Hong Kong aviation authorities are investigating two serious incidents in which two Cathay Pacific pilots, unable to continue to perform their duties because of impaired vision, had to hand over control to their first officers.

The Air Accident Investigation Authority released two reports on Tuesday, saying that due to the physical discomfort of two pilots during separate flights, the aircraft had to resort to single-pilot operation with first officers taking over the controls for the remainder of the flights, according to a government release.

In January the captain of a flight from Sapporo, Japan to Hong Kong experienced vision problems while the aircraft was near Taiwan.

According to a preliminary report, the captain advised his first officer about his loss of visual acuity and informed him that he was relinquishing control. The first officer took over and requested priority landing at Hong Kong Airport. The plane landed safely.

The second incident happened in February when the captain of a flight from Perth, Australia to Hong Kong felt a shortness of breath and impaired vision while the aircraft was over the Philippines.

He then declared that he was incapacitated, and the first officer assumed command of the aircraft.

The crew sought medical assistance from a medical professional on board who was a passenger and advice from the company doctor in Hong Kong via radio. The captain’s condition stabilized after he was administered medical oxygen.

A spokesperson for the aviation authority said they will continue to collect and study all relevant information in order to determine the circumstances and causes of these serious incidents, and expect to finish the probe within a year, Sing Tao Daily reported.