Boeing’s (BA) efforts to get its grounded 737–9 Max back in service hit a snag after the Federal Aviation Administration (FAA) said the company's instructions to airlines on how to inspect the planes were inadequate.

Earlier this week the FAA ordered the temporary grounding of some Boeing 737 Max 9 jets due to a door “plug” that flew off an Alaska Airlines plane midair on Friday. Upon inspection, other 737-9 Max jets in the fleets of Alaska Airlines and United Airlines were found to have “loose hardware.” Specifically, the inspection found that tightening was needed in some bolts connecting the plugs or panels that fit onto the jet’s fuselage when the openings aren’t used for emergency exits.

Boeing submitted plans for servicing the jets and getting them back in service, but the FAA just issued a statement claiming all planes should stay grounded until new guidance is issued and approved by the FAA.

“Every Boeing 737-9 Max with a plug door will remain grounded until the FAA finds each can safely return to operation. To begin the process, Boeing must provide instructions to operators for inspections and maintenance. Boeing offered an initial version of instructions yesterday which they are now revising because of feedback received in response. Upon receiving the revised version of instructions from Boeing the FAA will conduct a thorough review,” the FAA said in a statement on Tuesday afternoon.

“The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service,” the FAA said in a tweet accompanying its statement.

Alaska Airlines and United Airlines operate the only 737 Max 9 jets in the US. Yahoo Finance earlier reported the grounding order from the FAA will impact 171 planes.

The grounding order led to flight cancellations that are adding to the ongoing disruptions of flights across the country due to a winter storm across the Midwest and Eastern Seaboard. Per FlightAware data, more than 200 United departures were canceled as of Tuesday morning, accounting for 8% of United daily flights, as well as 108 Alaska Airlines flights, or 18% of Alaska’s flight load.

On Monday, National Transportation Safety Board (NTSB) Chair Jennifer Homendy said the agency was “not shy” about broadening its investigation into other Boeing Max aircraft, if necessary.

“We will go broader if we need to,” Homendy said.

The NTSB also confirmed on Monday that two “stop fittings” at the top of the door plug had fractured guide plugs on the Alaska plane’s fuselage. Four bolts that secure the door plug, Homendy said, were missing during the agency’s inspection of the aircraft. Homendy said the agency had not yet determined if the four missing bolts had ever been installed or if they were lost in the plug failure.

Twelve total fittings secured by bolts on the door plug are meant to keep the plane’s internal pressure from shifting the plug upward and disengaging it.

“The exam to date has shown that the door did in fact translate upward. All 12 stops became disengaged, allowing it to blow out of the fuselage,” NTSB aerospace engineer Clint Crookshanks said.

Last Friday's incident with Alaska Airlines’ Boeing 737 Max 9 isn't the first time the 737 Max has been under scrutiny. Two fatal crashes in 2018 and 2019 that together killed all 346 passengers and crew onboard led to a 20-month grounding of planes as safety changes were made.

Pras Subramanian is a reporter for Yahoo Finance. You can follow him on Twitter and on Instagram.

Alexis Keenan is a legal reporter for Yahoo Finance. Follow Alexis on Twitter @alexiskweed.

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