DALLAS (CBSDFW.COM) – Southwest Airlines says it has finished inspecting planes grounded since the weekend.

The airline says 5 of its planes are now out of service because of what it is calling “minor sub-surface cracking,” but the government is expanding emergency inspections of some Boeing planes.

The Federal Aviation Administration isn’t taking any chances. On Tuesday it issued an Airworthiness Directive on protocol on how to address the issue.

Southwest Airlines says it completed inspections on 79 of its Boeing 737-300 series aircraft which includes the plane that had a hole rip open in it mid-flight Friday forcing pilots to make an emergency landing.

Federal investigators are now examining a dismantled piece of fuselage from Friday’s flight.

“This is our surprise…we do not expect aircraft in service today to rapidly decompress in flight,” said National Transportation Safety Board Chairman Deborah Hersman.

Now the FAA is ordering other airlines to examine some of their aging 737’s, about 100 planes worldwide.

“We want to make sure that this fleet of aircraft is as safe as possible,” said Hersman. “If we identify any issues that need to be addressed, we have the ability to issue urgent recommendations.”

The 15-year-old plane with the hole in it has nearly 40,000 flight cycles.

“This aircraft is probably best characterized as in its mid-life,” said transportation expert and former NTSB chairman Mark Rosenker. “We start looking to retire aircraft somewhere in the 75,000 to 80,000 cycle category.”

In an email Southwest Airlines says the planes in question were manufactured between 1993 and 1996.  That email goes on to say, “Our average age is 11 years – which is a young fleet for an airline. The issue with these aircraft is not age – it is related to design.”

And while Southwest says its fleet is young, Rosenker says the amount of use of Southwest’s 737’s is significantly different compared to other aircrafts and airlines.

“The 737 has primarily been used by Southwest Airlines for short hops which gives it anywhere between 3 to 5 cycles a day, where if you  compared that to a 777 which goes from New York to London, it might only go on two cycles a day.”

Under the FAA’s new mandate, crews will use an electro-magnetic device that can detect cracks not seen by the human eye and will have to check for them regularly.