* FAA mandated urgent inspections for heavily used planes
* Cracks also found in aircraft requiring less-urgent inspections
* Grounded planes will cost around $275,000 each to repair - IBA
* Fewer than 5% of planes at inspection threshold cracked - Boeing (Adds Boeing update on inspections to date)
By Jamie Freed and Tracy Rucinski
SYDNEY/CHICAGO, Oct 31 (Reuters) - Qantas Airways Ltd and Southwest Airlines Co are stepping up checks for structural cracks on Boeing Co 737 NGs after discovering problems with planes that did not require urgent inspections, airline sources said.
The cracks are on what is known as the "pickle fork" - a part that attaches the plane's fuselage, or body, to the wing structure.
Repairing the cracks requires grounding the airplane and costs an estimated $275,000 per aircraft, according to aviation consultancy IBA. The are currently thousands of 737 NG planes in use by airlines around the world.
The issue surfaced while the newer 737 MAX model is grounded globally following two deadly crashes.
The U.S. Federal Aviation Administration (FAA) on Oct. 2 mandated checks of 737 NGs with more than 30,000 take-off and landing cycles - which typically correspond to the number of flights - within seven days.
But Qantas said it has discovered cracks in a plane with just under 27,000 cycles that had been removed from service for repair. While no Qantas jets have yet reached 30,000 cycles, the airline plans to inspect 33 planes with more than 22,600 cycles by the end of this week.
A source with knowledge of the matter told Reuters that cracks have also been found on a second Qantas jet with nearly 27,500 cycles in an inspection on Wednesday evening. Qantas did not respond to a request for comment on Thursday.
The FAA said jets with 22,600 to 29,999 cycles must be inspected within 1,000 cycles. That, however, could take several months before an inspection is required.
Southwest found cracks in one with about 28,500 cycles, a separate source with knowledge of the matter said.
A Southwest spokesman said the airline has pulled three jets from service for pickle fork repairs but he could not confirm the number of cycles. The airline has complied with the FAA directive on inspections but is expanding checks to its entire 737 NG fleet, he added.
A Boeing spokeswoman said on Thursday that just over 1,000 planes had met the threshold for inspections to date, and of those fewer than 5% had issues.
In a filing with the Securities and Exchange Commission last week, Boeing said all 737 NGs with more than 30,000 flight cycles and about one-third of planes with over 22,600 flight cycles had been inspected for pickle fork cracks.
The manufacturer said additional assessments were underway to determine the cause and potential implications for planes with fewer than 22,600 cycles.
"Depending on the results of these assessments, additional inspections or repairs may be required," Boeing said.
It said that so far the repair costs were not big enough to affect its bottom line. But the company added that it could not estimate the potential financial impact because the inspections were ongoing.
American Airlines and United Airlines, whose 737 NG fleets have less than 30,000 cycles, are also inspecting their entire fleets but have not found any pickle fork issues so far, representatives for both airlines said.
Virgin Australia said it had already inspected all 19 of its 737 NGs with more than 22,600 cycles and did not find any cracks.
South Korea's transport ministry said all nine 737 NGs grounded in the country with cracks had more than 30,000 cycles. (Reporting by Jamie Freed in Sydney and Tracy Rucinski in Chicago; additional reporting by Eric M. Johnson in Seattle, Heekyong Yang in Seoul and Subrat Patnaik in Bengaluru; Editing by Kenneth Maxwell and Edwina Gibbs)