Tom Earnshaw
Flight experts have issued a warning to those taking to the skies for their holidays over secret messages you might hear on your journey, and the issue with some of them is that you really don't want to hear them on yours.
Spring has sprung, and so has a growing desire for millions of people around the world to journey to somewhere new, whether it's checking out a cultural hotspot in the heart of Europe, or heading on an all-inclusive package holiday that's going to keep the kids entertained and you stocked up on that limitless bar.
More and more of us are taking to the skies, too, with air travel in the USA and Europe soaring; a record 5.2 billion passengers are expected to fly in this year alone, according to the latest estimates.
And while most flights go smoothly, cabin crew are trained for when things happen at 30,000 feet, using secret codes and language to discreetly communicate situations ranging from lighthearted quirks to serious, even life-or-death emergencies - often without passengers ever noticing.
And according to travel experts at Wander, two codes you really won't want to hear are 'Code 300' or 'Squawk 7500'.
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If you hear the words 'Code 300' - or even 'Angel', it usually means there has been a tragedy on board.
Sadly, it means someone has died and cabin crew are now working to respond to the situation, from where to put the body to providing support to loved ones who might have been travelling with the deceased.
This is a serious one. According to the experts, 'Squawk 7500' means there is a hijacking of your plane.
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"If you ever hear this code mentioned by the flight crew or air traffic control, it signals a potential security threat on the flight," Wander say.
"The aircraft’s transponder will send a signal with this code to alert authorities that the plane is in danger."
If it's not a hijacking, it could also mean there is a general unlawful experience taking place on the plane, and immediate assistance from air traffic control or police is needed.
'Code 300' and 'Squawk 7500' aren't the only two terms used by staff if needed.
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Alongside this, 'Code Yellow' means there is a minor medical problem on the plane, and staff will respond accordingly, such as providing a sick bag.
Then there's 'Pan Pan'. This flags a serious issue that falls short of life-threatening, from issues with the plane to a person's health.
'Mermaid' is also used by staff to nickname a passenger who deliberately sprawls out across empty seats to deter others from sitting in their row.
"This behaviour is particularly common on flights with spare seats, where a passenger may stretch out or scatter belongings to claim extra space," they say.
One code you could hear on the ground in an airport is 'Code Adam', which refers to a missing child. One that we hope we never hear!