UA finds loose bolts on Boeing 737 Max 9s

CHICAGO, Jan. 8 (Xinhua) -- Chicago-based United Airlines (UA) has found loose door plug bolts on at least five Boeing 737 Max 9 planes as it begins inspecting planes following the Federal Aviation Administration's (FAA) temporary grounding order.

The loose bolts were found during initial inspections of what is known as the "mid-cabin door plug," located partway down the fuselage of Boeing 737 Max 9, local media reported Monday.

It appears to be a manufacturing quality issue as opposed to a design issue. "The issue could be an assembly error by Boeing, an install error by supplier Spirit AeroSystems, or an issue further down the supply chain," local media quoted Bank of America analyst Ronald Epstein as saying on Monday.

The mid-cabin door plugs on a Boeing 737 Max 9 operated by Alaska Airlines became dislodged from the aircraft following an abrupt depressurization shortly after departure on Friday. As a result, a piece of the fuselage was expelled at an altitude of 16,000 feet.

The plane, bound for Ontario, California, executed an emergency landing in Portland just 20 minutes after taking off.

The FAA on Saturday temporarily grounded 171 737 Max 9s on an international level with door plugs pending inspections.