HONG KONG — Hong Kong Airlines was in the spotlight again after Japanese aviation authorities found its planes to be missing aircraft parts — ranging from screws to brake mechanisms.

None of the lost objects were critical for the safety of the planes, authorities said, and their absence raised no concerns about airworthiness. Still, the thought of missing aircraft parts — or worse, falling parts — raises questions. Here’s a look at how common this is, and what experts have to say.

WHAT HAPPENED?

Multiple inspections by Japanese authorities between May 26 and May 28 found items and parts missing from HKA aircraft. Parts were found to be missing from seven consecutive flights over four days with a total of 17 parts unaccounted for, according to a Japanese newspaper.

The initial discovery prompted runway inspections at Tokyo’s Narita International Airport, according to Japan’s public broadcaster. Narita officials feared the missing parts could present a risk to other aircraft.

Then, after the reports had emerged, a Hong Kong Airlines flight returning from Tokyo was delayed for six hours on May 29 after two screws under a wing panel were discovered to be missing.

Two days later, the Civil Aviation Department in Hong Kong announced that since May 26 it had recorded eight incidents of the carrier’s aircraft missing parts, with 19 items in total unaccounted for.

HAS THIS HAPPENED BEFORE?

Earlier this year, a Cathay Pacific Airways flight to Tokyo on March 17 was sent back to Hong Kong soon after take-off after one of the front-wheel door hatches detached from the plane. The missing component was not recovered.

In July 2015, a metal piece of the landing gear broke away from an Air France Boeing 777 and fell over the suburbs of Shanghai just after take-off. The metal piece fell 3,700 metres (12,139 feet) and hit a factory, but no one was injured. Boeing and Air France admitted at the time that they were aware the metal part had a “fragility issue”.

HOW COMMON ARE SUCH EVENTS?

There are no public records of missing aircraft parts in Hong Kong, so it is a challenge to assess the frequency. Mr Warren Chim Wing-nin, a spokesman for the Hong Kong Institution of Engineers, said a day could go by with no reports of a missing part.

“At Hong Kong airport, we have more than 1,000 [aircraft] movements a day. With that amount of flights, for the reported missing parts, one day might not have any reports,” Mr Chim said. “It is not that common, but of course it’s not that rare.”

CAN PLANES FLY WITHOUT MISSING PARTS?

Yes, it is perfectly safe. “Lots of bits, panels and screws can be missing from an aircraft, provided they are not part of the basic structure of the aircraft,” said Mr David Newbery, a pilot and former president of the Hong Kong Airline Pilots Association.

“Flying an aircraft under a CDL [configuration deviation list] is perfectly safe. However, it is undesirable because there may be additional maintenance items … additional fuel burn and possibly more noise in the cabin. It is, ultimately, in the best interests of everybody to repair CDLs as soon as possible.”

WHAT IF PARTS GO MISSING FROM A PLANE?

When parts go missing, the CDL manual is key, Mr Newbery noted, because it gives clear instructions in the event of a problem. “This document details what can be missing from an aircraft and what precautions or actions have to be taken before the aircraft is flown,” he said.

Mr Chim, the aviation engineer spokesman, said missing parts had a direct affect on airworthiness.

“So, for an aircraft to be not airworthy, and it doesn’t matter if it’s a small screw or a big component — anything missing from the aircraft, unless it’s a non-structural part listed in the CDL, it will not meet the type design for the aircraft. In any situation when the aircraft doesn’t meet the type design requirement, it will become unairworthy,” said Mr Chim.

WHAT’S HAPPENING NOW?

Japan’s Civil Aviation Bureau wants Hong Kong Airlines to analyse the cause of the problems and “develop countermeasures” to ensure that aircraft parts do not fall in its airspace.

Mr Albert Lam, who served as civil aviation chief in Hong Kong from 1998 to 2004, took a sterner view. “Any missing parts are definitely not acceptable, a crucial part or not a crucial part.”

On Thursday evening, Hong Kong Airlines management met with the city’s Civil Aviation Department, which has demanded that the airline launch a thorough investigation.

The CAD said that while the missing parts had not compromised safety, what had caused the parts to go missing needed to be understood. It also vowed to strengthen on-site supervision and inspection of the carrier’s aircraft. SOUTH CHINA MORNING POST