The inspections are high-tech and labor-intensive.
Mechanics using a device that sends magnetic signals through metal to detect unseen cracks will scan about 50 feet of the twin metal seams running along the top of each airplane. The task takes two experts in aircraft service about eight hours. Repairs on any fatigue cracks will take a day or two at most. The checks will have to be repeated every 500 flights.
Boeing redesigned the lap joint on 737s in the early 1990s and thought airlines wouldn't need to inspect them closely until 60,000 flights. That was a mistake, a top Boeing engineer acknowledged this week, and the company was surprised by the failure of the 15-year-old Southwest jet that had flown fewer than 40,000 flights.
Indeed, Continental and Alaska Airlines are inspecting airplanes that are years from the new FAA threshold as an extra precaution, the companies told The Associated Press.
Continental, now merged with United Airlines, has 32 of the 737s in question, none with more than 30,000 cycles that would make them subject to the immediate inspection order. Nonetheless, the twin joints that hold the skin together along the top of the airplane will be inspected as they come due for major maintenance in the coming 18 to 24 months.
The first Boeing 737 entered commercial service in 1968, and 6,725 have been delivered since then. Very few of the early models, with their distinctive cigar-shaped engines, are still flying.
A 737-200 model flying for Aloha Airlines in 1988 had one of the most spectacular aviation incidents in modern history when its roof ripped off while flying from Hilo to Honolulu. A flight attendant was sucked out of the plane and plunged to her death, and dozens of passengers were injured.
That tragedy was blamed on the failure of the same type of metal joint that forced Southwest Airlines Flight 812 to make an emergency landing near Yuma, Ariz.