Around 40 minutes from landing on the service from Miami, US in October last year, the passenger, who had been flying in premium, was woken and told to put her lie-flat seat in an upright position. After doing so, she went to the toilet to freshen up and one of the attendants, who was putting away the flyer's bedding, noticed a smell of "sulphur" coming from the seat.
It was then she spotted smoke, according to the AAIB report.
"At this point they heard a 'hissing' sound and a large plume of grey smoke emitted from the seat in a 'tornado' motion. They remembered seeing an orange glow in the seat area amongst the smoke."
A senior member of the crew put out the fire with an extinguisher and the phone was found trapped in the seat. The fire service met the aircraft on landing into Heathrow, but there was no more damage to the aircraft.
The UK Civil Aviation Authority has reported 166 previous events of Portable Electronic Devices (PEDs) becoming trapped in passenger seats in the last five years. Of these, 42 resulted in a fire or smoke in the cabin.
Despite it being a known problem, there is no requirement to design seats to prevent damage to PEDs that become accidentally trapped.
In its final recommendation, the AAIB said that passenger seats in commercial air transport aircraft should be designed to minimise the chance of PEDs becoming crushed in mechanisms.