updated 4/8/2011 6:03:22 PM ET 2011-04-08T22:03:22

The crown prince of Thailand has one. So do the presidents of Peru and Chile. The Chinese Air Force relies on it, as do airlines in Russia, Indonesia, Australia and Romania.

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The Boeing 737 is a workhorse of international aviation. And the accident in which the roof of Southwest Airlines jet ripped open at 34,000 feet has brought scrutiny to the hundreds of older-model 737s around the world that could be similarly vulnerable because of tiny, hard-to-find stress fractures in the aluminum skin.

The planes will now be subjected to repeated examinations as the problem revealed by the fuselage crack on the Southwest flight resonates through the world's 737 fleet for years to come.

Many of their owners are now giving the planes a closer look after what happened April 1 in Arizona when a 5-foot section of the fuselage tore apart and forced pilots to make an emergency landing at a desert military base. Light-headed passengers were banged around the cabin and had to quickly put on overhead oxygen masks as pilots made a rapid descent.

Story: Older planes safe, but maybe less convenient

The incident has forced airlines and governments around the world to take swift action.

The governments of Japan, Indonesia, South Korea and others ordered airlines to beef up inspections. Scandinavian airline SAS is performing similar checks on some of its 737s. Qantas Airlines in Australia is checking four of its planes and Air New Zealand is looking at 15. Airlines said the inspections have not disrupted air travel.

Southwest and Continental Airlines have the most planes on the list of 737-300s, 737-400s and 737-500s prone to the fuselage ruptures, but a large number of the planes are owned by overseas carriers. UTAir in Russia, Garuda Airlines in Indonesia, Air New Zealand and three major carriers in China are among the biggest. Alaska Airlines has 17.

6,000 in operation around the world
Southwest finished inspecting all of its affected planes by Tuesday. They found five that had cracks in the same lap joint that tore open during last week's flight, and were working with Boeing to make repairs. Alaska Airlines is going a step beyond a Federal Aviation Administration directive this week that ordered inspections when the planes reach a 30,000 takeoffs and landings; the airline will inspect all its planes in the coming weeks.

"We're not required to inspect them right now, but we felt it was the prudent thing to do, and to help the industry determine the proper interval," spokesman Paul McElroy said.

There are about 6,000 737s in operation worldwide, and an emergency FAA order on Tuesday only covers 579 that have the type of "lap joint" that failed during last week's flight. Lap joints are used in many places on an aircraft fuselage and get their name because it is the spot where the aluminum skin of the aircraft overlaps and is secured with rivets. The FAA order focuses on a Boeing joint design on planes made between 1993 and 2000.

Experts say that all of the planes around the world will be covered by the FAA order because of international agreements between civil aviation regulators globally. Many of the inspection orders handed down by foreign governments mirrored the one issued by the FAA.

"Some airlines may not always maintain the records that they need to and certainly not all airlines will not maintain their airplanes to the highest levels of safety," said Henry Harteveldt, aviation analysis at Forrester Research, Inc. in San Francisco. "But I would make very clear that the top tier U.S. and foreign flag airlines do this. Airlines like British Airways, Qantas and so on, those airlines maintain their airplanes to the highest standards and the best record-keeping."

The FAA said all of the planes have to undergo inspections when they reach the threshold of 30,000 takeoffs and landings. The 175 of those planes that have already reached the threshold are getting immediate inspections.

For example, the planes owned by the crown prince of Thailand and Chinese Air Force have only flown about 5,000 cycles each, meaning their planes have a long ways to go before an inspection. One of the Swedish planes has more than 40,000, requiring an immediate examination.

Time and know-how needed
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.

The Aloha incident
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.

The Aloha incident triggered a decades-long effort to prevent similar stress-related failures that came to a conclusion in January when new FAA regulations went into effect mandating closer inspections.

On Thursday, FAA Administrator Randy Babbitt ordered a review of the new safety regulation, saying the agency needed to find out why the cracks in the latest incident were not detected.

Independent aviation consultants were also worried by the failure.

"The one thing we have to be worried about this is that we were surprised by that crack," said Hans J. Weber, president of San Diego-based aviation consulting firm TECOP International, who worked extensively on the aging aircraft program. "After all that work, that we again have an aging aircraft surprise, that bothers me."

It remains important to remember, Harteveldt said, that all airplanes have problems, and have ever since modern jet transportation changed travel in the mid-1950s.

"To Southwest's credit, when that skin ruptured on that plane a week ago or so, that pilot got that plane down from 34,000 to 10,000 feet or so in (four) minutes. No one was severely hurt, no one died, and I think that is an important point to keep in mind," Harteveldt said. "I think people will view this as a hiccup, I don't believe that it will have a long-term effect on the 737."

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Video: Southwest struggling to recover from air scare

  1. Closed captioning of: Southwest struggling to recover from air scare

    >>> good evening. while they are fortunate to have avoided a disaster in the air tonight southwest airlines has a big problem. so does the maker of the 737. so do thousands of people with plans to fly. the issue is cracks in the fuselage of some boeing 737s. most of the cracks too small to see, but one of them grew large enough to open up a hole in the roof of a southwest jet a few days back. that incident has now triggered the inspection of more 737s across the country. this is going to bring new attention to our short-haul aircraft in this country. in the air for multiple flights per day and carrying a lot of the passenger load. nbc's tom costello who covers aviation for us, starts us off from washington tonight. good evening.

    >> reporter: hi, brian. the f.a.a. is talking about more frequent inspections while southwest says of the 79 planes grounded over the weekend three were found to have small cracks and 64 have been returned to service already. nearly 72 hours after southwest flight 812 made the emergency landing in arizona the f.a.a. is ordering checks on specific groups of the 737 300s, 400s and 500s with at least 30,000 cycles. the plane involved in friday's emergency had nearly 40,000 cycles on it. investigators were surprised to find signs of pre-existinging cracking under a lap joint and along a rivet line. a large chunk of the fuselage will undergo metal fatigue tests at the lab in washington .

    >> it was not believed that this was an area that could fail until we see it now.

    >> reporter: today's f.a.a. inspection order affects 175 planes worldwide. 80 in the u.s. the vast majority at southwest . investigators are wondering whether planes used on short haul flights with multiple takeoffs and landings like southwest routes incur greater metal fatigue as the skin of the plane expands and contracts.

    >> i think part of this is not only inspecting the plane but looking at how often the planes are used, how often they take off, how often they land.

    >> reporter: the 737 involved had a 14-year maintenance overhaul in march of 2010 . those overhauls involve stripping the plane to its frame and going over every centimeter looking for corrosion or cracks but inspectors rarely look under the lap joints where this was found. a former chief for the f.a.a. says it's stunning how quickly this crack turned into an emergency.

    >> this is the worst decompression pilots plan for. getting down as quickly as you can.

    >> reporter: after cancelling several hundred flights over the weekend southwest cancelled another 70 today as it continued inspecting its planes. southwest says it hopes to be back to normal service by tomorrow and says the planes the f.a.a. today ordered to be inspected are the planes it has been inspecting, no more. brian?

    >> tom costello in washington tonight.

Photos: Made in America: Boeing 737

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  1. A Boeing 737 fuselage rolls into Renton, Wash., on Monday, April 5, 2010. The fuselages are manufactured by Spirit Aerosystems in Wichita, Kansas. (Jim Seida / Back to slideshow navigation
  2. 737s move along the rolling production line. The rudder is the only part of the airplane that is painted before the plane makes its first test-flight as the balance of the rudder is critical to proper flight, and even the weight of a paint job is enough to put it out of balance. (Jim Seida / Back to slideshow navigation
  3. Brightly-colored tennis balls protect employees from sharp objects attached to the fuselage frame. (Jim Seida / Back to slideshow navigation
  4. Using a flashlight and a small mirror, a worker closely inspects the the fuselage of the aircraft before insulation and wiring is installed. Boeing employees inspect the fuselage this way three times. (Jim Seida / Back to slideshow navigation
  5. An employee installs some of the 36 miles of wires into a 737. A 737 contains 367,000 parts. (Jim Seida / Back to slideshow navigation
  6. A 737 cockpit sits empty, awaiting seats and instruments. (Jim Seida / Back to slideshow navigation
  7. Equipment sits outside a 737 in production. The green color is a protective coating that keeps the aluminum from oxidizing. It is washed off with pressure washers before the aircraft is painted. (Jim Seida / Back to slideshow navigation
  8. Ready to be installed, bathrooms sit on the factory floor. (Jim Seida / Back to slideshow navigation
  9. Hundreds of seats sit in front of the assembly line of aircraft that crawl along behind them at the rate of two inches per minute on their rolling production line. The production time of a 737 was cut in half in part by Boeing switching from building the planes in stationary positions to the assembly line in 2000. (Jim Seida / Back to slideshow navigation
  10. In the engine shop the exhaust ducts are installed and the engines go through final inspection before being hung beneath the aircraft's wings. (Jim Seida / Back to slideshow navigation
  11. Machinists prepare an engine for installation. The engines make up one-third of the cost of the 60 million dollar aircraft. (Jim Seida / Back to slideshow navigation
  12. Only 11 days after entering the other end of the factory as a bare fuselage, a finished 737 is readied to head to the flight line. Boeing produces 737s at the rate of 1.5 planes each workday. (John Brecher / Back to slideshow navigation
  13. Before being painted, a brand-new 737 goes on a test-flight just outside the Renton 737 plant. There are so many 737s in service that, according to Boeing, one is taking off or landing every 2.2 seconds. (Jim Seida / Back to slideshow navigation
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