We’ve seen this before: A Boeing airliner crashes, killing all aboard. Investigators believe a design flaw in the aircraft played a major role in the accident, but Boeing blames the pilots. Eventually, the design flaw is corrected, but not before another plane crashes, leaving more deaths in its wake.

In our time at the National Transportation Safety Board we saw this happen — long before the two Boeing crashes in the past year.

On March 3, 1991, a United Airlines Boeing 737 crashed on approach to Colorado Springs, killing all 25 people aboard. After an investigation of almost two years, the N.T.S.B. concluded that one of the two likely causes was a malfunctioning rudder power control unit, which moved the rudder in the opposite direction to that intended by the pilots. The agency recommended that the Federal Aviation Administration require airlines to install a modified part, to prevent future rudder reversals, as soon as Boeing made them available, but Boeing failed to do that.

On Sept. 8, 1994, a USAir 737 crashed as it neared Pittsburgh, killing all 132 people aboard. Despite the obvious similarities between the two crashes that were revealed during the investigation, Boeing insisted even to the final stages of the second inquiry that there was nothing wrong with the design of the aircraft, and the company again pointed to improper pilot rudder commands as the cause.