The Hidden Engineering of Runways

[00:01] September 2025 was an unusually bad month for  runway overruns in the US. On the night of   September 24th, an Embraer 145 with 53 people  on board landed long at the Roanoke-Blacksburg  

[00:16] Regional Airport in Virginia, overshooting  the end of the runway. Just weeks earlier,   on September 3rd, TWO similar  incidents occurred on the SAME DAY,   one a Gulfstream at Chicago Executive Airport  and another a Bombardier at Boca Raton. In all  

[00:32] three cases, the surface at the very end of the  runway crushed under the weight of the planes’ tires. You look at the photos, and it looks  like a mess, but these systems worked exactly  

[00:44] as they were intended, preventing fatalities  and serious injuries in all three cases. We’ve all seen a runway before. At  first glance, there’s not much to it:   a strip of concrete or tarmac planted on the  landscape with some extra markings and lights.  

[00:59] It basically looks like a short section of  highway. But if you look under the surface,   there is a tremendous amount of engineering  that makes these facilities entirely unique   from anything else we build. I want to  peel back the layers and show you what  

[01:14] really goes into building a runway. I’m  Grady, and this is Practical Engineering.

[01:27] A fully loaded semi truck usually weighs on the  order of 80,000 pounds (or 36 metric tonnes) and,   depending on what state you’re in, legally maxes  out at 60 to 80 miles per hour. Our highways are  

[01:41] carefully engineered for vehicles in that weight  and speed regime. Compare that to modern heavy   jets that can weigh more than 500 tonnes or a  million pounds, with takeoff and landing speeds   around 180 miles per hour. Just like highways,  the design decisions for runways - from length,  

[01:59] to width, to shape, to materials and beyond -  all have major implications on public safety.   There is a long list of crashes and incidents  that could have been avoided by better designs,  

[02:11] and actually, a lot of the reasons we do  things the way we do is because of lessons   learned through previous tragedies. Maybe  better than any other industry, the aviation   world strives for continuous improvement through  the understanding of past failures, and you can  

[02:27] see evidence of that just about everywhere  you look, including resources like SKYbrary. The thing is, building a runway is an extremely  costly endeavor.

[02:39] to the amount of money you can spend making one  incrementally safer. So there’s always a balancing   act between cost and capability. One of the most  fundamental decisions that affects both sides is  

[02:52] length. A longer runway can accommodate larger  aircraft, but it can dramatically increase costs   by requiring more land and more infrastructure.  It can even affect the siting decisions,  

[03:04] pushing an airport farther outside a city. It’s  a pretty important choice. So important that FAA   has a 40-page guidance document on length alone.  Based on what you want to accomplish - whether  

[03:16] it’s basic general aviation at a municipal field,  air cargo, medevac, or serving as a backup to the   Space Shuttle program - you first have to pick  a critical aircraft: the one that requires the  

[03:29] longest runway. But it’s more complicated than  that, since takeoff and landing performance   depends on a lot of factors. High temperatures  and elevation reduce the density of the air,   requiring more speed for the same amount of  lift, which results in longer takeoff distances  

[03:44] and landing rollouts. Slopes affect both takeoff  and landing as well. Uphill takeoffs are harder   because the engines have to fight gravity;  downhill landings require stronger braking.  

[03:56] The FAA says that for each percent of downhill  slope, landing distance is increased by 10%.   Manufacturers of aircraft can tell you the runway  requirements for a specific make and model, or FAA  

[04:09] has developed curves that can help you take these  factors into account to decide a runway length. When you’re driving on the highway, direction  isn’t that important. Obviously, you have to get   to where you’re going, but other than that, there  aren’t many engineering requirements that change  

[04:24] with the direction of the roadway. With runways,  that’s not true. Whether taking off or landing, airplanes work best when facing directly into the wind. And in fact,  

[04:36] they might not be able to land or take off at  all under certain crosswind conditions. So the direction of a runway is a consequential decision.  Prevailing winds vary a lot by location. In fact,  

[04:48] one of my favorite types of diagrams, the wind  rose, is specifically designed to show this at   a glance. And if you look at enough wind roses,  you’ll notice that, in some places, there’s not a   prevailing wind direction at all. That’s why most  large airports have perpendicular runways. Again,  

[05:05] this is aircraft-dependent. Every airplane has its  own crosswind limits. FAA generally expects runway   orientation to provide about 95% wind coverage  for the airport’s design aircraft, so in places  

[05:19] without a strong prevailing wind direction,  it takes a second runway to meet that target. Length and direction are easy to notice, but  there’s more to the geometry of a runway. In 2019,  

[05:31] a Miami Air International Boeing 737 touched down  in Jacksonville during heavy rain. The aircraft   skidded off the runway and came to a stop in  the St Johns River. 21 people were injured,  

[05:44] but thankfully, nobody was killed. When  the NTSB investigated the accident,   one of the main contributors was that  the runway was ungrooved. The water   instead building pressure in the contact patch  between the tire and runway. It’s hydroplaning:  

[06:03] the tires ride on the water instead of the ground,  wiping out friction and directional control. Just like in a car, planes need friction to stop.  Larger jets have the benefit of aerobraking,  

[06:16] using devices that reverse the thrust of  the engines, but regular-old wheel brakes   still do most of the work. And just like for  cars, water makes that much more challenging,  

[06:28] so there are a lot of engineering decisions that  go into maintaining good friction on the runway   surface. Like highways, most runways have a gentle  crown at the centerline that drops off to the   sides. This cross-slope helps shed rain and stops  water from pooling on the surface. Larger airports  

[06:46] install grooves in the runway surface that give  water an escape path from beneath the tires,   reducing the chance of hydroplaning in bad  weather. And this isn’t just a one-time   decision. Airports use friction-measurement  equipment to monitor operational conditions.  

[07:02] If the surface gets too polished from use or  built-up rubber from the countless touchdowns,   they have to clean the surface or even  retexture with shot blasting to roughen it up. Runways are a bit unusual because, when  you think about it, they really have two  

[07:17] very different jobs. Taking off and  landing are pretty similar; one is   essentially the reverse of the other. But  in some ways, they’re entirely different.   And so they drive the requirements for runway  engineering in different ways. For example,  

[07:32] it may feel like landing is the most dynamic  moment in a flight, but it’s actually takeoff   that usually governs runway length and strength.  That’s mostly because of weight. A big part of   the weight of a fully loaded airliner is fuel.  An Airbus A380, the largest of commercial jets,  

[07:50] has a max gross takeoff weight of over  550 metric tonnes. For a long-haul flight,   nearly half of that weight can be in fuel. When  an airplane touches down, even though the moment  

[08:02] the wheels hit may feel impactful, the plane  is much lighter. In fact, landings are so much   less damaging to pavement than takeoffs that they  usually don’t even count in load cycle tracking  

[08:15] for the engineering design. It’s all about  takeoffs, and to support those enormous loads,   airport runways have some of the most heavily  engineered pavement systems in the world. This is something that you’ll almost never be  able to see, but the amount of consideration and  

[08:31] engineering below the surface is incredible. The  FAA even has its own engineering software package,   complete with a wonderful government acronym:  the FAA Rigid and Flexible Iterative Elastic  

[08:46] Layered Design or FAARFIELD. Just like highways,  you basically have two choices for runway pavement   materials. Rigid pavements generally use  concrete. Flexible pavements use hot-mix  

[08:59] asphalt. Their behavior and performance are pretty  different, so the engineering is different too.   Asphalt has a small but significant measure  of give to it, which causes the effective   width of aircraft tires to spread out in a  cone underneath the surface into the deeper  

[09:15] layers. This contrasts with rigid pavement, where  a tire's effective width is its actual width. Asphalt is a cheaper material, so it's used  in the vast majority of paved airfields in   the US. Concrete is stronger and stiffer,  so most large-scale commercial airports use  

[09:32] rigid surfaces. The tradeoff usually comes with  volume. A rigid pavement has a longer design life,   so the additional cost is offset  by reduced maintenance and a longer   interval before replacement. But in both  cases, there’s a lot under the surface.  

[09:48] It’s basically a layer cake of materials  that all serve different functions. Everything sits on the subgrade, which  is the natural soil at the site. The   quality of the subgrade really decides  everything else. The soil strength,  

[10:02] its potential for shrinkage and  swelling, the depth of the frost line,   and the depth of the water table will drive  the design. If it’s really soft and mushy,   the subgrade can be amended with sand,  lime, cement, or geosynthetic materials.

[10:16] Some pavements put a drainage layer on top of  the subgrade. This is a permeable material,   like gravel, that lets water get out of the system  so it doesn’t soak the soils below, which might   lead to softening and weakening over time. A  runway is one place you don’t want a pothole.

[10:33] Above that, many pavement systems (especially  flexible ones) use a subbase. This is a layer   of course material (sometimes even crushed  up bits of an OLD runway). Practically,   the sub-base adds thickness cheaply. Stress  from wheel loads drops quickly with depth,  

[10:50] so a layer of material that doesn’t have tight  engineering specifications can accomplish the   depth without driving up the cost too much.  Plus, the subbase serves as a working platform   so you’re not mucking up the subgrade  with heavy equipment during construction.

[11:05] Then comes the base course. This is the  structural workhorse of a pavement system.   It’s usually a mixture of high-quality  crushed and uncrushed aggregates,   specifically designed to lock together when  compacted into a high-strength support. The  

[11:20] goal is to distribute the point forces of wheel  loads into the layers below. Lower stress mean   less movement, which results in less cracking of  the surface layer and a smoother ride over time.

[11:33] On top of all that is the surface course  that provides the friction and texture.   Concrete pavements distribute forces, so  they don’t require quite as much engineering   underneath. For asphalt, friction  is essentially its only purpose. The  

[11:47] layers below do the heavy lifting. And if the  surface course degrades, you can often mill   it and overlay it with new material without  having to rebuild the entire system below. about finding the right balance between  performance, cost, and constructability.  

[12:06] You could just build a 10-foot-thick layer of  concrete and be done with it, but eventually   those costs flow to the airline tickets,  and no one would be happy to pay for that! Since runways are essentially a connection  to the sky, there are some quirks in their  

[12:21] engineering to account for that too. One is  the use of displaced thresholds. Sometimes,   surrounding obstacles don’t allow  for a gentle glide slope to the   end of a runway. You don’t want airplanes  diving steeply into a landing, so instead,  

[12:36] we displace the touchdown point farther down  the runway, while still allowing takeoffs   to use the full length. Takeoff lengths are  usually longer than landing lengths anyway,   so this is a compromise worth making to take  the best advantage of the surrounding airspace.

[12:51] You can only displace a threshold so much, though.  Sometimes design choices and sacrifices are made   to accommodate unavoidable restrictions caused  by nearby terrain or buildings. Airports have  

[13:04] to exist in the broader context of developed  areas. So, airport designers and managers have   to ensure that imaginary zones called “obstruction  surfaces” are free of buildings, trees, towers,  

[13:17] and anything else you don’t want to get hit  by a plane. These imaginary surfaces extend   farther than you might think into the air space,  providing safe approach and departure paths with   comfortable margins of safety. Airports  don’t usually have land-use authority,  

[13:33] though, so keeping the airspace free  from obstructions is a collaborative,   and occasionally contentious, process between  regulators, cities, landowners, and developers.

[13:45] There are also areas of pavement at the ends of  runways that aren’t intended to have planes on   them at all. For example, larger runways include blast pads. This is one of my favorite elements of runway engineering.

[14:00] The powerful wakes produced  by jet engines pick up grit and scour away the   land behind them. If this is just loose soil  or grass, the endless parade of planes will   eventually dig a huge hole at the back of the  runway! I’ve spent a lot of time working with  

[14:17] concrete structures meant to curb erosion from  flowing water, but there just aren’t that many   pieces of infrastructure that are purpose-built  to mitigate aerodynamic erosion. Blast pads can’t   carry the weight of a jetliner, so they’re painted  with yellow chevrons to tell pilots ‘stay off!’

[14:35] Even when a runway is long enough to accommodate  the air traffic it sees on a regular basis,   accidents happen, and sometimes airplanes  overshoot the end of the runway on takeoff   or landing. Runways are required to have a certain  amount of space beyond the pavement on all sides,  

[14:52] called runway safety areas or RSAs.  Like the clear zones along highways,   RSAs provide an airplane with room to  safely come to a stop without obstacles.   There are some instances where space is tight,  though. Urban infrastructure, a body of water,  

[15:09] or other stuff can get in the way, making it  less feasible to maintain so much open space   around a runway. Luckily, there’s another option:  Engineered Materials Arresting Systems, or EMAS.

[15:22] These systems are manufactured from crushable  material like lightweight concrete or foamed   glass. In an emergency situation, they  can dissipate a plane’s kinetic energy,   quickly slowing it down so it doesn’t  crash into whatever lies beyond. EMAS  

[15:38] saved the day in all three major overrun  incidents in September 2025. You can see   just how effective it is in this footage  from the September incident in Boca Raton.  

[15:52] It’s like a much more sophisticated and carefully  engineered runaway truck ramp for airplanes. There’s so much more going on in the engineering  and design of runways than I can possibly cover  

[16:04] in one video. I’ve tried to focus on the hidden  stuff: construction techniques and requirements   that you don’t really notice when you’re a  passenger looking through the window and may   not even be familiar with as a pilot. I really  love knowing how much goes into that stuff that  

[16:19] most of us never have to think about. It makes me  feel safer as a passenger. It’s a reminder that   smooth and boring is usually the goal, and  it takes a lot of work to keep it that way.

[16:31] If you’re a fan of airports and runways, I’m sure you know my friend Sam’s channel, Wendover Productions,   which has some of the deepest dives into topics on  air travel. I love that stuff, but a lot of people  

[16:44] don’t know about one of my favorite of Sam’s side  projects, the Logistics of X. And honestly, it’s   one of his best. My favorite is The Logistics of  Search and Rescue. I had no idea that most search  

[16:56] and rescue teams don’t have their own helicopters.  It’s such a fascinating variety of topics,   from mining to commercial fishing, and if you want  to check it out, it’s only available on Nebula. You’ve heard me talk about Nebula before. It’s  a streaming service built by and for independent  

[17:13] creators, including a lot of my favorites like  Neo, Wendover Productions, the Coding Train,   and Branch Education. I don’t know about you, but  independently-produced content is most of what I   watch these days. I just like the authenticity  and thoughtfulness of videos that haven’t been  

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[17:41] Nebula’s totally ad-free, with tons of  excellent channels and lots of original   series and specials like the Logistics of X.  Sign up for a free trial first and see what   it’s all about. After three days, I think you’re  going to feel like it's worth a subscription,  

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[18:10] go.nebula.tv/Practical-Engineering. Thank you  for watching, and let me know what you think!