The Real Reason United Airlines Grounded All 757s

In a rapidly developing story, United Airlines essentially grounded all 96 of its 757 aircraft late Tuesday. Initially, the airline said it was because of a software glitch.

Actually my reporting suggests it was more serious than that and it wasn't a sudden problem.

It dates back nearly seven years.

In June 2004, the FAA was paying attention to the 757 air data computer, which measures air pressure and other atmospheric conditions to determine speed and altitude. Back then, the FAA issued an "airworthiness directive" (AD) ordering that these computers be replaced and that mechanics perform a check to ensure they were working properly.

The FAA issued this emergency order in 2004 requiring that the computers be replaced because of a flaw that could cause pilots to erroneously believe they were flying too fast or too slow. This is critical information, especially since misinformation could easily put a plane into a stall.

Did the mechanics perform that follow up check required by the FAA? Apparently not.

FAA'S Strong Directive to United Was Not Heeded
First, some background: The FAA issued what's called a "modified airworthiness directive," which means it informed United of the needed fix, the required solution and the deadline. If the airline misses the deadline, the FAA considers it noncompliance and subject to fines, or worse, grounding.

No one knows the exact deadline that the FAA had established for the emergency order set in 2004, but clearly it was looming when we consider United's last-minute scramble to inspect its 757s on Tuesday.

FAA could have issued an airworthiness directive, which grounds planes until the problem is fixed, but that is hardly ever imposed because the FAA is always been sensitive to airlines' pleas of economic hardship.

Of course, the argument can always be made that if you know there's a serious problem, and you know what the solution is, and you can implement the solution, then delaying that implementation is tantamount to criminal negligence if that delay results in a fatality.

The Case with United
United apparently realized this week that it hadn't performed the required check after replacing the units.

My sources tell me that apparently the airline, worried about being assessed a stiff fine by the FAA for noncompliance, made the decision to ground all of its 757s immediately so that the inspections could be finally completed.

In fact the, FAA was not even aware of this the lack of compliance until United notified the agency.

In the case of the 757s, this is not the first AD about the airplane. The FAA has already issued a specific AD last month that affected 680 older 757 models across several airlines, ordering enhanced structural inspections. The directive was designed to prevent dangerous fuselage cracks that could result in an aircraft essentially disintegrating in flight because of rapid decompression.

That directive was prompted by a sudden rupture and rapid decompression onboard an American Airlines 757 on a flight from Miami to Boston last October. The plane in question landed safely.

According to United, there were 15 cancellations yesterday due to the grounding, but all flight schedules should resume back to normal today.

More from CBS News