4 ms·

Not answering your question, but a bit of interesting trivia for you. Large passenger airplanes routinely take off with too much fuel to land safely. If one too

by sowbug 3y ago

Not answering your question, but a bit of interesting trivia for you. Large passenger airplanes routinely take off with too much fuel to land safely. If one took off, circled the airport once, and landed, then it might actually break apart on landing because of the mass of the fuel being sloshed around as the airplane touches down. That's why planes preferably dump or burn off excess fuel before an unscheduled early landing. There are also rules about exactly where a plane can touch down on the runway. Only some of a runway is engineered to be strong enough to bear the weight of a massive plane dropping onto the pavement. The rest is called a "displaced threshold" and is OK for parking, taxiing, and taking off.

danpalmer 3y ago
Oooh, this is something I know a little about. At university we were tasked with building software for estimating the usable area of a runway given its configuration and obstacles. Runways have 3-4 different defined areas, each with different properties and usable in different ways. For example, the grass at the sides is important for safety, must be cleared a certain distance back, must not rise above a certain level above the tarmac, etc, and all those rules are distinct from the rules that apply to the concrete at the ends of the runway, distinct from the rules regarding surrounding buildings and trees, and so on. The area managed as a runway can extend kilometers out on every side. Now, if you have a plane break down at one end, or a support vehicle crash on the grass or something, you don't actually have to put the runway out of action, but you do have to re-declare the usable distance, and that's based on the location, size, and importantly height of the obstacle. You then need to plot the angle of landing/takeoff based on that height, accounting for the segment of the runway it's in, and the given runway's pre-approved angle (varies by city/terrain). It was a fascinating project and a good example of how problems are often so much more complex and nuanced than they initially appear.
plegresl 3y ago
Overweight landings are a thing and it's very, very rare for the airplane to break apart or even be damaged when performing them: https://www.boeing.com/commercial/aeromagazine/articles/qtr_3_07/article_03_1.html https://www.boeing.com/commercial/aeromagazine/articles/qtr_...
- sowbug 3y ago
  You're totally right. I was in "believe it or not!" mode, which means picking the most dramatic factoids. :)
lambda 3y ago
Generally, the displaced threshold is not because of the load of touching down, but because of clearance of obstacles. When landing, where you are aiming to land, and a standard glideslope, puts you over obstacles at a certain height. In some cases if you landed on the very end of he the runway, that would cause you to collide with (or have too low a margin of safety) over obstacles before the runway. The displaced threshold moves the glideslope further forward, allowing for better clearance over obstacles. Source: Advisory Circular 150/5300-13B Airport Design, 3.5.3 (https://www.faa.gov/documentLibrary/media/Advisory_Circular/150-5300-13B-Airport-Design.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...) When it is impractical to locate a threshold at the beginning of the runway, it may be necessary to apply a displaced threshold. A displaced threshold reduces runway length available for landings in one direction. The portion of the runway prior to the displaced threshold typically remains available for takeoffs. Depending on the circumstances surrounding the displacement, operations from the opposite runway end may or may not be affected. Refer to Appendix H for related information on declared distances. Generally, a runway threshold displacement provides: 1. A means for obtaining additional RSA prior to the threshold. 2. A means for obtaining additional ROFA prior to the threshold. 3. A means for locating the RPZ to mitigate incompatible land uses. 4. A means for obstacle clearance prior to the threshold. 5. Increased arrival capacity with certain parallel runway approach procedures. See FAA Order 7110.308, Simultaneous Dependent Approaches to Closely Spaced Parallel Runways Glossary: RSA: Runway Safety Area ROFA: Runway Object Free Area RPZ: Runway Protection Zone Note that all of the given reasons are for clearance of obstacles or various other mandatory safety areas based on clearance from taxiways or other runway traffic, not for durability of the runway.
mulmen 3y ago
I don't think the airplane is going to break up, just that it would need inspection and perhaps repair if touching down over weight. Blancolirio covers this: https://www.youtube.com/watch?v=4D2Kj0t4t9s https://www.youtube.com/watch?v=4D2Kj0t4t9s

by sowbug 3y ago

danpalmer 3y ago

Oooh, this is something I know a little about. At university we were tasked with building software for estimating the usable area of a runway given its configuration and obstacles. Runways have 3-4 different defined areas, each with different properties and usable in different ways. For example, the grass at the sides is important for safety, must be cleared a certain distance back, must not rise above a certain level above the tarmac, etc, and all those rules are distinct from the rules that apply to the concrete at the ends of the runway, distinct from the rules regarding surrounding buildings and trees, and so on. The area managed as a runway can extend kilometers out on every side. Now, if you have a plane break down at one end, or a support vehicle crash on the grass or something, you don't actually have to put the runway out of action, but you do have to re-declare the usable distance, and that's based on the location, size, and importantly height of the obstacle. You then need to plot the angle of landing/takeoff based on that height, accounting for the segment of the runway it's in, and the given runway's pre-approved angle (varies by city/terrain). It was a fascinating project and a good example of how problems are often so much more complex and nuanced than they initially appear.

plegresl 3y ago

Overweight landings are a thing and it's very, very rare for the airplane to break apart or even be damaged when performing them: https://www.boeing.com/commercial/aeromagazine/articles/qtr_3_07/article_03_1.html https://www.boeing.com/commercial/aeromagazine/articles/qtr_...

sowbug 3y ago
You're totally right. I was in "believe it or not!" mode, which means picking the most dramatic factoids. :)

lambda 3y ago

Generally, the displaced threshold is not because of the load of touching down, but because of clearance of obstacles. When landing, where you are aiming to land, and a standard glideslope, puts you over obstacles at a certain height. In some cases if you landed on the very end of he the runway, that would cause you to collide with (or have too low a margin of safety) over obstacles before the runway. The displaced threshold moves the glideslope further forward, allowing for better clearance over obstacles. Source: Advisory Circular 150/5300-13B Airport Design, 3.5.3 (https://www.faa.gov/documentLibrary/media/Advisory_Circular/150-5300-13B-Airport-Design.pdf https://www.faa.gov/documentLibrary/media/Advisory_Circular/...) When it is impractical to locate a threshold at the beginning of the runway, it may be necessary to apply a displaced threshold. A displaced threshold reduces runway length available for landings in one direction. The portion of the runway prior to the displaced threshold typically remains available for takeoffs. Depending on the circumstances surrounding the displacement, operations from the opposite runway end may or may not be affected. Refer to Appendix H for related information on declared distances. Generally, a runway threshold displacement provides: 1. A means for obtaining additional RSA prior to the threshold. 2. A means for obtaining additional ROFA prior to the threshold. 3. A means for locating the RPZ to mitigate incompatible land uses. 4. A means for obstacle clearance prior to the threshold. 5. Increased arrival capacity with certain parallel runway approach procedures. See FAA Order 7110.308, Simultaneous Dependent Approaches to Closely Spaced Parallel Runways Glossary: RSA: Runway Safety Area ROFA: Runway Object Free Area RPZ: Runway Protection Zone Note that all of the given reasons are for clearance of obstacles or various other mandatory safety areas based on clearance from taxiways or other runway traffic, not for durability of the runway.

mulmen 3y ago

I don't think the airplane is going to break up, just that it would need inspection and perhaps repair if touching down over weight. Blancolirio covers this: https://www.youtube.com/watch?v=4D2Kj0t4t9s https://www.youtube.com/watch?v=4D2Kj0t4t9s