Cathay Pacific flight CX170 had taken off from Perth International Airport on February 21 and was flying over the South China Sea when the incident occurred.

A preliminary report has found the Airbus A350, with 270 passengers and 13 crew members aboard, was about 500km west of Manila when the captain announced to his crew he was unwell. He told his co-pilot he was short of breath and his vision impaired.

At an altitude of 38,000 feet, and with an hour and 15 minutes to go before arrival at Hong Kong, the captain declared himself incapacitated.

The report says the co-pilot - a first officer - assumed command of the aircraft while flight attendants asked passengers for medical assistance.

The co-pilot called Hong Kong air traffic control and declared a pan-pan, an emergency call one level below a mayday. He also descended to a lower altitude to assist the captain's breathing.

One passenger, described as a medical professional, assisted the captain while a company doctor in Hong Kong radioed advice via the aircraft's Satcom communications system.

The captain's condition stabilised when medical oxygen was administered and he remained conscious throughout the ordeal.

Air traffic control gave the plane a priority shortened approach to Hong Kong International Airport where it landed safely. Paramedics were on hand at the airport to treat the captain.

Cathay Pacific was involved in a similar incident in January when the captain of a Boeing 777 en route from Hokkaido to Hong Kong also suffered a loss of vision and became incapacitated.

- SMH/The Age