A Korean Air flight to Taiwan was forced to return to Incheon airport west of Seoul after a sudden depressurization on the plane, a Boeing 737 Max 8, the transport ministry said Tuesday. Separately, Malaysia Airlines said one of its flights en route to Bangkok on Monday made a U-turn back to Kuala Lumpur after the Airbus A-330 experienced a “pressurization issue.”

South Korea’s transport ministry said 19 of the 133 people aboard the flight Saturday were sent to hospitals due to ear pain and nosebleeds, but none suffered serious injuries.

The airline and the ministry said the cause of the problem was under investigation. The aircraft was grounded and the ministry ordered South Korea’s 11 airlines to examine pressurization systems in all their 400 aircraft.

The sudden depressurization occurred about 50 minutes after the flight’s departure.

Malaysia Airlines said its pilots initiated an emergency descent even though the aircraft had not reached the altitude of 8,000 feet and oxygen masks were not deployed on that flight. Flight MH780 was carrying 164 passengers and 12 crew members.

An investigation is underway.