The US Federal Aviation Administration has ordered airlines to inspect nearly all Boeing 737NGs flying in the US for structural cracks.

An FAA airworthiness directive (AD) published on 2 October responds to cracks found on hardware connecting the wing and fuselage of 737-800s.

The order comes as airlines and Boeing address a separate 737 issue – getting regulatory approval for the now-grounded 737 Max to return to service.

The FAA confirms the structural cracks found in 737-800s involve a piece of hardware known as a “pickle fork”, which connects the wing to the fuselage.

“Cracking… if not addressed, could result in failure of a principal structural element,” says the FAA’s 737NG order. “This condition could adversely affect the structural integrity of the airplane and result in loss of control.”

The order requires airlines to inspect 1,911 US-registered 737NGs, including -600, -700, -800 and -900 series aircraft. Of those, 165 aircraft must be inspected within seven days. The order does not apply to 737 Max.

The 1,911 aircraft represent nearly the entire US in-service 737NG fleet, which stands at about 1,930 aircraft, according to Cirium fleets data.

The order requires airlines, within seven days of 3 October, to inspect aircraft that have accumulated more than 30,000 flight cycles. Aircraft that have logged between 22,600 and 29,999 cycles must be inspected within 1,000 additional cycles, the order says.

Airlines must report their inspection results to the FAA.

“The inspection reports that are required by this AD will enable the manufacturer to obtain better insight into the nature, cause and extent of the cracking, and eventually to develop final action to address the unsafe condition,” says the FAA’s directive.

The inspections take about one hour per aircraft, though the findings will likely determine whether the issue will have a more disruptive impact on airlines, many of which are still coping with the grounding of the 737 Max.

Boeing did not respond immediately to a request for comment about the FAA’s order.

But last week the airframer issued a statement saying it had notified the FAA and operators about “a cracking issue discovered on a small number of airplanes undergoing modifications”.

“Safety and quality are our top priorities,” Boeing said. “Over the coming days, we will work closely with our customers to implement a recommended inspection plan for certain airplanes in the fleet.”

Delta Air Lines, which operates some 220 737NGs, says it is “working closely with Boeing and the FAA after they informed 737 operators about additional inspection work on certain 737NG aircraft models”.

“No structural fatigue has been observed on any of our 737 aircraft in the subject area in the fuselage section behind the wing specified by Boeing and the FAA,” Delta adds.

American Airlines, which operates some 300 737NGs, says none of its aircraft will need inspections within seven days. The carrier anticipates 80 aircraft will need inspections within eight months, but it anticipates no operational impact.

Other US 737NG operators Southwest Airlines and United Airlines did not respond immediately to a request for comment.

“Boeing notified the agency of the matter after it discovered structural cracks on an aircraft undergoing modifications in China. Subsequent inspections yielded similar cracks in a small number of additional planes,” the FAA says.

Cracks were found on the “left- and right-hand side outboard chords” of “frame fitting and failsafe straps” on multiple 737-800s, the order says.

Those aircraft were undergoing passenger-to-freighter conversions in China and had accumulated between 35,600 and 37,300 flight cycles, it says.

The FAA issued the order as an “immediately adopted rule” that takes effect 3 October.

It did not follow the typical process of first posting a proposed rule and then collecting comments before issuing a final rule. The agency can take such urgent action to address critical safety concerns.

The FAA will still accept comments about the rule for 45 days and could later revise the order, according to its website.