A pilot who was forced to ditch his plane in the sea at last year’s Herne Bay Air Show became trapped because he was wearing a “totally unsuitable” life jacket, an investigation has uncovered.

A piece of balloon lodged in the pilot’s engine caused the aircraft’s loss of power, the probe by the Air Accidents Investigation Branch (AAIB) has revealed.

To prevent future accidents, guidance has now been issued on the correct life jackets for flying. A modified part to stop debris entering an aircraft’s engine has also been approved.

The incident, witnessed by thousands of spectators, occurred at about 2.30pm on Saturday, August 14 2016.

The single-seater plane hit the water approximately 18 strides from the shore and overturned, with the pilot becoming trapped in the cockpit by his lifejacket.

Onlookers rushed to the pilot’s aid and managed to right the aircraft and help him out.

The 71-year-old, who gained a private pilot’s licence in 1988 and was a regular member of the display team, escaped with only a small cut to his face and had remained conscious throughout the ordeal.

The aircraft was recovered from the sea intact with just some damage to the wing.

Further examinations found a piece of balloon, approximately 50mm in diameter, lodged in the air path of the carburettor - part of the engine.

The balloon had come from the balloon-bursting section of the display routine, whereby the aircraft aim to pop the targets with their propellers.

The balloon fragment caused a significant restriction to the airflow through the carburettor, causing the loss of engine power experienced by the pilot.

A document published by the Air Accidents Investigation Branch (AAIB) stated: “An approved modification has since been developed to fit a screen to the intake of the carburettor, to prevent ingress of debris similar to the balloon fragment.”

Investigators also discovered that the pilot was wearing a 150 Newton life jacket of a design intended for use in boats.

He had selected an automatic design as he perceived this to be a desirable feature and wore it whenever flying over water.

The CAA has published General Aviation Safety Sense leaflet which provides the following advice: “Many automatically inflated life jackets, used by the sailing community, are activated when a soluble tablet becomes wet.

“This type is totally unsuitable for general aviation use as it will inflate inside a water-filled cabin, thus seriously hindering escape.”

Realising something was wrong, the pilot planned to stall the aircraft on to the water and had chosen the area of the beach where there were fewer people.

The section of the beach was cleared for a coastguard helicopter to land and the air ambulance was also scrambled.

Eyewitness Nigel Hancock photographed the moment the plane crashed into the sea.

He said: “The display team was coming towards the end of its routine, and I saw a plane coming down low and slowly, beyond the parameters of its normal flight routines.

“I thought he was in trouble, so I took a series of shots of what happened. It looked to be a textbook forced landing.

“I saw about a dozen people going into the water which was very heroic, bearing in mind the pilot was underwater and struggling to get out.

“If they hadn’t flipped the plane over quickly it could have been a lot worse. Due to their quick actions the pilot was relatively unharmed.”