As the wing detached, it struck the fuselage and the right horizontal stabilizer, leading to a total loss of control. The discovery of similar cracks in other Northwest Martin 2-0-2s led to a major grounding and a lawsuit against the Glenn L. Martin Company.
Final Conclusions
The Civil Aeronautics Board determined the probable cause was the loss of the outer panel of the left wing due to a fatigue crack in the front outer panel attachment fitting. This crack was induced by faulty design and aggravated by severe turbulence. The board noted that the design resulted in high local stress concentrations that the material could not sustain over time.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Airframe & Maintenance
Crew Experience
Captain
Robert Leslie Johnson
First Officer
David Franklin Brenner
Systems & Failure Modes
Wing Spar Attachment Fitting
The critical joint connecting the outer wing panel to the center wing section; it failed due to fatigue cracks.
Front Spar
The primary structural member of the wing which suffered the initial separation.
Interesting Facts
- 01The crash remains the deadliest accident involving the Martin 2-0-2 aircraft type.
- 02Rescuers had to form a human chain to recover remains from a 150-foot deep ravine.
- 03Inspections following the crash found identical fatigue cracks in three other Martin 2-0-2 aircraft.
- 04The aircraft was less than a year old at the time of the accident.
- 05Northwest Airlines later sued the manufacturer, Glenn L. Martin Company, for $725,000 over the design defects.
- 06One of the victims was a woman found still holding a baby in her arms, which may account for manifest discrepancies.
Safety Actions & Advisories
Mandatory Wing Inspections
The CAB recommended frequent and rigorous inspections of all Martin 2-0-2 wing root fittings for fatigue cracks.
Structural Reinforcement
Recommendations were made to increase the thickness of the wing skin and spar components where they attached to the fuselage.
Operating Speed Reduction
The Board recommended a 10% reduction in operating speeds for the Martin 2-0-2 to reduce structural loads.