Israel Aerospace Industries (IAI) on Wednesday advised customers not to fly some Boeing 737 (BA.N) freighters converted by IAI pending a review by Israel’s aviation authority.

IAI said it had detected an “apparent irregularity” in the production process of a rigid barrier installed in some of the conversions, 47 of which it has delivered.

It said it expected the results of a review by the Civil Aviation Authority of Israel (CAAI) of an interim solution proposed by IAI later on Wednesday.

“IAI has recommended to the aircraft’s operators that they not operate the aircraft until the results of the CAAI’s review are known,” it said.

IAI, a market leader in converting passenger planes into freighters, declined to say how many companies operate these planes.

Australia’s Qantas Airways Ltd (QAN.AX) said it had taken four 737 freighters out of service pending further guidance from IAI.

IAI converted four Boeing 737-300 planes for Qantas in 2006 which the airline flies on domestic cargo routes.

The rigid barrier installed on some aircraft is there to provide emergency support under extraordinary circumstances but does not affect flight under normal conditions, state-owned IAI said.

“IAI’s tests indicate that the rigid barrier may not provide the support it was designed to provide in such extraordinary conditions,” it said in a statement to Reuters.

“The issue does not affect aircraft flight but may limit the way the aircraft can be safely loaded for flight.”

IAI said it has developed an interim solution which will be presented later on Wednesday to the CAAI.

IAI said that to the best of its knowledge there has never been a flight incident in connection with the rigid barrier.

IAI’s converted 737 aircraft entered service in 2003.