Boeing has revealed that some other parts of its 737 aircraft could be faulty, which is set to cause further headaches for the manufacturer, airlines and regulators.

Hundreds of leading-edge slat track parts manufactured by a Boeing supplier could be affected by the fault, Boeing and the Federal Aviation Administration (FAA) said, as reported by CNN and ABC News.

The slats are movable panels that extend along the wing’s front during take-offs and landings to provide additional lift. The tracks guide the slats and are built into the wing, and the agency has ordered the parts be quickly replaced.

One batch of slat tracks with specific lot numbers produced by the supplier was found to have a potential nonconformance. Boeing has identified 21 737 NG series aircraft most likely to have the part in question, while it has advised airlines to check an addition 112 NGs.

Working with the FAA, Boeing said it had reached out to airlines flying its 737 planes and that it advised them to “inspect their slat track assemblies on Max and NG aircraft”. The 737 NG series includes the 737-600, -700, -800 and -900 planes.

“We are committed to supporting our customers in every way possible as they identify and replace these potentially non-conforming tracks,” Kevin McAllister, president and CEO of Boeing Commercial Airplanes, said.

Boeing is now staging replacement parts at customer bases to help minimise aircraft downtime while the work is completed. Once the new parts are in hand, the replacement work should take one to two days.

Boeing will also issue a safety service bulletin outlining the steps to take during the inspections.

The announcement came on Sunday while the International Air Transport Association’s annual general meeting was underway in Seoul, where director general Alexandre de Juniac addressed the press on the return to service of the 737 MAX, which he said was unlikely to happen before August.

He said trust in the aircraft certification system “has been damaged” following the grounding of the Boeing 737 MAX, and that the prolonged grounding of the Boeing 737 MAX had taken its toll on regulators and airlines.

De Juniac’s comments follow discussions behind closed doors between the FAA, Boeing and the International Civil Aviation Organization’s governing council over the return to service of the 737 MAX following its software update.

It has been reported that sources close to the matter believe the FAA will approve the jet to be returned to service by late June.

Airlines would then be required to perform maintenance checks on grounded planes, update their software and train pilots, according to Reuters.

However, even if the 737 MAX is approved in the US, aviation authorities from other parts of the world reportedly may not follow suit in approving it.

The jet has been grounded internationally following two devastating crashes – one in October last year in Indonesia, another in Ethiopia in March – that resulted in the loss of a combined 346 lives.

De Juniac’s comments also come after Boeing CEO Denis Muilenburg publicly apologised on live television on behalf of the company, in his first sit-down interview since the crash of Ethiopian Airlines flight 302.

In separate reports, it has also been revealed that an Ethiopian Airlines pilot told senior managers at the carrier months before flight 302 crashed that “more training and better communication” was needed to avert a repeat of Lion Air flight 610, as reported by The Sydney Morning Herald.