| ▲ | jld 9 hours ago |
| 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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| ▲ | WalterBright 9 hours ago | parent | next [-] |
| 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. |
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| ▲ | psunavy03 8 hours ago | parent [-] | | 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. | | |
| ▲ | krisoft 5 hours ago | parent | next [-] | | 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. | | |
| ▲ | mikepurvis an hour ago | parent | next [-] | | 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. | |
| ▲ | bobthepanda 4 hours ago | parent | prev [-] | | 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. |
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| ▲ | refibrillator 7 hours ago | parent | prev | next [-] | | 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… | | |
| ▲ | HPsquared 7 hours ago | parent [-] | | "Spread spectrum" is used in EE to spread out the frequency ranges used and reduce interference. The extreme version being CDMA. | | |
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| ▲ | cucumber3732842 5 hours ago | parent | prev [-] | | 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). | | |
| ▲ | Cerium 4 hours ago | parent | next [-] | | 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. | |
| ▲ | Onavo 5 hours ago | parent | prev [-] | | Not sure about the ocean industry but carrier landings have full autoland support for a long time (see e.g. magic carpet). |
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| ▲ | metadat 7 hours ago | parent | prev | next [-] |
| 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. |
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| ▲ | username135 7 hours ago | parent | next [-] | | If I had to guess, all things equal, its probably much thicker than your average driveway. | | |
| ▲ | stephen_g 2 hours ago | parent [-] | | 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. |
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| ▲ | blauditore 7 hours ago | parent | prev | next [-] | | 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). | | |
| ▲ | Arainach 7 hours ago | parent [-] | | 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. | | |
| ▲ | amluto 2 hours ago | parent | next [-] | | 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. | | |
| ▲ | Arainach 2 hours ago | parent | next [-] | | 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. | |
| ▲ | thejazzman 2 hours ago | parent | prev [-] | | 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 |
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| ▲ | absurddoctor 6 hours ago | parent | prev [-] | | 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. | | |
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| ▲ | quickthrowman 7 hours ago | parent | prev [-] | | 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... |
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| ▲ | gerdesj 5 hours ago | parent | prev | next [-] |
| "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... |
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| ▲ | mannykannot 3 hours ago | parent | next [-] | | 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. | |
| ▲ | laurencerowe 5 hours ago | parent | prev [-] | | Isn’t the acceleration here just the difference in velocity between the plane and the runway so 0.7 m/s/s? | | |
| ▲ | petschge 5 hours ago | parent [-] | | 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. | | |
| ▲ | adgjlsfhk1 4 hours ago | parent [-] | | 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. |
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| ▲ | pbreit 8 hours ago | parent | prev | next [-] |
| 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? |
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| ▲ | duskwuff 8 hours ago | parent | next [-] | | > 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. | | |
| ▲ | absurddoctor 6 hours ago | parent [-] | | 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. | | |
| ▲ | ExoticPearTree 9 minutes ago | parent [-] | | 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. |
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| ▲ | rtkwe 8 hours ago | parent | prev | next [-] | | 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. | | |
| ▲ | tadfisher 8 hours ago | parent [-] | | 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. | | |
| ▲ | krisoft 6 hours ago | parent [-] | | > 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.” |
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| ▲ | pc86 8 hours ago | parent | prev | next [-] | | 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. | |
| ▲ | tasty_freeze 7 hours ago | parent | prev [-] | | Watch the video. He says for long range flights, fuel is half of the total weight of the plane. |
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| ▲ | cmurf 8 hours ago | parent | prev [-] |
| 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. |
