When the jet was assembled 15 years earlier, the drilling of the rivet holes along one side of the fuselage skin panel that tore away "showed a lack of attention to detail and extremely poor manufacturing technique," the report concluded.
The work also "was not in accordance with Boeing specifications or standard manufacturing practices."
The NTSB said evidence indicates the hidden cracks emanating from the rivet holes had been slowly growing with each take-off and landing, and had started "approximately when the airplane entered service" in 1996. But the safety agency's report suggests this may have been a one-off error by a mechanic.
Boeing pointed to the NTSB finding that subsequent inspections of other 737s found no similar damage in the same fuselage panel joints.
The NTSB concluded that therefore it's "unlikely that there was a systemic QA (quality assurance) error at the Boeing facilities."
The report reveals that a panel above the one that ripped away was replaced at some late stage of the assembly process and that the join between these two panels -- three rows of rivet holes along the overlap -- showed serious discrepancies, including non-circular holes, double-drilled holes, gaps between the rivets and the holes, and metal burrs protruding from under the rivets.
"The crown skin panel and the upper left fuselage panel were misaligned, so most of the lower rivet row holes were misdrilled," the report states.
At the time, the inflight incident raised concern that aging jets might be more susceptible to metal fatigue cracking sooner in their life cycle than previously believed. Hans Weber, an aviation technical expert, president of Tecop International in San Diego, said the NTSB analysis dispels that concern.
"The workmanship was just terrible," Weber said. "This has nothing to do with a typical fatigue fracture due to aging."
"I'm relieved that it's not because of some aging process we didn't understand," he added.
On April 1, 2011, as Southwest flight 812 climbed to 34,000 feet out of Phoenix, Ariz., with five crew members and 117 passengers aboard en route to Sacramento, Calif., the cracks finally opened into an 8-inch-wide gash in the jet's ceiling.
Air rushed from the passenger cabin, and the rapid decompression caused an immediate loss of oxygen. As oxygen masks deployed, the pilots declared an emergency and descended quickly.
A flight attendant lost consciousness while trying to make an announcement over the public address system, falling and fracturing his nose.
Another off-duty Southwest employee on board who tried to help the flight attendant also lost consciousness, sustaining a cut above his eye as he too fell.
No one else was injured as the pilots landed the plane safely at Yuma, Ariz.
The NTSB report, noting that a person can become unconscious in as little as six seconds after a rapid decompression, said the flight attendant didn't follow procedures in not immediately putting on an oxygen mask before trying to help anyone else.
Weber said that the developing cracks in the jet that ripped open would not have been visible during standard maintenance checks for a jet that was considered roughly mid-life, at 15 years old and less than 40,000 take-off-and landing cycles.
He said the cracks formed in an area -- on the inside of the overlapping skin panels and along the lower row of rivets -- that was hidden from view and is normally not highly stressed.
The NTSB investigation was hampered by the fact that at the time Boeing retained its manufacturing records for only six years after assembly.
As a result, it could not be determined whether the poor job of installing the replacement skin panel, and the subsequent quality checks, occurred during the initial fuselage assembly at Boeing's plant in Wichita, Kan. -- now Spirit AeroSystems -- or during final assembly in Renton.