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The hidden engineering of runways
Here's the one-minute version from the FAA.[1]

Runway overrun areas marked with diagonal stripes have an Engineered Materials Arresting System. There are several different materials used. One is pumice embedded in styrofoam, with a thin concrete layer on top. Large aircraft weigh enough to break through, and the pumice is crushed to powder, absorbing energy. This yields a surprisingly short stopping distance. The aircraft landing gear will be damaged, but the rest of the aircraft is usually intact. The overrun material comes in prebuilt blocks, and after an overrun, only the ones damaged need to be replaced.

It gets a lot of use. The FAA has logged 25 overruns stopped by EMAS, out of 161 runway ends so equipped. That's surprisingly high.

It's a simple, clever system.

[1] https://www.youtube.com/watch?v=hVSvU06_NGE

That's only a small portion of what the video is about.
That’s pretty cool. I’d assumed there would be something similar to the run-away truck ramps you see on steep grades — basically a deep gravel pit. However, actually thinking about it, something along the lines of a gravel pit would likely cause significant damage to engines (not to mention risks created by engine damage) but also seriously impede emergency services.
These overrun areas aren't that long. This trick is used where there's not enough room for a long overrun area. The high drag has to start fast. With the styrofoam/pumice material, the wheels quickly drop all the way to the hard bottom of the arresting material, and then plow forward, dissipating energy by crushing pumice. With something more solid, the wheels may skim the surface for a while before digging in.
Gravel flies. You can't have it anywhere near an airport runway.

(Some planes have no problem with it, though. You can make even the entire runway of gravel if you only fly those.)

the distinction here is mainly jets vs props.
Gravel will happily fuck up a prop just as much as a turbine; it's more about ground clearance. The engines on modern airliners are quite low to the ground, which is why they tend to ingest gravel kicked up by the nose gear.
What's the ground clearance of a flying piece of gravel! (in a gust of wind for example)
  • kmarc
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Hah, I guess this is the same then as in formula 1 (and possibly other motorsports). After the end of a straight (speeding) section, just before the curve they have 150/100/50 (m?) distance indicators. Sans the concrete block at the top. That would obviously shave the driver's head off.

When a driver hits these, they evaporate as dust.

It's not quite the same thing - the EMAS is the overrun surface itself, rather than the signs warning of it. However you are quite close to the money on another aspect of airfield design.

Lots of obstructions near the runway - signs, lights, aerial masts, meteorological equipment, fences - are supposed to be "frangible" [1]. They must break into pieces less likely to cause damage to an aircraft in a high-energy collision. There's a heck of a lot of GFRP used in lieu of metal around an airfield.

1. ICAO Doc 9157, particularly part 6.

  • jld
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I saw a talk a long time ago about the structural aspects of runway design. The most interested fact I remember was that the stresses on the runway generated by departures was higher than those of arrivals, as departures repeatedly stress the same part of the runway, while jets land on a much more distributed area of the runway.

Plus jets weigh a lot less at arrival than at departure.

When I worked at Boeing, I talked about autoland systems with my lead engineer. He said the autoland was too perfect, as the airplanes would touch down at the same place every time.

This caused that place in the runway to suffer severe fatigue damage.

IIRC, there was a similar problem on aircraft carrier flight decks, where they had to induce some randomized amount of dispersion to keep the tailhook from hitting the same spot over and over again.
I work at a self-driving car company and we observed a similar problem when we did some off-road testing on dirt tracks. The cars were too precise and they were cutting deep ruts into the soil. We too solved it by adding a pseudo-random offset to the track.
I believe Google Maps adds a bit of a rng in which route it will recommend when two otherwise similar in distance/time. Obviously the traffic input also affects this, but that's a slower feedback mechanism; better to distribute the cars all leaving the airport for downtown across the 2-3 possible routes upfront rather than dumping them all onto route A until it's a jam and then all onto route B until it's a jam, etc.
Annealing.
Before the current wave of automation there was a previous technology to automate buses using optical sensing and lines in the road which had the same issue.
If you want rails: build rails.
  • bdamm
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There are entire subway systems built with tire-on-concrete where the trains ride precisely the same routes down to the millimeter. Montreal’s is a famous one. These systems are not as efficient as rail, but they are quieter and gentler than the typical subway.
  • namibj
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That's still rail. It's just not steel-on-steel.

Otherwise you'd have to seriously limit what systems you call "monorail".

Hmm in distributed computer systems similar patterns exist, e.g. adding jitter to avoid thundering herd effects.

This feels like an essential pattern of the universe or something…

"Spread spectrum" is used in EE to spread out the frequency ranges used and reduce interference. The extreme version being CDMA.
Intriguing
Similar problem with catenary wires on electric trains wearing into the contacts. The wires zigzag to distribute the wear.
Citation please. Doesn't pass the sniff test.

I suspect the ocean in its various states provides quite a bit of dispersion. Replacing deck plates on a ship is a normal part of maintenance. I find it very hard to believe they'd induce randomness rather than having just that one plate get a different hardness (I know some people will screech about that but trust me, the warship industry is well practiced at such things).

I interpreted the effect here not to be on the deck plates but to be the point of impact between the cable and the hook. Sudden bends in cables can wear them fairly quickly in the immediate vicinity. I'm sure you can extend cable life proportionally to the spread of the loading.
That makes far more sense.
  • Onavo
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Not sure about the ocean industry but carrier landings have full autoland support for a long time (see e.g. magic carpet).
Do you know how they keep the concrete from cracking? All the pads in general are in way better shape than my driveway, and the driveway has decent support underneath and is subjected much, much less load.

Maybe they use plentiful jagged interlocking sharp granite as the base l? (like railroad track foundation)

Next time you're at SFO, SJC, or any other major hub sitting in the plane before it backs out of the gate take a second to gaze upon and admire how pristine all the concrete pads are, it's really impressive.

Most concrete cracking you will see in residential construction and private driveways are either because the ground wasn't compacted well enough before the pour, or more often they didn't put a thick enough layer of stone to prevent the ground from moving. Cutting out depth from the base of crushed stone is often the easiest way to cut costs because it means less material brought in and less material to dig out.

Granted private driveways don't need to be absolutely perfect, but if you want it to last for a really long time you need deeper base layers.

Same with any roadway. The base is everything. I visited some European contries and noticed that the roads seemed to have fewer cracks and potholes than many roads in the US. I had assumed it was better maintenance, but the reason I was told is that they spend a lot more on preparing the base than is typical in the US.
If I had to guess, all things equal, its probably much thicker than your average driveway.
Yes, for example taxiways and aprons designed to take the weight of large aircraft like A380s can be ~470mm (almost half a metre) thick, and that's only half of the structure with the subgrade and sub-base together being about as thick! Whereas the standard for driveways where I live is 125mm thick.
Not sure if that's a serious question, but your driveway might lack a proper foundation, so the surface is moving and cracks. Also, it's likely not concrete, but tarmac (which is much softer).
A significant number of American driveways are concrete. I'm not going to look up numbers, but I would have to believe that more are concrete than asphalt/tarmac. Unpaved driveways could outnumber both, who knows, but most people with paved driveways have concrete.
I think that depends on the region and weather differences. In the northeast at least, asphalt seems to be a lot more common than concrete.
  • ghaff
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Or eveen gravel as in my case.
I’ve never understood why people so frequently choose poured concrete over cheap interlocking pavers. Where I live, it’s extremely common to see someone pour a concrete driveway then promptly cut it up because they forgot something.
In contrast I've never seen someone have to cut up a driveway.

There are many ways concrete is superior to pavers. One of the most important is that it is miserable and almost impossible to properly clear snow and ice from pavers.

They sink when a car repeatedly drives over them. I watched a neighbor redo their yellow brick driveway. Well. Only briefly…

Ruts were visible pretty quickly again

  • ekr
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Not when installed properly, i.e. on a level, compacted base. Where I live, in the Netherlands, a great portion of streets, driveways, sidewalks, bikepaths are from klinkers, or bricks. Very rarely do you see any indentations in them (mostly when there was some roadwork and a part of them removed and then reinstalled. The whole reinstalled section sinks a bit, probably because there workers were not careful and did not compact the substrate to the same degree).

Some of these klinker roads see heavy traffic and they're perfectly fine. It's also nice to see the automated machines they have for laying them.

  • rob74
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If the substrate below isn't compacted properly, bricks will sink, and concrete will crack (maybe not as fast, but eventually). So we're back at where the discussion started...
It’s about half the price. Paver driveways cost twice as much to install due to the increased labor. It’s not that expensive to sawcut concrete.
Mostly the extensive subgrade work, as I understand. Similar to a road, there’s a bed of sand and aggregate under the concrete surface.

The concrete they use is very precisely mixed to a specification and then it’s tested for adherence to the spec.

A runway is also going to be 3-4x thicker than a 4-6” thick driveway slab. Probably they also use fiberglass or PVC coated rebar instead of plain steel rebar.

Definitely not an expert here but I can read a civil site plan and hire civil site work subcontractors frequently.

Here’s some information on concrete testing: https://www.concrete.org/frequentlyaskedquestions.aspx?faqid...

Also, Grady is one of the best creators on YouTube, I can’t help but watch his full videos whenever they pop up. I always learn something, even if I’m familiar with the subject.

Edit: Granite is not one of the listed materials in Part 4 - Base Courses of the FAA runway construction guide, here’s the entire thing for reference - https://www.faa.gov/airports/engineering/construction_standa...

"stress" (in engineering terms) has a particular meaning and is not a generic term. It is not really a synonym for "forces" or "what makes other stuff break"!

Let's look at just the downward forces:

I need some quick figures 1 - an early Boeing 747: 330 tonnes (metric) fully loaded and 160 tonnes empty. A tonne is 1000 Kg.

According to 2: 240 feet per minute vertical is a hard landing which about 1.2m/s. 60 - 180 is considered ideal, so let's go for about 150fpm which is about 0.7m/s.

We have to estimate the maximum downward force on take off. At the point of just before lift off, the plane has rotated to say, let's say 45 degrees, and its engines are delivering enough force and its wings are delivering enough force to push it into the air. Surely at take off, that vertical force is simply the weight of the aircraft, which has remained the same all the time. It doesn't suddenly push down harder than its weight, that's just what it feels like for a passenger.

So let's allow our jet to be empty on landing and also let the acceleration due to gravity be 10m/s/s

So what is the instantaneous downward force of a mass of 160 tonnes dropping at 0.7 m/s compared to a dead weight load of 330 tonnes. Both are in a gravitational field of 10 m/s/s (or m^s-2).

Now this is where I get a bit lost because force = mass x acceleration and the landing plane is descending at a constant velocity of 0.7 m/s. Mind you, the ascending plane is also ... ascending, or will do but it does not have an instantaneous upward velocity so at wheels off it has a vertical acceleration of zero.

Help!

1 https://measuringly.com/how-much-does-boeing-747-weigh/ 2 https://aviation.stackexchange.com/questions/47422/what-is-t...

You might find these points helpful:

1) when an airliner lands, the undercarriage legs, which are telescopic sprung and damped struts, spread the vertical deceleration over a finite period (I cannot say how long it lasts, but I would say of the order of a second or so.)

2) At the point of touchdown, the wings are generating lift about equal to the aircraft’s weight. This decreases quite rapidly, largely on account of the decease in angle of attack as the nosewheel comes down and from the deployment of spoilers, but it would be mistaken to think that the runway is immediately supporting the full weight of the airliner after touchdown.

3) On takeoff, until the nosewheel is lifted to initiate rotation, a significant fraction of an airliner’s weight is being supported by the runway. During rotation, as the angle of attack increases, the lift increases [1] until it exceeds the weight, at which point the airliner lifts off.

4) If we ignore the fact that the undercarriage is sprung, then the airliner has no vertical velocity until it lifts off. Right at that point, however, when the lift exceeds the weight, it gains a vertical acceleration.

I hope this helps!

[1] Plus a vertical component of the engine thrust, but no airliner rotates to anything like 45 degrees - in fact, if it has not left the ground at a rotation angle equal to the angle of maximum lift coefficient (~10 - 15 degrees), it is not going to do so without going faster.

Wouldn’t the force be the force required to decelerate the plane’s vertical velocity to 0 m/s over whatever small amount of time?
Isn’t the acceleration here just the difference in velocity between the plane and the runway so 0.7 m/s/s?
IF the velocity would change from 0.7m/s to 0m/s over one second, the acceleration would be 0.7m/s/s. But if the time span over which that velocity change is (much) shorter, the acceleration would be (much) higher.
Is it much shorter? An airplane does take a while to settle down. You land tail first and the nose generally takes a second or so to come down.
  • bdamm
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Not typically. An airliner can and often does stall just above the runway and then falls the very small distance that remains. So the formula would be more like the airplane falling from a few feet up. The tail is still generating downforce (the opposite of lift) which is why the nose is in the air.
According to the video the much larger weight is the main or even only cause of takeoff exerting more load on the runway than landings.
Isn't it the opposite? Landing stress a sub-section of the runway while departures stress a larger portion?

I'd be surprised that a heavier plane on takeoff exerts more force on the runway than a lighter plane landing.

And as the departing plane goes faster, doesn't the lift take stress off the runway?

> And as the departing plane goes faster, doesn't the lift take stress off the runway?

Only for a short period between rotation and liftoff. Most of the takeoff roll is spent building up horizontal speed; the pilot doesn't command the aircraft to pitch up before it's ready to lift off.

There will be lift almost as soon as the plane begins moving forward, reducing the weight of the plane, which would seem to reduce downward stress.
You are half-right. Actually pilots push the stick forward to force the plane to stay on the ground until it reaches takeoff speed. If the plane would rotate naturally because od the air passing under the wings it would generate a lot of drag.
  • rtkwe
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Planes all start their take off from basically the same position and stress the whole runway, slowly lowering as lift increases, but at their highest weight.
And this is because pilots are trained to keep their nose gear on the centerline, and there are relatively few aircraft types in use which receive the "heavy" after their flight number over ATC. So wheels are going to roll over the exact same tracks repeatedly.
> pilots are trained to keep their nose gear on the centerline

Funily I was learning to fly at a grass strip and we were told to vary our positioning left and right on the runway for exactly this reason. In practice it meant that as we were taxiing to the runway my instructor would tell me “Today we are taking off left/right of center to avoid damaging the grass too much.”

  • pc86
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It's the same principle as walking on snow in normal shoes vs. snow shoes. Taking off is normal shoes, a lot of pressure concentrated at the very first part of the runway. Landing is snow shoes because it's distributed across more of the physical surface, and the plane weighs a lot less when it lands anyway.
Watch the video. He says for long range flights, fuel is half of the total weight of the plane.
  • cmurf
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Yeah, the higher departure stress due to greater fuel weight at takeoff was mentioned in this video.

I'm now curious about the engineering of the displaced threshold. This is a portion of the runway that aircraft can taxi onto and use for takeoff but not for landing. I thought (assumed) that the landing was harder on the runway surface than takeoffs, hence the displaced threshold wasn't designed for that force.

The displaced threshold could also be used to ensure obstacle and terrain clearance on landing - simply disallow that portion from being used in order to create an offset from the obstacle. But I don't know whether this is a very common reason for displaced threshold usage.

-- Video also mentions https://skybrary.aero/ which I'd not heard of previously. Looks neat. I'll have to check it out.

I absolutely love that Grady includes full transcripts of his videos.

It's much faster to read the article than watch the video, even though that hurts him by 1 view.

I just watched parts of the video after reading because I wanted to see his explanations.

One of the few really good creators out there.

Reading the transcript doesn’t even give you 10% of the information actually conveyed in the video which is why it’s so much faster.
If you already understand the concepts you're not gaining much by watching the video.

If you don't then I'm sure it's better than nothing but idk if stock footage is where you should be developing your mental model of how a tire hydroplanes or how a paved transfers load into what's below it.

Some of it is stock but Grady has a lot of custom animations and miniature scale replicas in his videos all the time.
Writing off Practical Engineering's videos as "stock footage" is utterly hilarious to me.
C'mon. I know everyone on HN just can't help but glaze anyone who deals with "real world things" rather than 1s and 0s but this is absolutely not one of his more original content videos. You can't simulate landing aircraft using a clear plastic box (one of his better props) like you can soil conditions so you get what you get.
Video is great, came up in my youtube recommendation cycle last week.

Honestly one of the better things youtube has pitched to me, the quality/relevance of the rest of its recommendations have been nose diving over the last year (or so it feels).

100% anecdata, but I think YouTube nudges your ad profile towards some averaged out cosign product of everyone’s ad profile at regular intervals.

I’ll discover something new, then get pushed a ton of things related to it, which is really good! After a very long break of ~4 years, I started playing oldschool RuneScape again, and that interest weaved its way into my recommended feed perfectly for a month. Felt like I was picking up where I left off, new folks making OSRS video essays, folks I remembered from a long time ago that I had unsubscribed from, exactly what I want out of an algorithmic feed when I’m freshly into something.

Then BAM, gaming content. Some sort of threshold gets hit and now I’m being pushed hyper popular gaming content regardless of RuneScape-y-ness. There’s still a nudge towards it, but I got placed in some “gaming” cohort and it totally crowds out my recommended feed. I don’t really do much gaming outside of this stupid old MMO!

All that’s to say: it might have been a year since you last had one of these inflection points where YouTube will let your ad profile exist as an outlier for a bit.

100%. I've always used youtube on desktop (connected to the TV usually), and it seems to me they're just making all versions of youtube into a mobile app optimized for shorts type content. Recommendations are hugely influenced by what you watched very recently. What used to be related videos in the side panel of the player is now just the home page again, but vertical. Even just generating a recommendation from "Whatever (part 2)" to "Whatever (part 3)" is hit or miss now. Some of the recommendations are actually quite good, but at least for the way I like to watch, it's only getting worse over time. The category labels on the home page are also pretty telling - horrible labels (e.g. I watch some cooking/recipe videos and the label will be "Baking sheets" or something like that), plus it emphasizes the recency bias when it shows 5 categories that are basically just the same content with different labels and forgets what I've always liked watching.
> Recommendations are hugely influenced by what you watched very recently.

Ah, well, I don't know that I fully agree.

I watch channels that are people building things, repairing tools, or goofing around in an easy-going way without a lot of product placement or sponsored content.

And yet, all of the recommendations I get are either sponsored unboxing videos with AI voiceovers or click-baity channels with ugly reaction faces in the video thumbnails. I guess those probably make more money for Google.

I suspect ad revenue isn't good for maker/hacker type videos. I watch a lot of woodworking and cnc projects, and I mostly get the kind of recommendations you're talking about
For me my home page is now 95% videos I've already watched. The side bar is the only reliable source of new content. Until recently I had no subscriptions but that never used to matter. I had to subscribe to Patrick Boyle in solidarity when they demonetized his Epstein video.
One of the issues of YouTube is there is "discovery" vs "what I want to watch". https://www.youtube.com is ok for discovery and pulls a lot of the "this is what I've subscribed to" in there too. Doing a subscribe to channels that give you consistent media that you want to watch and then going to https://www.youtube.com/feed/subscriptions as the "this is where I want to be when on YouTube" gives a completely different experience.

(I'm also quite free with the "don't recommend this channel to me" option if something disappoints me)

The way Youtube (and I've started to notice in other platforms) does recommendations, for every 5+ that are nonsense, I'll get one I like. Youtube will then start showing me more of that video's channel content and similar channels for a week or so, and then it'll just stop showing any of it to me quite randomly. Sometimes it's when I click on that random recommendation out of 5 from a different topic.

then the cycle starts again. sometimes youtube brings the content back and sometimes i really need to hunt for it.

it's almost like they base interests into like a top 3 or so list and if the third favorite one cycles out a lot (however they deem it is being cycled out) they'll stop recommending or otherwise showing it to me.

"Sign in to confirm your not a bot"...gone.
I absolutely love this channel. It provides a reasonably no nonsense view into parts of engineering I'm not super familiar with. It's made cooler by the fact that I occasionally interact with the systems being described. (Though hopefully not this one).

Does anyone have recommendations for similar high quality engineering adjacent content?

There's Farmcraft101 if you are interested in DIY builds and repairs. There was a series of videos repairing an excavator from scrap to finish, and he is currently doing a series of videos adding hydro generation to a pond. Interesting metal fabrication, hydraulics, power, etc content.
Farmcraft101 is my absolute favorite channel right now. I can’t wait for fridays when he puts out a new video. I found him a couple of months ago and binge watched all his playlists.
That sounds pretty interest, I'll take a look!
Every time I fly, I marvel at how much engineering and know-how went into making the airport that I'm using. From the oddly shaped trucks with various functions, to mundane elements (elevators, escalators, ...) to advanced technology (radio communication, radars...) to the sheer organizational feat (thousands of people coming in every day to execute their carefully planned tasks). This text will give me one more thing to think about :)
Same, and I also just marvel at the airplanes. This video made me think of the several grass runways that are in my area. They're literally just maintained by some guy mowing them, and yet people land on them in tiny planes as well as two-engine aircraft.
Glad I'm not the only who is fascinated with airports and the technology/engineering that makes them function at scale.
> The FAA says that for each percent of downhill slope, landing distance is increased by 10%.

This is accurate as to their recommendation, but it was surprising. I know it’s inaccurate, because at a perpendicular angle there would be no way to land. I know they don’t allow an angle that steep, but that means this is “gut feel” inexact math that seems unsafe.

The force required to stop on an inclined plane is proportional to the sine of the angle. At angles less than 10 degrees (~17% slope, far steeper than the FAA would allow for an airport), the small angle approximation of sine is within 0.5% of linear.
We had group coding projects at university, and the first one was always "sponsored" by the local airport. I think the ATC manager was friends with a lecturer. Every year the students built to the same spec in groups, being able to compare and contrast. It was great fun.

The year before me was all about runway markings: take a bunch of industry specified XML describing the runway and produce accurate diagrams in a GUI browser.

My year was runway "redeclaration", if a vehicle has broken down on the runway, you can still use the runway as a shorter strip, accounting for the onion layers of different zones radiating out from the tarmac itself, accounting for the height of the obstacle and angles of approach, accounting for all the necessary safety margins.

It was my first real exposure to working in a team and to solving a real world problem with a good spec. Of course it was an absolute shitshow, but I look back on it fondly.

  • ggm
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Something which still confuses me is the nature of the illuminations in the roadway. Because we can see edge elements, we see things on stalks. I don't see how that can work, for things the tires run over. But, the illuminations are there. They must be super-designed cats-eyes.

Also, the approach lighting has very good engineering to keep you in the safe slot for approach angle. The lights must have fresnel lenses or shading or something to keep a very narrow angle of approach lit up "best"

On take off if I have a window, I now look for the banding which I mentally model as "not yet.. " "almost .." "if you are doing <x> kph then YES" .. and "nope. don't wanna see this one"

Gate approach, there are clues that pilots drive by following lines. So many lines! marked by aircraft type: if you are a <this> then follow <this track> type markers.

You’re on the right track! Some of them are embedded, some of them are on stalks, and some of them are highly directional: https://flightlight.com/airfield-products/
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I can never watch just a minute of these guy’s video — it’s always the whole thing, always so interesting.
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He does such great videos and content. Might have to watch this one with my dad; he used to joke that his Eagle Scout project was putting in the north-south runway at MCI.