TROUBLE: The Malibu Mystery

It's a record to give pilots and passengers pause: seven Piper Malibu Mirage aircraft have broken apart in midair in the past 22 months. The manufacturer, Piper Aircraft of Vero Beach, Fla., insists that the six-seat, single-engine plane is entirely airworthy. The Federal Aviation Administration has suspicions to the contrary and last week issued a directive grounding the 518 remaining Malibus in foul weather and limiting the ways the planes can be flown.

Prompting the action was a mid-March accident in Florida that killed a family of four when their Malibu fell apart during a rainstorm. The FAA has not determined...