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.