More than six decades ago, when the federal standards on the strength of airplane seats and seat belts were written, government regulations specified that seats be designed for a passenger weight of 170 pounds. But now the average American man weighs nearly 194 pounds and the average woman 165. Now, some engineers and scientists have raised questions about whether airplane seats, tested with crash dummies that reflect the 170-pound rule, are strong enough to protect heavy travelers. “If a heavier person completely fills a seat, the seat is not likely to behave as intended during a crash,” said Robert Salzar,...