Flying high - 9 facts for frequent flyers (2024)

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Why does airline food taste so terrible? Are planes becoming fuel efficient? Take your mind sky high and give yourself something to think about next time you take off ...

1. At cruising altitude you are 10% of the way to being in space.

When you're cruising in a long haul flight at an altitude around 10,000 metres you have only 90km to go before you could officially be called an astronaut.

There's no strict definition of the beginning of space because the atmosphere doesn't abruptly end, but according to the Fédération Aéronautique Internationale (FAI) to call yourself an astronaut you need to fly above what's called the 'Karman' line, which is 100 kilometres above sea level.

At that altitude you'd be above 99.99 per cent of the Earth's atmosphere. In such thin air, to generate enough aerodynamic lift for the wings to work, an aircraft would have to fly faster than the velocity needed to put it into orbit. It would no longer be an "aircraft", but a "space craft".

The International Space station typically orbits much higher, at an altitude of 370Kms

2. Airliners are pressurised because your life depends upon it

The human body is not good at extracting and absorbing oxygen in low pressure conditions.

Sea-level dwellers will lose consciousness within 2 to 3 minutes of being exposed to altitudes above 8,500 m (27,900ft) - unless they are breathing oxygen-enriched air (or have spent months slowly acclimatising to high altitudes.)

For this reason, high flying commercial aircraft maintain cabin air pressure to the equivalent of what you would encounter at around 2430 metres (8000 feet). They constantly draw in and pressurise fresh air from outside the aircraft. The outside air is very cold and dry, so it is heated and partly mixed with filtered air from inside the cabin. The incoming dry air is the reason the relative humidity inside an aircraft is so low - usually less than 20%.

Enough outside air is drawn in to completely replace all of the cabin air every few minutes.

Recent aircraft such as the Boeing Dreamliner have been designed to operate with the cabin pressure set to a lower altitude of 1800 metres (6,000 feet). This requires a stronger airframe, but the benefit is greater passenger comfort.

You can see the effect of cabin pressure by sealing an empty plastic drink bottle when at cruising altitude and watching it partially crumple as the aircraft descends to land.

3. There are good reasons why airline food tastes awful

Your taste buds and your sense of smell are actually less sensitive when you're flying.

This is because the dry and low pressure cabin air dulls four of our tastes - saltiness, sourness, bitterness and sweetness - but doesn't affect the fifth taste - umami - found in tomatoes, fish sauces and glutimates.

To combat the effects of the dry cabin air, hot in-flight dishes are sealed in foil and served with sauces to keep them moist.

But solving the taste bud problem is more complicated as it's to do with human physiology at altitude, and the simple approach of merely adding more salt or sugar to the food would make it less healthy for the passengers.

And if you've ever wondered why the tea and coffee on airliners never seems very hot - it's also to do with low air pressure at altitude. At typical cruising cabin pressure (2430 metres/ 8,000 feet) the boiling point of water is only 90 degrees centigrade - 10 degrees colder than at sea level.

4. Fuel can account for up to 45% of an aircraft's take-off weight.

Jet fuel is mostly kerosene with some additives such as anti-oxidants and anti-freeze. It can be produced synthetically and some airlines are already exploring the use of jet fuel blends containing biofuel, partly to reduce their carbon footprint and partly to reduce their dependence on fossil fuel.

By 2020, the airline industry is aiming to get to the point where all future growth will be carbon neutral. A huge amount of research is being undertaken to produce biofuels.

The other kind of aviation fuel is Avgas, which is mostly used by light aircraft with spark ignited internal combustion engines.

5. Modern jet engines don't work like they used to

Early jet engines worked on the principle of sucking air into a turbine, where combustion with fuel resulted in a very high speed 'jet' of exhaust thrust.

However, modern aircraft are powered by 'high-bypass' turbofan engines. These still have a turbine and jet engine at their core but most of the power is used to spin the big fan at the front. This big fan acts like a propeller, accelerating air backwards and producing up to 70% of the thrust.

The turbine at the core of the engine still produces a high speed 'jet' of very hot exhaust gas, but this jet is surrounded by a plume of slower, cooler air pushed back by the fan. The effect makes turbo-fans far less noisy.

Most propeller-driven airliners are also powered by a gas turbine - so they are powered by 'jet' engines too. At lower speeds and altitudes, propeller aircraft are actually more fuel efficient than turbofans.

6. There's a small turbine hidden at the rear of some planes

Larger airliners also have a small gas turbine usually located in the tail cone of the aircraft.

It's called the Auxiliary Power Unit or APU and it's not used to push the aircraft through the air. Instead it provides electrical power and runs the air-conditioning system while landed.

The APU can also be used to start the main engines without assistance from the ground crew, which is very useful at smaller airports. If you look closely at aircraft which are docked at an air terminal, you can sometimes see the telltale exhaust plume from the APU.

The APU is not normally used during flight, but can be switched on in emergencies to provide power to the aircraft's control systems.

7. Airliners are getting greener

Measuring airliner fuel economy is complicated because there are many factors to consider such as the presence or absence of headwinds, the weight of cargo, how much fuel needs to be carried and traffic congestion.

However, both Boeing and Airbus claim their latest models consume around 3 Litres/100 km per passenger, which matches the most fuel efficient cars. Boeing claims that there has been a 70% reduction in fuel consumption and CO2 emissions since commercial jets were introduced in the 1950's.

While the most energy intensive part of any flight is the take-off and climb to cruising altitude, it doesn't necessarily follow that longer flights are more fuel efficient than shorter ones, nor that larger aircraft are more fuel efficient than smaller ones.

However it is generally true is that newer aircraft designs are much more fuel efficient than older ones.

8. Winglets make a big difference to aircraft fuel economy.

Aircraft wings produce lift by creating a higher pressure below the wing and a lower pressure above it, but this also results in a turbulent vortex at the wing tip - which reduces efficiency.

Winglets - those upward-swept extensions to aircraft wings - smooth this turbulence, which means they increase the lift of the wing and reduce drag. This means less power is needed from the engines, reducing noise and fuel consumption. Current winglet designs can reduce fuel burn by up to 6%, but there is considerable research on further optimising them. Expect to see a variety of new winglets types in the near future.

9. At 11,000 metres (36,000 feet) you can see for approximately 375 Kms

Here's a handy calculator to help you estimate the distance for other altitudes. The underlying formula depends upon the radius of the Earth. This varies because the Earth is not a perfect sphere, so we've used an average value. Even so, it gives a pretty good approximation. The in-flight entertainment system on most airliners has a screen which tells you your altitude.

If you are looking down at something on the ground not so far away as the horizon, a useful rule of thumb is that an object 45 degrees down is about the same distance from directly below you as you are above the ground.

Tags: science-and-technology

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Published 10 December 2014