After determining that a fuel sealant was not applied during manufacturing, the FAA proposed inspections for 25 Boeing 737 MAX9 aircraft. The FAA says the issue applies to a total of 25 MAX9 aircraft. These 25 aircrafts belong to various airlines:

• United Airlines, 14 B737MAX9
• Copa Airlines, 6 B737MAX9
• Flydubai, 3 B737MAX9
• Icelandair, 1 B737MAX9
• Turkish Airlines, 1 B737MAX9

These 25 aircraft had been delivered to the respective airlines between April 2018 and February 2019. 

According to the FAA, the fuel sealant is intended to act as a "fuel barrier" on "blowout" doors. A missing sealant could result in an unintended drain path allowing fuel to come into contact with the engine in the event of a substantial fuel leak from the wing box. This could result to a large fire. The FAA intends to require a fluid seal contact inspection and a detailed inspection for the missing sealants on the blowout door and apply the sealant if deemed necessary.