Slim down or sit down at the back of the plane: that is the message from Thai Airways International. The Thai airline has banned passengers with waists bigger than 56 inches from the business class cabin of its Boeing 787-9 “Dreamliner” fleet.

The airline, known as THAI, has fitted the safety belts in its premium cabin with airbags. The idea is to meet strict requirements on passenger survivability: basically, preventing the traveller’s head impacting the back of the seat in front in the event of sudden deceleration.

The airbag manufacturer, AmSafe of Phoenix, Arizona, says the device provides “compliance for difficult to certify seat placements”. The firm describes the airbag as: “A self-contained, modular restraint system specifically designed to improve occupant protection from serious head-impact injury during a survivable aircraft crash, and enhance the occupant’s ability to egress the aircraft.”

But passengers with a waist size above 56 inches cannot fit within the maximum travel of the belt.

Normally larger passengers are issued with a safety belt extender. But the airbags must be centred over the traveller’s waist to be safe and effective. The maker says: “The airbag deploys up and away from the seated passenger … providing protection to the head, neck and torso.” An extender would reduce the protection.

The same restriction excludes a parent travelling with an infant on their lap in business class, branded by THAI as Royal Silk class.

The Thai airline markets the premium product thus: “THAI invites you on a journey as smooth as silk in our elegant business class cabins that are inspired by the rich culture and heritage of Thailand.

“Whatever your needs, we assure you that you will be perfectly equipped to either ease off and relax or catch up on the world of business.”

Other airlines encourage larger passengers to travel in business class rather than economy, in order to be more comfortable.

THAI also warns passengers of the perils of lying on the floor on aircraft. The airline tells travellers: “Despite a clean and tidy floor, tiny dust particles in the carpet may cause allergic symptoms, and even affect the respiratory system.

“Moreover, the onboard oxygen may not be circulated fully and sufficiently down on the floor, causing difficult to breathe, and, upon getting up, the passenger may feel lightheaded and dizzy.