Washington (CNN) -- Boeing engineers had anticipated the aluminum skin and "lap joints" on certain Boeing 737s would need to be inspected for cracks after roughly 60,000 take-off and landing cycles, but the company drastically altered that estimate in the wake of the fuselage rupture on a Southwest Airlines flight with only 39,000 cycles.
Boeing this week issued a bulletin advising operators of similar aircraft to inspect the planes for subsurface skin cracks after only 30,000 cycles.
The Federal Aviation Administration on Tuesday mandated the earlier inspections and took it one step further, ordering the inspections be repeated every 500 cycles thereafter, at least until the cause of the April 1 rupture can be determined.
In a conference call with reporters, Boeing engineer Paul Richter said the company has long recognized cracks as a problem on the lower row of fastener holes on the skin panels, but believed the cracks would appear much later in the aircraft's lifespan.
"We did anticipate that there would be need to inspect the airplane at some point in time. Our plan previous to this event was to recommend inspections starting at 60,000 cycles, which was a very conservative number," Richter said.
"Obviously none of the planes ... are close to that threshold ... so I would say that it's regrettable that we had to accelerate our plans to recommend inspections based upon an event such as this."
Richter characterized the FAA's mandate of frequent repeat inspections as a cautious move, which the FAA may change once it fully understands the nature of the problem.
Boeing said 175 planes worldwide are affected by the service bulletin, and eventually a total of 570 planes will need the same inspections as they reach the 30,000 cycle threshold.
It was unclear Tuesday how many of the 570 planes belong to U.S. airlines.