The US Federal Aviation Administration (FAA) has published an airworthiness directive requiring that Boeing 737NGs, which have completed 30,000 flight cycles, be re-inspected on its hardware known as pickle forks.
Under the directive, carriers must inspect 737NGs that have completed 22,600 flight cycles within their next 1,000 flight cycles within the next 60 days. The order impacts US-registered 737NGs, including -600, -700, -800 and -900 series aircraft.
In response, Boeing has issued a statement, which said: "We regret the impact to our customers and have a repair plan in place to address any findings.
"Less than 5% of the 1,200 airplanes that have undergone the initial inspection were found to have the cracking issue. The secondary issue has been discovered on three in-service airplanes and one airplane that was undergoing maintenance in preparation for a modification."
IBA has said that it could cost up to $250,000 per aircraft to fix the pickle forks affecting Boeing 737NG variants -600, 700, 800, 900 and 900ER aircraft.
FAA ordered inspections on the pickle fork issues in October after the agency was notified of structural cracks.
Boeing discovered cracks on pickle fork hardware of 737-800s undergoing passenger-to-freighter conversions in China.
Airlines impacted include Qantas, Southwest Airlines, Gol and Indonesian carriers Sriwijaya Air and Garuda.
South Korea is the latest country to ground a number of Boeing 737NG found to have structural cracks.
The Ministry of Land, Infrastructure and Transport (MOLIT) inspected 100 737NGs and found that 13 contain cracks.
Nine of these were identified in a first round of inspection in October and had clocked more than 30,000 flight cycles.