Southwest Airlines attempted to push back on FAA-mandated inspections of aging Boeing 737s, like the one that lost part of the roof of its cabin Friday, complaining that the safety checks would “pose a significant burden,” a review of government documents reveals.

The FAA rejected Southwest's appeal, saying the inspections were needed "to maintain an adequate safety level" and also insisted the new inspection regime "will not increase the economic burden." They calculated that every periodic inspection would cost each carrier $1,275 per airplane.

This is revealed in the fine print of an order, called an Airworthiness Directive, which came into effect on February 1 this year.

On Saturday, Southwest grounded 79 of its 737-300s, all of them 15 years old or more, so that they can be inspected for undetected cracks in their skin—cracks that can lead to a sudden structural failure and loss of cabin pressure. These emergency inspections are being carried out according to guidelines set out in several FAA Airworthiness Directives, and CNN reported last night that cracks have been found in two more planes.

The earlier concerns about the structural integrity of the 737s was serious and urgent. The FAA was explicit: The checks were imperative, the agency said, “to detect and correct fatigue cracking of certain fuselage frames.” In view of Friday’s emergency, the words that followed were predictive: “This reduced structural integrity can increase loading in the fuselage skin, which will accelerate the skin crack growth and could result in rapid decompression of the fuselage.”

Southwest was the only airline that balked at the timetable in the proposal.

The directive that Southwest objected to covered not just the oldest 737s, but later models, the 400 and 500 series—affecting a total, according to the FAA, of 635 737s flown by U.S. carriers. Southwest was the only airline that balked at the timetable in the proposal.

Southwest representatives did not return multiple calls for comment.

Airworthiness Directives are part of a normally submerged and arcane surveillance of every airplane operated in U.S. air space. Scores of them are issued every year, often as a result of recommendations from both manufacturers and operators. They are part of the continual and highly detailed feedback from experience, aimed at the early detection of problems, no matter how insignificant, that might have a bearing on airline safety.

Southwest did not dispute the need for the inspections. It wanted to choose its own time frame in which to carry out them out, to lessen the risk of disrupting its schedules. (Schedules that are now in disarray.) In its response, the FAA was unusually tart: “No supporting data were submitted proposing alternative inspection thresholds to maintain an adequate level of safety for its fleet.”

As I reported after the emergency landing, Southwest’s whole economic model is built on the high utilization of every one of its 737s, which, on average, make seven flights a day. Frequent takeoffs and landings are a proven cause of the kind of metal fatigue that eventually produces cracks in the cabin’s outer skin. The new FAA inspection regime is a response to what is now better understood about the consequences of this kind of hard-pounding of an air frame.

On Sunday, a top official of the National Transportation Safety Board, Robert Sumwalt, speaking from the military air base where the stricken Southwest jet made its emergency landing, said “We did find evidence of widespread cracking across this entire fracture surface.”

Clive Irving is senior consulting editor at Conde Nast Traveler, specializing in aviation—find his blog, Clive Alive, at CliveAlive.Truth.Travel.