A Southwest Airlines 737NG is landing at McCarran Airport with mountains in background. File photo: IC

The Indonesian Transportation Ministry grounded three Boeing 737NG (Next Generation) planes operated by two airlines following findings of cracks in the aircraft, an official said on Tuesday.

The three grounded planes included one operated by the state-run airline Garuda Indonesia and two operated by the private airline Sriwijaya Air, according to Avirianto, the Airworthiness and Plane Operation Director at the Ministry.

The grounding of those planes will continue until further recommendations are issued by Boeing, he said.

Furthermore we advise all national airlines operating the Boeing 737NG, including Garuda Indonesia, Lion Air, Batik Air and Sriwijaya Air, to periodically conduct maintenance programs every 3,500 flight cycles in accordance with the Directorate General of Civil Aviation (DGCA) mechanism, he said.

Cracks were found in planes which already had a Flight Cycle Number (FCN) of 30,000, he said.

The cracks were found in one of three Boeing 737NGs operated by Garuda Indonesia, and in two of five of the same aircraft operated by Sriwijaya Air.

The grounding order was issued after inspections were carried out by the Ministry in line with airworthiness directives from international aviation agencies, which called for checks of Boeing 737NG planes operated by all airlines globally.

Southwest, Gol ground 13 737NGs

On October 9, the US' Southwest Airlines Co and Brazil's Gol Linhas Aereas grounded a total of 13 Boeing 737NG airplanes, the companies said, after US regulators ordered urgent inspections.

Southwest said it had taken two planes out of service, while Gol had grounded 11. Both airlines are major operators of the 737, including the NG variant and the more recent MAX, which has been grounded for months after two deadly crashes. 

The US Federal Aviation Administration earlier told aircraft operators to inspect 165 older 737NG planes for structural cracks within seven days, after cracking was found on a small number of planes. Nearly all the 165 jets were Southwest aircraft, officials said. 

First delivered in 1997, the NG is the third generation of the 737, preceding the grounded MAX, which is unaffected by the cracking issues. 

Southwest, which did not find any issues in the "vast majority" of planes, said it "removed the two aircraft from [its] operation and reported the findings to Boeing and the FAA." 

Gol, which runs a fleet of 115 737s excluding its grounded MAX jets, said it took 11 out of service after finding "evidence of the need to replace a specific component." The fleet change will affect 3 percent of passengers until December 15, it said. 

The inspections were ordered following the discovery of cracks on a part known as the "pickle fork," which attaches the fuselage to the wing structure and manages forces. 

Minor aircraft cracks are not unusual, an industry source said, but are not normally expected to occur on the pickle fork until near the end of the plane's lifespan, typically considered to be more than 90,000 FCN. 

The FAA mandated checks for "cracking of the left and right hand side outboard chords of frame fittings and failsafe straps," warning that the issue "could adversely affect the structural integrity of the airplane" and result in loss of control. More planes will eventually be inspected, it said. 

The initial inspection order covered a total of 1,911 US-registered planes. The checks can be done visually and take about an hour per airplane. 

Aircraft that have carried out more than 30,000 FCN, must be checked within seven days, the safety regulator said, while those with 22,600 to 29,999 FCN must be checked before completing a further 1,000. 

Boeing has been in touch with 737NG operators about the inspections, the manufacturer said earlier, emphasizing that "no in-service issues have been reported." 

United Airlines and American Airlines Inc are among other US carriers flying the 737NG, which includes the 737-600, -700, -700C, -800, -900, and -900ER variants.

Each airline has about 80 planes that will need inspections in coming months, United and American said, but none that fall into the seven-day requirement for more urgent checks. 

United and American said they have not seen any cracks on their airplanes.

European budget carrier Ryanair, which has a fleet of more than 450 737-800s, confirmed it was in the process of carrying out the inspections, without saying how many planes required them. 

"We are midway through the first part of this mandatory check program and don't expect it will have any impact upon our fleet or operations," a Ryanair spokesperson said. 

Transavia Airlines, a low-cost carrier owned by Air France-KLM, said it would examine all 42 of its 737s even though only 16 need checks under the FAA mandate - of which one was inspected under the seven-day requirement. KLM, which also operates 737NGs, has inspected all but one of 13 aircraft covered by the order and found no anomalies, a spokesperson said. 

Jeju Air, a South Korean low-cost operator, had also completed the majority of required checks, while inspections were ongoing at domestic rival Jin Air and had been completed at Eastar Jet. None of the three airlines had found any issues, they said. 

Cracks found on 38 jets

Boeing said on October 10 that airlines had inspected 810 of the company's 737NG jets around the world and found 38 structural cracks requiring repair and replacement of the affected parts.

The planes will be grounded until the repairs are made, Boeing and airline officials said. Nearly 5 percent of inspections have found cracks in the pickle fork.

The FAA had earlier told US aircraft operators to inspect 165 older 737 NGs for structural cracks, but it now appears the actual number of planes covered is more than 200. 

The FAA said late on October 10 that "a small number of aircraft based in the US have been removed from service while Boeing develops instructions for customers to repair or replace the affected parts." 

The agency added it "is working with the manufacturer and other international aviation safety regulators to better understand the factors that led to the formation of the cracks."

Southwest spokesperson Brandy King said that out of more than 200 high-use airplanes, it had found cracks on two planes and grounded them. She declined to say if cracks had been found on any other jets but said the airline planned to release an update next week. 

"We do not have a timeline for when the airplanes will be returned to service; we are working with Boeing to schedule the upcoming repairs," King said. 

Raymond James analyst Savanthi Syth wrote in a research note on Thursday that the findings from the 737NG inspections could "potentially take up to 4 percent of capacity off-line between mid-October and mid-December." 

Planes with cracks "may need to be taken out of the fleet for up to 60 days for maintenance," Syth said. 

The FAA last week said inspections would look for "cracking of the left and right hand side outboard chords of frame fittings and failsafe straps."