Boeing is grounding up to 50 of its popular 737NG planes due to cracks.

That is the second major blow to the US aviation manufacturer that was forced to ground the best-selling aircraft in its history, namely the Max 737. The NG is a precursor to 737 MAX, which was grounded since the two deadly crashes in Indonesia and Ethiopia.

Among the worst affected companies is Australia’s Qantas, which grounded 32 aircraft, which reported cracks on nine of its planes in October. The Boeing 737NG is also part of Ryanair’s fleet.

A Boeing spokesperson on Thursday told AFP that less than 5% of 1,000 planes had cracks, which were grounded for repair. Australia’s Virgin Airways conducted checks on its fleet of 17 Boeing 737NG finding no cracks.

Initially, the US Federal Aviation Administration demanded the grounding for checks of all Boeing 737NG planes that had flown more than 30,000 times. The call found the reaction of the US regulator “completely irresponsible,” noting that a crack does not immediately compromise the safety of the aircraft.