Both United and Alaska Airlines have discovered loose bolts on Boeing 737 Max 9 aircraft.
‘Additional tightening’ will be carried out after the airlines began inspections of the grounded planes.
United Airlines said ‘installation issues would be remedied’ before the jets return to service.
Alaska Airlines found ‘some loose hardware’ on grounded Boeing Max 9 planes, it said.
The airlines were ordered to inspect the aircraft after a fuselage panel blew out off an Alaska Airlines 737 Max 9 jet on Friday.
“Since we began preliminary inspections, we have found instances that appear to relate to installation issues in the door plug,” a United spokesperson said.
The fallen door plug landed in a resident’s backyard in Oregon.
NTSB chair Jennifer Homendy said the bolts may already have been missing but could have come loose after the door plug blew out.
Most US-based Boeing 737 Max 9s are operated by United Airlines and Alaska Air.
So far, hundreds of flights have been scrapped because of the grounding.
