Even though at first glance you might expect a runway to be flat for ease of take-off and landing, it’s important to remember that a runway is only built to allow an aircraft to do this in varying conditions and as safely as possible. Being exactly level is not necessary nor a requirement.
Runways aren’t flat because it allows rainwater to run off the sides and keep the surface as free of standing water as possible. Runways are built with this crown down the centreline to achieve this.
Additionally, many runways are also built on land with a slight gradient, so they can, therefore, have an uphill and downhill direction, or a high point or low point in the middle.
In some photos, the runway might appear to have a very hazy, wave-like appearance. However, that is just heat haze. Runways are perfect for absorbing a lot of heat on hot days which is then shown in the photos.
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Does the slope impact landing and take-off performance?
While the crown in a runway will make no discernible difference to an aircraft’s departure or arrival, the runway’s gradient or slope must be taken into account when calculating departure and arrival figures.
Runway slope is the height difference, or gradient, from threshold to threshold (each end of the runway), and this information is provided in airport charts so pilots can take this into account when calculating take-off and landing performance.
Generally speaking, the maximum slope for a major airport’s runway will be about 2%, or 1.2 degrees (though there are some extreme exceptions to this). If an aircraft is departing in the uphill direction, for example, it will need more thrust and a longer take-off roll than if it were to take off “downhill”.
Other Methods of diverting water off the runway
Runways are designed and built with a crown down the centreline, so rainwater can easily drain off either side of the runway, ensuring that little standing water is able to puddle and thus detrimentally affect an aircraft’s braking performance while landing.
Many modern paved runways are also grooved (perpendicular to the length), so that rainwater flows into the grooves thus allowing an aircraft’s tires to be in contact with the runway on the peaks, which should, in theory, be dry.
When building a runway there are also a lot of other considerations to be made depending on the expected traffic, surrounding terrain, local weather conditions, and local laws.
Let’s take a closer look!
What you need to consider before building a runway
Airport authorities have a number of things to consider when building their runways, and what may work for one airport may not work for another. Often, the planning process takes years of studies and proposals, and encompasses the following points:
Aircraft types
The main consideration when building a runway is the type of aircraft that will be using it.
For a major international airport, it can be safely assumed it will have to accommodate the arrival of an Airbus A380, the heaviest commercial aircraft in the world and a modern-day stalwart of many international airline fleets. With a maximum take-off weight of 575 tonnes, it is nearly 120 tonnes heavier than the queen of the skies, the Boeing 747.
To give a little more perspective, the Boeing 737 and Airbus A320 weigh approximately 80 tonnes each.
Small regional airports, on the other hand, only need to build a runway that is 2,000m long, rather than the 3,000m required for the larger jumbo jets.
Airport location
The location of an airport plays a significant part in runway construction as well. Some questions that need to be answered before starting to build an airport are:
- Is the airport high above sea level?
- Is it near the sea?
- Are the general conditions hot or cold?
Aircraft engines lose performance with less oxygen, and wings generate less lift in higher temperatures, so at high altitudes, in hot weather, with high humidity, and potentially a combination of all three, an aircraft needs more time to reach take-off speed. Runways will thus be built to allow for the reduced performance expected of the aircraft flying there.
There are occasions when aircraft will be unable to depart in extreme heat, and in cities like Las Vegas and Phoenix, it isn’t uncommon on very hot days with the temperature passing 49°C to have some flights canceled. Even the longest runways can’t help in such conditions.
Runway direction
Runway direction is determined by factors like:
- the land available
- the surrounding terrain
- prevailing wind conditions.
Crosswind conditions are avoided due to the greater difficulty conducting a crosswind arrival, as are runway directions that might conflict with mountains or highrise buildings.
Lawmakers will ensure this is avoided due to noise and safety considerations, and that is without mentioning the adverse environmental impact a new or redeveloped airport will have. There are many shareholders involved in the planning and building processes.
Runway Material: Concrete or asphalt?
If the location is geotechnically stable, concrete will normally be used because it does not need to be repaired or patched frequently. However, if some very minor ground movement (settling) is expected, asphalt is preferred as repairs can be patched quickly and easily.
For very small airfields and aerodromes the runway may not be paved at all, and simply a stretch of prepared gravel, grass, or even crushed coral, can be used if the area is free of obstacles.
So as you see, there are a lot of factors that need to be considered before the construction of an airport can begin.
Still, the work doesn’t end there. Maintaining an airport to ensure the safety of passengers requires a lot of work too!
Let’s look closer at this!
Maintaining safe runway conditions
A lot of work goes into the day-to-day and long-term maintenance of a runway. Runway inspections are a frequent, multiple-times daily occurrence and are completed by a vehicle to check the following things:
- that the runway is clear of any foreign object debris (FOD),
- that the runway surface is not damaged and had no oil or fuel spilled on it
- that all the lighting functions and the signs and markings are visible and in good condition.
FOD is perhaps the most important aspect of the runway inspection, as the repercussions from hitting it are huge and potentially fatal. Air France Flight 4590, the only Concorde accident during its time in service, was the result of hitting FOD.
A thin strip of titanium alloy from a preceding departing DC-10’s engine cowl was run over and blew up one of the Concorde’s tires, sending that tire debris into the underside of the left-wing and ultimately causing a fuel tank to rupture. Sparks from further damage to landing gear caused a fire, which led to a reduction of thrust from Engines 1 and 2. This, coupled with the landing gear that could not retract and the increased drag this created, meant the Concorde crashed into a hotel building with the loss of all 109 people on board, the aircraft plus an additional four people on the ground.
Inspecting the runway pavement for damage is as necessary as searching for FOD, as the consequences can be just as severe. Extreme heat is always problematic and as well as affecting aircraft performance it can also cause runways surfaces to crack or buckle.
A recent example occurred in Hannover, Germany in 2018, as sustained high temperatures reaching 38°C took its toll on the facilities and caused some 41 flights to be cancelled or diverted. Repairs can take hours or even days to complete, though if possible these runway works will be completed at night when most airports are at their least busy or even closed.
Dramatic Sloping Airports
There are some interesting airports that highlight how sloped a runway can be while still being safe.
1.Birmingham Airport (U.K)
.Birmingham Airport in the UK has three noticeable dips between the thresholds which make for some impressive photos.
2. Courchevel Airport (France)
Birmingham airport is still relatively flat when compared to Courchevel Airport in the French Alps. Its incredibly short runway of 537m has a gradient of 18.6%, or a very significant 10.5 degrees. Needless to say, it is an airport only the most experienced pilots dare fly in and out of.
3. Lukla Airport (Nepal)
Lukla’s Tenzing-Hillary Airport in Nepal, and the gateway to Mt. Everest is frequently listed as the most dangerous airport in the world.
With an 11.7% gradient on its 527m-long runway, it might be a little flatter than Courchevel, but the much higher terrain north of the runway and a steep valley at the other, mean missed approaches are unlikely to be successful. The long list of accidents makes for somber reading.
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