US AIRLINE SOUTHWEST has cancelled about 100 flights from its schedule today after investigators examining the state of the airline’s fleet found three planes with ‘sub-surface cracks’.

The investigations followed a major safety scare during a flight on Friday, where a hole measuring about one foot in width and five feet in length appeared in the roof of a jet mid-flight.

That plane, a Boeing 737, was 18 minutes into a flight from Phoenix to Sacramento when the hole opened, with the result that the air pressure in the cabin dropped significantly causing the plane to have to return to ground, CNN reported.

A flight attendant was slightly hurt as a result of the incident, which had began with a loud bang, passengers said. There were 118 passengers on board the affected flight.

The scare triggered an automatic review of the rest of the airline’s fleet – with the BBC now reporting that three other planes have been shown to have similar cracks to the ones that are assumed to have been present on the first problematic plane.

600 flights over the weekend were cancelled until the soundness of each respective plane had been guaranteed, with Southwest hoping to have the review of its fleet completed by tomorrow evening.

Boeing has said it will issue a ‘service bulletin’ to describe its safety checks on the 737-300 plane, which is a popular model with commercial airlines in both the US and Europe.

Shares in Southwest fell by around 2.7 per cent in early trading in New York.