Around 30 passengers on a Jet Airways flight suffered nasal and ear bleeding on Thursday, allegedly because cockpit crew “forgot” to turn on a switch that helps maintain cabin pressure.

The Jaipur-bound Flight 9W 697 was forced to return to Mumbai shortly after the take-off. Several passengers were given first aid at Mumbai airport and five were taken to Nanavati Hospital, where they were treated and discharged.

An official with the directorate-general of civil aviation said: “During the climb, the crew forgot to select a switch because of which cabin pressure could not be maintained.”

The Aircraft Accident Investigation Bureau has started a probe. A Jet Airways statement said the flight had turned back “due to loss in cabin pressure” and the “cockpit crew has been taken off scheduled duties pending investigation”.

The statement did not specify how many passengers fell ill but news agency PTI quoted a Jet “spokesperson” as acknowledging “30 passengers are affected... some have nose bleeding, few have ear bleeding and some are complaining of headache”.

Doctors at Nanavati said the five brought in were suffering from “barotrauma” of the ear, an injury caused by a change in air pressure that can cause bleeding from the ear as well as the nose.

Dr Amol Patil, the ENT specialist who examined them, said they had “mild conductive deafness”, a temporary condition likely to heal in a week to 10 days, and had been advised not to fly till then.

Aviation experts said pilots are supposed to apply the correct cabin air pressure settings before take-off as part of the pre-flight checks. The pilots turn off the cabin-pressure-maintaining switch during take-off for full power and flick it on after cruising to a certain height.

“While it is hard to say what went wrong, it does look like a case of pilot error,” said Arunava Sengupta, aviation sector analyst. Sources said the flight had turned back from an altitude of 11,000 feet.

Experts said a loss of cabin pressure is a serious emergency. It reduces the flow of oxygen and, in severe cases, can cause hypoxia.

The Boeing 737 had taken off with 166 passengers and a crew of five. PTI quoted a passenger, Ankur Kala, as saying the crew were like “trainees” who did not know how to handle the situation.

“As soon as we took off, the air conditioners (stopped) working. It was suffocating,” said Kala, a jewellery firm employee. A few minutes later, oxygen masks got deployed but there were no instructions, he said.

“The crew members panicked more than the passengers,” he said. “Suddenly, it smelt like a wire was burning. The pressure was so high that we were covering our ears and holding the oxygen mask with our hands but it was still intolerable,” Kala said.

The pressure was actually low, but the passengers may have experienced it as “high pressure” because the Eustachian tube connecting their middle ear to the atmosphere would have got blocked, trapping air bubbles inside.

Kala said that during landing, there was “extreme pressure” in the flight and he kept covering his ears and holding the mask, and fell unconscious for 10 to 15 seconds.

“When I removed my mask it was full of blood. I told my co-passengers, ‘I am bleeding’ and waved to the airhostess, but she told me to remain seated and said all was normal,” Kala said.

Jet Airways said the airline “regrets the inconvenience caused to its guests” and was “making alternative flight arrangements” for them.

Apart from the five taken to hospital, 17 other passengers refused to take the alternative flight arranged by the airline.

Civil aviation minister Suresh Prabhu has asked the aviation regulator to prepare a comprehensive safety audit plan for all scheduled airlines, aerodromes and flying training schools within 30 days.