A British Airways jet on its way to Chicago plunged about 28,000 feet before landing safely back at Heathrow on Monday.

Flight BA295, a 30-year-old Boeing 777-200ER, climbed to 37,000 feet after takeoff and then dropped to around 9,000 feet in just seven minutes over Ireland. Oxygen masks were deployed as the plane descended rapidly.

The pilot issued a 7700 emergency squawk and turned the aircraft around. It then spent about four hours circling over the Bristol Channel, burning fuel before landing safely at Heathrow at about 5 pm.

No injuries were reported. British Airways said the plane, registration G-VIIA, was met by emergency services “as a precaution” and cited a technical issue. The flight had been due to reach Chicago at 11:05 am local time.

Tracking data showed the steep descent clearly as the plane turned back over Ireland. Emergency services were on standby at Heathrow when it landed. Passengers were rebooked on later flights to Chicago. The aircraft will undergo a full technical inspection before returning to service, and investigators will review the flight data.