SINGAPORE - Passengers on a Scoot flight from Singapore to Hat Yai, Thailand, first realised something was amiss when the plane suddenly descended mid-flight on Thursday afternoon (April 5).

Things got even more puzzling when they saw through the windows two Republic of Singapore Air Force (RSAF) F-15SG jets flying alongside the plane, which had left Singapore at 1.20pm.

However, instead of panic, the "atmosphere was calm", a passenger onboard flight TR634, who did not want to be named, told The Straits Times.

This was despite the passengers not being told what was happening.

The Scoot flight was carrying 173 passengers and six crew members.

The passenger added that "most people were more amused than alarmed" by the sight of the fighter jets, not knowing that the jets were part of security measures that kicked in because of an alleged bomb threat made by a fellow passenger.

"The cabin crew did not want to give answers when asked what was happening," he said.

He initially thought there might have been a radio failure which could have caused the pilots to lose contact with the air control tower, or that the pilots unknowingly deviated from the flight path.

A 41-year-old man who triggered the bomb scare, and believed to be Singaporean, was detained alongside his two travelling companions after the Scoot plane returned to Changi Airport and landed without incident at 3.23pm.

The man was arrested under Regulation 8(1) of the United Nations (Anti-Terrorism Measures) Regulations, which states that it is an offence for a person to make false claims that a terrorist act has been, is being or will be carried out.

Those found guilty can be punished with a fine not exceeding $500,000 or with imprisonment for a term not exceeding 10 years, or both.

According to the police, the suspect told a flight crew member that he had a bomb in his carry-on baggage. The pilot was then quietly alerted and he decided to turn the plane back to Singapore.

A thorough security search was carried out on board the plane and the baggage of the suspect and his two travelling companions were examined. But no suspicious articles were found.

"The cabin crew wanted to move his bag but he said no because (there was a) bomb in the bag," said the passenger who declined to be named.

The passenger, who was seated in the middle part of the plane, said he found out about this from other people who were seated in the front with the 41-year-old man.

No announcement was also made about the plane returning to Singapore. The passenger said that when the plane was about to land, the pilots said that "we are landing shortly at our destination", without saying what the destination was.

But the passenger said he knew they were back in Singapore because, among other things, he recognised Batam through the windows as the plane was descending, and that Singapore fighter jets can usually only fly within the country's airspace.

Many other passengers did not realise this until the plane had touched down, he said.

"We only found out what was happening after we were on the ground. Even until then, the pilots said nothing. We were on the ground (in the plane) for some time. Many passengers were unhappy because they wanted to walk around to stretch their legs and go to the toilet. But the crew didn't allow them. Everyone had to be seated," he said.

The passenger added that he thought the bomb hoax was merely "meant as a joke".

"Even if the crew wanted to keep the reason (for turning back) a secret, the passengers should have still been informed of the plans ahead or the intentions," he said. "Our lives are also at stake. We have the right to know. They can always say they will let us know the reason later."

The passenger said that even if the flight's crew did not want to inform other people onboard for fear of alerting the aggressor, he was certain the aggressor would have been able to see the F-15 jets beside the plane and realise something was wrong.

"No information was shared with the passengers and I think that's not very right. We could all face this together," he said.

Passengers on flight TR634 were finally able to depart Singapore for Hat Yai again at about 6.30pm on Thursday, after the aircraft was declared safe following investigations and associated procedures were concluded.

"We were delayed for 5½ hours in total. And, yes, my travel plans for the day had to be changed," the passenger who declined to be named said, adding that he was on holiday and had to skip a place he planned to visit.

"Others chose not to continue to Hat Yai."

When contacted by The Straits Times, Scoot declined to share details of the incident, citing confidentiality of its security protocols and that investigations were still underway.

The airline added that it would offer all affected passengers on the flight a $50 flying voucher, due to the delay they experienced.