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This episode of Real Engineering is brought to you by Brilliant, a problem solving website
that teaches you to think like an engineer.
Last year there were over 4 billion passengers in airlines around the world, a figure that
grew from about 2.5 billion just 10 years earlier.
The airline industry is big business with a total revenue of 834 billion dollars expected
in 2018.
[1] With this kind of money for taking governments and private companies want to get their share
by designing an airport that can facilitate AND encourage air traffic to pass through
But if we take a look at the footprints of some of the busiest airports in the world,
there are some patterns, but nothing immediately jumps as the go to design for air traffic.
So let’s demystify some of this mysterious world of aviation and figure out how to optimally
design an airport.
In the early days of aviation runways we often nothing more than a cleared field.
The Wright Brothers choose Kitty Hawk, an isolated strip of beach, because it had plenty
of space and more importantly strong winds to help get their planes off the ground.
Today, that practice isn’t all that different.
Airports are some of the largest plots of land allocated for a single use in any city,
and wind still dictates their design.
Once again, taking a look at runways around the world, you may not see a pattern at first,
but if you overlay the prevailing winds in their area the pattern becomes clear.
[2] Most airports in the Northern Hemisphere, are alined east to west, which coincides with
the most consistent wind directions.
Inspect at any airport and it’s likely they have followed this design principle.
This is done to take advantage of the wind, just as the Wright brothers did all that time
ago, because a head on wind adds lift reducing the power required for take-off, and reduces
landing speed.
It also maximises the operational hours of the airport in windy conditions.
The alternative is landing with heavy crosswinds, which is not particularly fun for the passengers
or easy for the pilot, if the pilot can land at all.
The crosswinds a plane can tolerate differ with plane design, with planes with larger
vertical surfaces like winglets and vertical stabilizers being more susceptible to crosswinds
pushing them off course.
A typical plane like a Boeing 737, the most common airliner on the planet, can tolerate
a crosswind of about 60 km/h [3] with a dry runway and 55 on wet.
Anything exceeding that and planes need to hold until winds calm down, or divert to an
alternative airport.
Tailwinds are even less tolerable with winds from 18-25 km/h making it too dangerous to
land at any, but the longest runways.
Some Airports, like London City Airport, have to enforce their own crosswind tolerances
below plane tolerances, as their runways are narrower than average.
Fortunately tailwinds are easy to counteract by landing in the opposite direction.
NATs, formerly known as ‘National Air Traffic Services’ , illustrated how these shifting
winds affect air traffic with UK Air Traffic data from February 14th 2014.
On that day winds of up to 110 km/h were recorded, making it impossible for aircraft to land
all over the UK, causing towering holding stacks to open over London airspace, with
the lower aircraft waiting for a break in the wind to land.
Yellow flight paths here are delayed planes, which were approaching two hours, red flight
paths are diverted planes.
This is an extreme case, but this incident cost these airports and airlines massive amounts
of money.
Designers of airports will analyse decades of wind data to minimise any possible operational
shutdowns like this.
This is our first design principle to maximise traffic, to simply minimise shutdowns due
to wind.
Now that we have chosen our runway direction, let’s pick a location in our city to place
our airport.
With this wind alignment in mind, let’s say East to West for this example, most city
airports will attempt to place the airport on the Northern or Southern edge of the city,
so low flying aircraft coming in to land don’t have to fly over the city.
This is the case for most airports.
But Heathrow airport is a special little butterfly located smack in the middle of London.
This is fantastic for accessibility with London city centre only a short train ride away,
but it creates problems of its own.
The first is noise.
In the 1950s the owners of Heathrow signed an agreement with the residents of Cranford
to not allow planes to take off to the east, which is often needed as the wind blows from
the East about 30% of the time in London.
This was done to reduce noise over the most populated area neighbouring Heathrow.
This agreement is no longer in place, but it still affects how Heathrow operates.
It runs a policy of runway alternation.
From 6 am to 3 pm, planes will land on the Northern runway and take-off from the Southern
Then the moment the clock strikes 3 they switch, with planes taking off from the Northern runway
and landing on the South.
This order also flips every second week.
All of this is done to give the residents around Heathrow some relief from the constant
blaring of jet engines over their homes.
Not an ideal situation when trying to run a busy airport.
Parallel runways like this are great for traffic, as two planes can land and take-off simultaneously.
Once again maximising traffic.
You can see two planes landing at the same time at Heathrow, typically between 6 am and
7 am when departures are quiet, but you do need space between the runways The FAA specifies
that parallel runways with centrelines spaced 760 to 1300 metres apart must use staggered
approaches, meaning planes cannot land side by side Runways with centre lines spaced between
1300 metres and 2700 metres can land simultaneously with air traffic control monitoring.
Seeing a flight land alongside your own is a pretty common sight at LAX for this reason
with it’s runway pairs 1.4 kilometres apart,
and even though Gatwick Airport has two runways it operates as a single use runway as they
are too close to each other to work simultaneously.
The alternative to parallel runways are intersecting runways, and while these are more space efficient,
and can provide alternative approaches with a shift wind patterns, they come with their
own risks and require careful monitoring by air traffic control to prevent crashes.
In general a single runway operating both take-offs and landings can achieve a similar
throughput of aircraft if wind conditions are favourable.
So when looking to increase air traffic volumes, placing additional parallel runways at least
1.3 kilometres apart is best.
This is where Heathrow runs into its next design issue.
It’s location has made it near impossible to expand.
Heathrow is now operating at 98% capacity, and being the UK’s hub international airport
increasing capacity is a major concern.
So where can we place another runway?
Let’s see.
Hmmm nope, no, nope, definitely not, that won’t work…..or will it.
Amazingly this was the proposal set forth earlier this year that will require a village
to be bulldozed and a tunnel dug to reconnect the M25.
This will cost 3.3 billion dollars for compulsory land purchases alone, with a further 18.4
billion for the expansion itself, though the British Government has promised this bill
will be entirely privately funded.
Under its current format, Heathrow is constrained to about 480,000 flights a year, but they
have managed to continually grow passenger numbers by increasing the numbers of large
long haul flights passing through it, but this is not an option for all airports, as
their runways are too short.
Take Dublin airport as an example, it currently operates two intersecting runways.
One 2623 metres long and another 2072 metres long.
To see why this is a problem let’s analyse runway length requirements.
Basic runway length is determined by airplane performance, and to calculate it we analyse
the critical moments in an aeroplanes take-off sequence.
A plane hits 6 critical speeds during take-off.
The first is the stall speed, this is the minimum speed at which a plane will remain
This is not used as the take-off speed, as any decrease in speed due to fluctuations
in wind or orientation of the plane will cause the plane to fall.
The next critical speed is the minimum control speed, this applies to multi-engined aircraft
If a multi-engined aircraft loses an engine, the uneven thrust between the wings will cause
the plane to turn, this is called yaw in aviation.
To counteract this the rudder will be deflected to provide the opposite yawing moment.
The rudder needs air passing over it to work, and thus the minimum control velocity is the
velocity at which the rudder can provide enough of a yawing moment to keep the plane straight
in the event of an engine failure.
The next speed a pilot needs to worry about is V1.
V1 is a line in the sand for pilots making a decision whether to abort a take-off.
If something happens before V1, like an engine failure, the pilot must abort the take-off.
If it happens above V1, they must continue with the take-off, as it would be unsafe to
This is the most important speed for runway designers.
At this speed the plane will need enough distance on the runway to safely bring the plane to
a stop, which is exactly the same as the distance needed to reach V1.
The resulting total runway length is thus called the balanced field length.
Back to that in a moment.
The 4th critical speed is Vr, or the rotation speed, this is the point the plane can begin
to lift its nose up and begin it’s ascent.
The next speed, which results in some of the coolest testing videos, is the minimum unstick
speed, Vmu, this is the speed the plane can take-off at its maximum pitch, which is actually
the point where the tail skid hits the ground.
This is a video of a test pilot testing this speed.
Since this would be incredibly uncomfortable, the actual take off-speed is at least 10%
higher than the minimum unstick speed.
At this point no part of the plane is touching the ground, and it is officially airborne.
It must then accelerate to it’s climb speed V2, which it must achieve with a minimum clearance
of 10 metres from any obstacle.
With all this in mind we can begin designing our runway length.
Planes are typically designed to use standard runway lengths, and not the other way around,
but these speeds can vary between different aircraft, so let’s start our calculation
with the world’s largest plane the Airbus a380.
Here we will be assuming a maximum take-off weight at sea-level with the international
standard atmosphere model for weather conditions, and no wind.
A typical decision speed of a fully laden a380 is about 280 km/h, or about 78 metres
per second.
This, along with other critical speeds, do vary with flight conditions and will vary
for the runway itself.
The pilots have a flight computer to output the relevant critical speeds for this reason,
and gives them an appropriate thrust percentage to provide the acceleration needed.
This is just an example.
Assuming an average acceleration of about 2 metres per second squared we can calculate
the distance needed to reach v1 by employing one of the fundamental kinematics equations
every high school student learned in physics, specifically this one.
Initial velocity is zero and we can rearrange the equation to find distance.
Applying our variables and we get a distance of 1521 metres to reach v1.
In the event of an aborted take-off the plane will need an equal distance to bring the plane
to a stop, this is called the balanced field length.
Which is simply double this distance at 3042 metres.
Again this value varies wildly and v1 is dictated by the runway available, not just the plane
This graph provided by airbus, shows the various runway lengths needed for the a380 at various
take off weights and altitudes, and agrees roughly with our calculation [8]
So, as you can see, Dublin Airport’s runways are too short to accommodate fully laden planes
like this.
Large long haul planes can and do land here when needed, but cannot take-off with a full
tank of fuel and passengers on board, which prevents any large long haul carriers from
operating from Dublin.
Thus a new runway is being built to run parallel to the existing longer runway to the South,
but even this may be too short.
As winds and weather will increase the runway distance needed, on top of this Dublin airport
is 75 metres above sea level, which would add about 2% to runway length requirements,
as the thinner air reduces the lift provided by the wings, and thus increases the take-off
The longest runway in the world in Tibet at an elevation of 4334 metres or 14,219 feet
is 5.5 kilometres long for this reason.
The temperature of the air at the airport also has a significant effect on runway length
requirements, with an additional 1% of runway length required for every 1 degree celsius
over the standard atmosphere measurement we used earlier at 15 degrees celsius.
Once again this is a result of reduced air density reducing lift capabilities.
Last year this actually resulted in flights being delayed and cancelled out of Phoenix
Arizona when temperatures rocketed to 49 degrees celsius.
Clearly, designing airports is a tricky and expensive business.
If money and space wasn’t an issue the ideal design would simply be multiple parallel runways
spaced about 1.3 kilometres apart.
The busiest airport in the world the Atlanta International Airport runs 5 parallel runways.Beijing
comes next, running 3 parallel runways each far enough apart to run simultaneous operations,
and long enough to accomodate any plane.
Dubai Airport coming 3rd with it’s two parallel runnings each over 4000 metres long due to
the heat of Dubai, allowing it to be one of the world’s most important stop over points
for long haul carriers.
This pattern reoccurs all over the world.
International airports with parallel runways long enough to accomodate large planes are
consistently the busiest, but with limited space available some alternative designs have
been proposed to increase capacity, like this circular runway design.
Which would not only be a nightmare for air traffic control trying to direct airplanes
AND make it even more difficult for a pilot to land and take-off, but would also only
be useful in calm weather with no wind dictating take-off direction.
These are the kinds of issues that are only found when engineers carefully analyze a problem.
Without paying close attention to detail, it’s easy to fall into the trap of thinking
a design that looks promising on the surface will work.
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