If you spend long enough in the air, you'll become aware of things which might escape scrutiny by the more casual leisure flyer.

Though gone are the days when a sliver service or animals might roam the aisles of your passenger aircraft, but there are still some mysterious artefacts and hidden features that might surprise you to find.

Here are twelve aircraft features designed to be invisible to passengers, and which might change the way you fly.

The extra legroom button

This could be a game changer. The armrest next to the aisle may appear to be an immovable object, but if you search underneath and towards the back you will be rewarded with a button.

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Extra legroom button: This could be a game changer. Photo / Getty Images
Extra legroom button: This could be a game changer. Photo / Getty Images

Press away to gain precious extra millimetres of chair width.

William Shatner Seat: A triangle marks the best view of the wing. Photo / Getty Images
William Shatner Seat: A triangle marks the best view of the wing. Photo / Getty Images

The black triangle

The windows aren't just for wingtip holiday photos, they are part of the plane's safety features.

The easiest way to check up on a wing mid flight is to look out the window. The best view for assessing the airworthiness of the wing is marked by an innocuous looking black triangle.

"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 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."

Holey plane cabin, batman! There's a hole in our aircraft! Photo / Getty Images
Holey plane cabin, batman! There's a hole in our aircraft! Photo / Getty Images

The bleed hole

Holey plane cabin, batman! There's a hole in our aircraft!
This might not be as problematic as you think. I, the middle of the triple-glazed reinforced airplane windows, there is often a hole.
As the air cabins are pressurised, the plane needs some sort of safety valve for the enormous pressure on aircraft windows. The hole you might see is only there to take pressure of the inner pane of glass.

The secret bedroom: A retreat for cabin crew. Photo / Getty Images
The secret bedroom: A retreat for cabin crew. Photo / Getty Images

The secret bedroom

When they're not carting drinks trolleys or dancing along to safety announcements, have you ever wondered where the crew go to get away from the passengers?
Most long-haul aircraft are designed with crew compartments for attendants to catch 40-winks, take a break or just hide from the advances of their airborne charges.

Secret latch: Best not to think about while making a toilet trip at 35,000ft. Photo / Getty Images
Secret latch: Best not to think about while making a toilet trip at 35,000ft. Photo / Getty Images

The emergency latch to open the loo

Best not to think about while making a toilet trip at 35,000ft, but – in the event of an emergency – cabin crew can access occupied toilets by use of a small button.
The hidden latch – often behind the no smoking sign – allows crew to access the toilets in an emergency.

Hand rail A more considerate way to traverse the cabin. Photo / Getty Images
Hand rail A more considerate way to traverse the cabin. Photo / Getty Images

The hand rail

If you need to traverse the cabin without grabbing the backs of sleeping passengers headrests, there is a more considerate way. Along the skirting of the overhead bins there is often a handrail moulded into the fuselage. Take a grasp of the rail and proceed with confidence.

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Arrest: Crews have to be prepared for a whole range of unlikely scenarios. Photo / Getty Images
Arrest: Crews have to be prepared for a whole range of unlikely scenarios. Photo / Getty Images

Handcuffs and defibrillators . . . guns

Stories of unruly passengers are nothing new, but should they overstep the mark they could find themselves banged up in cuffs and being handed over to airport police.
We're not talking cops and robbers style handcuffs though. It's far more likely that cabin crews are issued with cable ties as restraining tools.

Since 9/11 sky marshals on flights to and from the US have been allowed to carry concealed weapons on planes. Sky marshals function as undercover armed escorts, ready to leap into action should the need arise. Though quite who is 'packing heat' is kept a secret from even the cabin crew. In August a United Airlines steward caught a glimpse of a marshal's gun which caused emergency landing – when the careless marshal was arrested after being mistaken for a rogue passenger with a gun.

Arrests from sky marshals aren't the only sort to worry about.

A defibrillator is also found on most aircraft, not for calming down unruly passengers but reviving those that suffer from the very real threat of cardiac arrest.

Yellow wing hooks

Hopefully you will never have to make use of these curious little features. However, in the event of landing on water these hooks on the wing are rudimentary handholds. As in captain Sully's 'Miracle on the Hudson' rope can be attached to the hooks for easier escape by passengers over the wing.

The mask: Should you need one you're unlikely to think about the complexities of onboard oxygen. Photo / Getty Images
The mask: Should you need one you're unlikely to think about the complexities of onboard oxygen. Photo / Getty Images

Chemistry sets

Swirling above your head is a cocktail of chemicals ready to explode into action. Should your cabin depressurise and those yellow face masks drop into your lap – you won't have much time to think about the details. However, it's an incredibly complicated process.
It wouldn't be space or weight efficient to provide every passenger with an aqualung. Instead there is a mixture of barium peroxide, sodium chlorate and potassium chlorate – powders and liquids which react violently together to rapidly produce oxygen.

Why are there still ashtrays on planes? Photo / Getty Images
Why are there still ashtrays on planes? Photo / Getty Images

Ashtrays

Since the ban of smoking on most airlines these features are somewhat redundant.
So why are they still there? Due to health and safety regulations the designs stipulate that there must be a safe place to stub out a cheeky (and illegal) fag midflight. Should someone ignore the rules (and they still do) it's better to have the trays in the bathrooms than have smoking embers disposed of in the bin.

Grab handles on the emergency exit

The large handle on the emergency door might be considered over-engineered, but this is a"calming measure."

Evacuating passengers tend to be a bit panicky. Providing a handle to hold onto slows down the escape to an orderly queue for the emergency slides. It's also something for attendants to hold on to, to avoid being barged out of the way.

Gas masks

Flight attendants are issued with gas masks. Instead of those restrictive yellow masks on strings which passengers get, gas masks free them up to move through the aircraft and address any problems.