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Most of us consider ourselves pretty au fait with the business of air travel. We could recite the safety briefing by heart, we understand turbulence, and we've even got to grips with Ryanair's overcomplicated "table of fees".
The following little-known features should surprise even the most frequent of fliers, however.
1. The black triangle
Windows aren't there just to give passengers a good view – they are also a safety feature. If the crew believe there is an issue with a wing - such as a problem with the flaps, the slats or even the engines - looking out of the window may be the best way to check. And to do that they will head to the seat with the best view, usually marked by a small black triangle on the wall.
"The black triangle marks the location of what has been called the 'William Shatner Seat', or the seat with the clearest view of the wing," explained retired aerospace engineer Lee Ballentine after the question was posed on the website Quora. "The Shatner reference is to one of the strangest Twilight Zone episodes, Nightmare at 20,000 Feet. In it, Shatner's character sees a gremlin on the wing of the plane he's a passenger on."
2. The bleed hole
Surely we shouldn't have holes in the windows of planes? On the contrary, those holes, which are only present in the middle pane of our triple-glazed planes, are actually there to keep us safe. Without them, passengers could be in trouble.
That's because air inside a plane is pressurised, which places an enormous strain on the windows. The inside pane – the one you may have woken up against from time to time – is largely superficial, meaning the middle pane and outer pane are taking the brunt of the pressure. Cue the "bleed hole", which relieves some of that pressure.
3. The secret bedroom
If you thought there wasn't enough space on a plane for a secret bedroom where cabin crew can catch 40 winks, you'd be wrong.
Most aircraft designed for long-haul services include a compartment where flight attendants are able to nap, read a book, or simply hide from slurring, passive aggressive customers who've had one too many G&Ts.
4. A button to give you more space
The armrest next to the aisle appears to be fixed in place. But should you require a few more millimetres of space for your hips, it can be raised. Simply seek out the small button underneath the armrest, towards the back.
The button, while allows the armrest to be raised normally, is particularly useful for passengers with disabilities, and makes it mildly easier for all fliers to get out of their seats.
5. The hidden latch that opens the loo door
Plane loos can actually be unlocked from the outside – which should give you something to think about the next time you relieve yourself at 35,000 feet. A hidden latch, sometimes behind the "no smoking" sign, allows flight attendants to access a locked lavatory in an emergency.
See also: 10 of the biggest myths about air travel
6. The nifty hand rail
Isn't it annoying when an air passenger uses your headrest to steady themselves while walking down the aisle, jolting you awake or spilling your merlot down the front of your shirt? They really needn't do so. That's because many planes feature a rudimentary hand rail. Often built into the overhead bins is a narrow groove – just the right size for gripping while you shimmy back to your seat.
7. Little yellow wing hooks
This curious feature found on many plane wings could help save you in the event of an emergency landing on water. If passengers evacuate the aircraft to stand on the wing, as was the case during the so-called Miracle on the Hudson, a rope may be secured between the door hatch and these hooks. It provides something for fleeing passengers to hold on to so they don't slip off the wing and into the water.
8. A cocktail of chemicals
Oxygen masks are found on all large aircraft, but what you're supplied isn't exactly oxygen – nor is it not compressed air in the scuba diving sense. Oxygen tanks are heavy and bulky so aircraft use a more complicated system. The panel above each seat actually contains a cocktail of chemicals that, when burned, release oxygen. They might include barium peroxide, a fine white powder used in fireworks, sodium chlorate, more commonly used as a weedkiller, and potassium chlorate, a staple of school science lab experiments (it reacts violently with sugar).
Are all these burning chemicals dangerous? Sort of. The crash of ValuJet Flight 592 in 1996, in which all 110 on board perished, was caused by an oxygen generator fire. However, the devices to blame were in the cargo hold, out of date, and missing their safety caps. A jolt lit one, and the rest followed. Understandably, masks are not deployed if there is an on-board fire as the production of oxygen may make matters worse.
9. Pointless ashtrays
They aren't exactly hidden, but ashtrays are a surprising feature of modern aircraft. After all, smoking at 35,000 feet has been banned for around 20 years.
So why are they still there? Because if someone were to have, illegally, a cheeky fag, they'd still need to stub it out, and it's best they have somewhere to do that rather than cause a fire by dropping it in the bin. Therefore an ashtray is a mandatory requirement in most countries.
10. Grab handles on the exit
Plane doors feature a seemingly pointless handle on the inside. What's it for? Well, in the event of an emergency evacuation, passengers tend to get a bit panicky. So the flight attendant overseeing their rapid departure, out of the door and down the emergency slide, is given something to hold on to, lest they be barged out of the way and down the slide before the evacuation is complete.
See also: What happens if you open a plane door?
11. Gas masks (but not for you)
After taking part in one of BA's cabin crew training days, Lizzie Porter discovered that flight attendants are given gas masks to wear in the event of a fire. Why don't passengers get them? Apparently they are reserved for those actually fighting the blaze.
12. Handcuffs and defibrillators
Unruly passengers are nothing new. So it shouldn't be a shock to learn that airlines routinely carry handcuffs for restraining the most unruly. Don't expect metal, police-issue cuffs – more likely are the plastic zip-tie types. They'll be tucked away somewhere in the cockpit.
Another piece of kit some airlines carry is a defibrillator – even Ryanair has them on board.
The Telegraph, London
See also: Why planes fly at 35000 feet
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