When passengers were given a pre-flight safety briefing onboard a Qantas A380 flight from Sydney to New York last week, they were “told to ask the crew for help if they lost their phone… and not, repeat not, to try to find it themselves.”

The strange announcement, which was witnessed by a reporter from online technology website The Register, was issued in response to the fire risk that lithium-ion batteries – which are used to power mobile phones – pose if they are damaged by the moving mechanisms in reclining seats. Since then, similar announcements have been witnessed on British Airways and Cathay Pacific.

But why the sudden warning? In May, on a flight from Sydney to Dallas-Fort Worth, Qantas cabin crew were alerted to the “presence of smoke in the cabin”. The Australian Transit Safety Bureau (ATSB), which has recently released its investigation into the incident, notes that the source of the smoke was traced to seat 19F, in business class – to “a crushed personal electronic device [a phone] wedged tightly in the seat mechanism”.

The remains of the phone which started smoking after being "crushed" in a seat mechanism Credit: Qantas/ATSB

The phone was no longer emitting smoke when it was retrieved, but “a strong acrid smell remained in the cabin”.

The ATSB, which investigates all Australian aviation safety incidents, said that the crew “placed the [device] in a jug of water,” before putting it in a metal box for the rest of the flight. This tactic was deemed “an excellent example of an effective response to an emergency situation” by the ATSB, and the flight continued as scheduled – landing in Dallas-Fort Worth two hours later.

The crew “placed the device in a jug of water,” before putting it in a metal box for the rest of the flight

When pressure is applied to a lithium-ion battery, it is susceptible to short-circuiting. This causes the battery to overheat – and, in some circumstances, start smoking or combust. Aircraft seats must be fire-retardant by law, but that doesn’t stop the heat or smoke.

“We’re asking people to keep track of their phone in their seat, and if they do lose it down the side, to let a crew member know and to not move their seat,” a Qantas spokesperson told the Telegraph. “We find this tends to be more of an issue on longer flights and on our Business Class skybeds, where people might have their phone next to them as they relax and it slips down the side of the chair.

“Our crew are trained to deal with this scenario and they’ve done a great job on the odd occasion where we’ve had a phone break and start to smoulder. But obviously, we’d much prefer if we could avoid this happening altogether.”

Dozens of aircraft fires have been linked to lithium-ion batteries: In 2014, a battery short-circuited in a passenger's bag on-board a Fiji Airways flight, according to an ATSB incident report.

Samsung mobile phones, Tesla electric cars and hoverboards have also been recalled because of fire risks.

So if you drop your phone on a plane, don’t go shifting your seat to look for it. Not unless you want to risk causing a fire – and have your phone handed back to you as a burnt-out, soggy souvenir of your flight.