US regulators are seeking to identify flawed slat-track assemblies from Boeing 737 Max jets in order to avoid checks on older 737 variants.

The FAA states that certain track assemblies were manufactured incorrectly and are affected by hydrogen embrittlement, which reduces their strength and could ultimately result in failure and the loss of a slat.

Boeing has indicated that 148 such assemblies were incorrectly produced but the FAA points out that, because the parts are rotable and not serialised, they could be removed from 737 Max 8 and Max 9 jets and reinstalled on previous 737 variants such as the -800 and -900.

"The current shortage of non-discrepant parts makes this removal and reinstallation more likely to happen," says the FAA in a directive.

"If a discrepant part were to be removed from a [737 Max] and installed on [an older 737] before all of the discrepant parts have been identified, it could expose the entire [older 737] to the unsafe condition."

It states that this spread of affected parts would probably then require a fleet-wide 737 inspection to find them, adding: "The FAA therefore considers the prompt identification and removal of these parts from [737 Max jets] to be an urgent safety issue."

Operators are being instructed to conduct maintenance records checks to establish whether any main slat-track assembly has been removed, and inspection of track assemblies for either a suspect lot number or a lot number that cannot be determined. Some 33 US-registered aircraft are covered by the directive.