Chicago, Jan 9 (IANS): Chicago-based United Airlines said that it found loose hardware, including bolts in need of "additional tightening", on a number of their Boeing 737 Max 9 fleet during inspections.

The Federal Aviation Administration (FAA)-mandated inspections began on January 5 after an Alaska Airlines Boeing 737 Max 9 had to make an emergency landing the same day in Portland, Oregon, with a window and a part of the fuselage missing.

As a result, certain Boeing 737 Max 9 aircraft still remain grounded across the US.

In a statement on Monday, United Airlines confirmed it found the loose bolts on an undisclosed number of its 737 Max 9 aircraft as the company is still conducting the inspections.

“Since we began preliminary inspections on Saturday (January 5 ), we have found instances that appear to relate to installation issues in the door plug –- for example, bolts that needed additional tightening,” it said.

The door plug is a piece of fuselage with a window that can be used as an emergency exit in certain configurations, reports the BBC.

It was this part of the Alaska Airlines plane which fell off mid-flight and left a gaping hole on the side of the plane as it flew at 16,000 feet shortly after taking off from Portland, carrying 177 people.

The refrigerator-sized Boeing 737 Max 9 fuselage door plug eventually landed at a teacher's backyard.

The vast majority of Boeing 737 Max 9s used in the US are operated by United Airlines and Alaska, while Turkish Airlines, Panama's Copa Airlines and Aeromexico have also grounded aircraft of the same model for inspections.

Also on Monday, Alaska Airlines issued a statement saying that technicians who were preparing to conduct their inspections had seen “some loose hardware was visible on some aircraft”.

The formal inspection process of Alaska Airlines’ fleet hasn’t started yet as they wait for final documentation from Boeing and the FAA, CNN quoted the statement as saying.

As of Monday, United Airlines, which has more Max 9s than any other US carrier, has canceled more than 470 flights due to the ongoing inspections.

It expected significant cancellations on Tuesday as well.

The Boeing 737 Max has been described as "the most scrutinised transport aircraft in history" after a series of safety issues.

In late 2018 and early 2019, two of its aircraft were lost in near identical incidents, off the coast of Indonesia and outside the Ethiopian capital Addis Ababa, reports the BBC.

A total of 346 people were killed in both the crashes which were caused by flawed flight control software, which ultimately forced the planes into catastrophic dives, despite the best efforts of the pilots.