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Comment by jld | Hacker News Reader

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.

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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.

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

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.

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.

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.

That's still rail. It's just not steel-on-steel.

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

The problem is that the optical guided bus was built with the intention of reducing cost, since painting lines is a lot cheaper than building rails.

The moment you have to build rail-like things you lose most of the cost advantages.

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.

I'm sure Google Maps has had to put their thumb on the scale in numerous instances. I recall reading articles about it "discovering" more optimal routes between Point A and Point B only to find things like the new "optimal" route being down a neighborhood street, and then the locals started squawking.

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).

Not sure about the ocean industry but carrier landings have full autoland support for a long time (see e.g. magic carpet).

I wouldn't call it a "long time." MAGIC CARPET per se, especially as a default mode of operation, did not become mainstream until the last 10 years, and that's only for the aircraft that have it.

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.

Citation is my professional network as a former Naval Flight Officer who flew off aircraft carriers, and it's not the deckplates, it's the nonskid coating that goes on top of them. It's expected to wear and be replaced every so often, but when you have to do it more often than that, it becomes a pain logistically and operationally.

And the entire point of the system is to counter the dispersion the ocean provides, much like a Naval Aviator is already required to in order to trap successfully. You are literally flying through a very narrow window that is moving in three dimensions.

Similar problem with catenary wires on electric trains wearing into the contacts. The wires zigzag to distribute the wear.

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.

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...

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.

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

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.

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.

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.

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.

Most of Europe sees far fewer freeze-thaw cycles than most of the US does, which are a huge killer of roads.

The color scales aren't equivalent here but you can see the difference:

Europe - pretty much only unpopulated northern Scandinavia + up in the Alps/Pyrenees getting over 64 days, most of the most densely populated areas with lots of infrastructure below 32 days: https://www.atlas.impact2c.eu/en/climate/freeze-thaw-days/?p...

US - https://www.semanticscholar.org/paper/Climatology-of-Freeze-... (Fig 4.2) - Probably more than 50% is over 75 cycles, and large chunks breaking 100 cycles a year (almost all of New England and some other scattered patches, the Rockies/interior West/Western Plains).

"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...

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.

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.

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.

OK so let's look at a second. I think I can get away with this analysis:

On take off f = 330 x 10 = 3300 (units etc)

On landing f = 160 x 10.7 = 1712

So, if you are gentle enough on landing and the aircraft is nearly half the weight it was on take off then the downward force on landing is very much less than that on take off.

That 160 tonnes empty also implies I've thrown the passengers, crew and luggage out too, which is a bit rough. Let's try total fuel at "about 180 to 213 tonnes" and allow that we need a factor of safety, so let's say 40 tonnes of fuel left over on landing.

On landing f = 200 x 10.7 = 2140

So, I'm still going to need some convincing about landing aircraft causing more damage than those taking off.

I was only waffling about rotation angles whilst trying to get to grips with what is going on. I now don't think the engines have anything to do with this analysis. Mind you I am just about old enough to remember watching Lightnings (https://en.wikipedia.org/wiki/English_Electric_Lightning) taking off. Imagine a large silver firework ...

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?

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.

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?

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.

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.”

> 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.

Watch the video. He says for long range flights, fuel is half of the total weight of the plane.

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.

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