Hong Kong airport began operating three runways simultaneously on Thursday in a significant capacity expansion, but flight growth looks set to be measured as the international hub is still struggling to get back to pre-pandemic passenger numbers.

Hong Kong International Airport is the world's busiest cargo airport, but has fallen from its pre-pandemic status as the third busiest for international seats.

Pandemic-related travel restrictions in Hong Kong and mainland China persisted longer than elsewhere, resulting in a slower aviation recovery. Hong Kong's anti-government protests in 2019-2020 and a new national security law have also had a lingering impact on sentiment, and many expatriates left after the three-year COVID shutdown.

In October the airport handled 27% fewer passengers than in October 2018, and 13% fewer flights, airport data shows.

New runways are long-term projects that take time to be utilised.

The number of flights set to depart Hong Kong in August next year will be 10% below August 2018, albeit 15% higher than August this year, according to Cirium airline schedule data.

Hong Kong leader John Lee said at a launch event he expects the third runway to boost economic development.

Beijing has earmarked Hong Kong, a special administrative region of China, to grow as an aviation hub as part of an economic plan for Southern China's Greater Bay area, a major manufacturing and export region.

The HK$141.5 billion ($18 billion) three-runway system, which includes upgraded cargo and passenger facilities, will allow the airport to handle up to 120 million passengers and 10 million metric tons of cargo a year by 2035.

This compares to a 2018 peak of 74 million passengers and 39 million in 2023, airport data shows.

The three-runway system will be able handle up to 102 flights per hour, compared to 69 per hour with two runways, airport authorities say.

Planes began landing on the new third runway in 2022 but only two runways operated while one was closed for upgrades.

Hong Kong's flight and passenger numbers have been increasing since mid-2022 but growth is slowing.

Airport Authority Hong Kong in July started offering financial incentives to attract carriers and routes - especially for specific long-haul destinations such as North America, Europe, Australia and New Zealand.

But some airlines have cooled on the city.

British Airways from October reduced twice-daily flights to one a day. Virgin Atlantic had intended to resume its flight from Heathrow in March 2023, but did not.

Hong Kong's main carrier group Cathay Pacific welcomed the start of three runway operations and said it would continue adding more flights and destinations.

Cathay exemplifies Hong Kong's struggles. It made heavy losses and layoffs during the pandemic and has struggled to find pilots and restore flight capacity.

Cathay on Thursday re-stated its aim to restore the number of pre-pandemic flights from January 2025.

($1 = 7.7819 Hong Kong dollars)

(Reporting by Lisa Barrington Editing by Mark Potter)