An MD-80 aircraft crashed, killing all on board. Individuals filed lawsuits against the aircraft's operator, Northwest Airlines (Northwest) (plaintiff), and the aircraft's manufacturer, McDonnell Douglas (MD) (defendant). Northwest and MD filed counterclaims relating to their respective liability. The trial court ruled in favor of MD and Northwest appealed to the United States Court of Appeals for the Sixth Circuit. Northwest argued the trial judge erred in excluding evidence that: (1) Northwest rewired its MD-80 fleet after the crash because the fleet's original wiring was defective; (2) contrary to an MD witness's testimony that Honeywell equipment on the MD-80 was safe, Honeywell's pre-crash repair of that equipment on other MD aircraft showed it was unsafe; and (3) MD staff debated recommending backup checks on an MD warning system involved in an earlier crash.