FAA plans to order emergency inspections for metal fatigue on older 737s. Officials say aircrafts' aluminum skin can be stressed by changes in cabin pressure from an average of six flights a day.Southwest Airlines' older aircraft plus its famously efficient short-haul operations — requiring planes to fly an average of six times a day — probably contributed to the fuselage rupture that forced a jetliner carrying 118 passengers to make an emergency landing in Arizona last week.