Sitting in storage and awaiting delivery, the Boeing 737 MAX has been grounded for almost a year now; however the company has identified another problem with the aircraft that may affect its return to service.

During scheduled maintenance, various foreign objects were located in the wing fuel tanks of some 737 MAX aircraft. Included in the list of items discovered are rags, tools, metal shavings and other production equipment, which pose a serious safety risk in flight and on the ground in operational service.

The discovery has prompted Boeing to launch inspections of some 400 737 MAX aircraft that have been built, awaiting delivery or are already delivered.

Foreign Objects Discovered in Boeing 737 MAX Fuel Tanks
Stored Boeing 737 MAX aircraft

Leeham News and Analysis, who was first to report on this case, says it takes up to three days to inspect each aircraft, as fuel must be drained and vapours must be vented from the tanks.

From an internal memo, that a Boeing spokesperson confirmed to be authentic, Mark Jenks, who is the general manager of the 737 program, said the debris was “absolutely unacceptable” and the company was taking steps to address the issue in its production system.

Boeing adds that the discovery of the foreign object debris is not expected to delay the 737 MAXs return to service.

This isn’t the first time Boeing has been caught up in a foreign object debris debacle. The company in recent times has been under fire about the production quality of its 787 Dreamliners, which has included the identification of poorly fitted components or left over manufacturing equipment.

Additionally the United States Air Force twice halted deliveries of the 767-based KC-46 tanker, after finding problems with some of the systems on board and locating production equipment inside the airframe.

Foreign Objects Discovered in Boeing 737 MAX Fuel Tanks

Whilst the quality control issues aren’t part of the reason why the Boeing 737 MAX is grounded, it comes as part of a thorough inspection of the aircraft design and manufacturing process; after it was involved in two fatal crashes.

The timeline for the 737 MAX certification remains in limbo, however Boeing says they plan to conduct the demonstration flight “soon”. It was reported last week that the flight might not take place until April or later. Boeing has declined to answer questions about the certification timeline.