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 it. 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. [4] 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 Runway. 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. [5] 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. [6] 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. [7] 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 airborne. 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 only. 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 stop. 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 performance. 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 speeds. 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. And if you want to improve your own analytical
skills, a great place to start would be this course on logical thinking on Brilliant. Logic is the foundation of all problem solving
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Immediate thought upon seeing title "Oh sweet a new Wendover Productions video.".