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Designing the Perfect Airport Runway

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15:27   |   Aug 10, 2018

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Designing the Perfect Airport Runway
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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
  • 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
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References:
[1] https://www.iata.org/pressroom/pr/Pages/2016-12-08-01.aspx
[2] https://www.windfinder.com/windstatistics/dublin
[3] http://www.b737.org.uk/limitations.htm
[4] /watch?v=brX_VhOU3qQ
[5] https://www.heathrow.com/noise/heathrow-operations/runway-alternation
[6] https://jdasoc.files.wordpress.com/2015/04/pm-edit-jda-crosswind-diagonal-runway-final.pdf
[7] https://www.wired.co.uk/article/heathrow-third-runway-plans-expansion
[8] Actual Airbus Requirements: https://www.airbus.com/content/dam/corporate-topics/publications/backgrounders/techdata/aircraft_characteristics/Airbus-Commercial-Aircraft-AC-A380-Dec-2016.pdf
https://www.statista.com/statistics/564717/airline-industry-passenger-traffic-globally/

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