The inspections are aimed at preventing a disaster of the kind that tore apart TWA flight 800.
The FAA is primarily concerned with tubes that carry electrical wires through 727 fuel tanks. Officials Saturday said holes were found in the tubing, exposing frayed wires in two older 727s. A spark could cause an explosion.
"This condition, if not corrected, could result in ignition of fuel vapors in a fuel tank and a fuel tank explosion," the FAA told 727 operators.
The "telegraphic airworthiness directive" issued by the agency gives airlines five days to make the inspections on airplanes with more than 50,000 hours of flight time.
On planes with less flight time, the inspections must be made within 10 days or before a plane accumulates 30,000 hours of flight time, whichever is later.
The airplanes in question are flown by the major passenger carriers and cargo companies such as FedEx and UPS.
FAA officials were unclear what effect the order would have on the traveling public, since about one-third of the planes were cargo carriers and the inspections could be spread between five and 10 days.
"We certainly hope the impact will be limited, but we can't be sure," said FAA spokesman Les Dorr.
Boeing and FAA officials estimated that the tests would take about four hours per aircraft and cost a total of $252,000 for the U.S. fleet.
The FAA ordered similar inspections last May of the 727's cousin, the Boeing 737.
That order came after mechanics found both exposed wires and holes in the conduit used to carry power lines through the fuel tank on a 737.
The chafing was believed to have been caused by engine vibration and the holes were believed to have been caused by electrical sparks.
While investigators in the TWA-800 crash three years ago did not find conclusive evidence that frayed wiring was the cause, the latest FAA action on the 727 shows how this type of disaster - where fuel and electricity mix - is now at the top of the list of concerns of aviation safety officials.