Sloped Runways: What? Why? When?

Where do I begin?


Well, it’s a hard problem to solve, but you probably figured that out since we haven’t tried to create sloped runways yet. There are a couple of considerations that make it hard. Number one, we ship the world…all of it…all 24,000 airports that we get data on. That’s a lot of data! Number two, the data we get isn’t so great and comes from a bunch of different sources that don’t always agree with each other. The two data sets most important to this issue are DEM (Digital Elevation Model) and AFD (Airport and Facility Database).


Let’s take DEM. It’s a regular grid of elevation data at varying levels of granularity, based on the source. In Flight Simulator our DEM resolutions varies from 30m to 1km. We use different sources to control data size and quality. (Many people supposedly ‘in the know’ would suggest we use the new SRTM data obtained by the Space Shuttle for everything. Unfortunately this data has lots of problems due primarily to oblique radar imaging.)


Now let’s consider the AFD data. We get data from three primary sources: Jeppesen, the FAA and NGIA (formally NIMA; through its DAFIF product). What data we get varies based on the source, as does its quality. For instance, for many airports we only get a single elevation for the ARP (airport reference point). On the other hand, for some airports we get precise latitude, longitude and altitude data for each end of each runway—but that’s as good as it gets.


The first major hurdle is getting these data to play nice with each other. It is exceedingly rare that the DEM and AFD data agree precisely. This could be due to differences in the survey timing, technique or datum used. It could also be that ARP is not very close to the runways. And some airport data is just plain wrong. This is particularly true in the U.S. where the FAA maintains a self-reporting policy for private airstrips. You gotta strip of gravel in your back yard you want to list as an airport? Heck, just phone up the FAA and give them the location and elevation! Who cares if it’s just a guess?! We’ve actually found airports listed several dozen miles from their actual location, probably because the owner mistook a 3 for an 8 when reading the USGS chart.


But let’s say for a minute that you had perfect data (or were willing to take creative license—i.e., hand edit it—in order to make this work. The next struggle is with the complexity of the generated terrain mesh. Note: If you’ve taken computer graphics 101 you can skip ahead.)


Almost all 3D images displayed on a computer are created using massive numbers of connected triangles (a.ka. polygons). The more triangles you have the smoother the generated image will appear. (Those of you who indulged in role playing games with multifaceted dice as kids will appreciate this. A 20-sided die looks much ‘rounder’ than a 4-sided one.) However, the more triangles you have the more work the computer has to do. So at some point replicating a nicely sloped surface would require *a lot* of triangles.


At this point I can hear you saying, “So what? With today’s beefy GPUs that should be no problem.” Perhaps, but pure mesh complexity is only half the issue. The other half has to do with how you subdivide the terrain and airport polygons.


To illustrate, consider the simplest of cases: a single runway contained completely within a single DEM cell. If you’re not familiar with how DEM works, consider a square, 30 meters on each side. Each corner of the square has a value indicating its height relative to a fixed point (sea level in most cases). If those four values are different the surface of the square will be sloped. So far, so good. Plop a runway somewhere on the surface and you’re good to go. (Well, not quite. What if the data—which is usually full of errors, remember—produces a severe slope? Good luck taxiing on that!)


Now, expand the surface to two collinear squares with the runway spanning the common edge. You now have 6 elevations values. What does the runway look like? Depending on the elevation differences it would not be pretty—probably something like two boards laid over a sawhorse— but you could add more triangles to smooth it out. But now add a crossing runway and some taxiways—and expand the overall area to a few hundred grid cells. Are you getting the picture? You’ll end up with something that—worse case—looks like the world’s largest skateboard park. In practice you would hope the data wasn’t *that* bad but consider all of this complex segmentation has to be done at runtime. We could cheat and pre-compute everything but what would happen if someone swapped out the DEM?


That’s the bulk of the problem (at least as I see it, but I’m not the terrain developer and he might have a different spin). Things like getting aircraft to react correctly already basically work now. (What, you mean you’ve never gone off-roading in the Cessna?) The last thing I’ll mention (it was also mentioned in the thread) is the problem of getting buildings and other objects to stay grounded. If you’ve ever seen a 3D object on a slope (check out San Francisco) you’ve probably noticed the ‘foundation’ sticks out into space on the downhill side. Our models have a single center point used to locate them and so don’t really like sloped terrain. We could build ‘basements’ but that would just introduce the same shadow problem we have on radio towers and that’s a topic for another day.


So to summarize, we’d also dearly like to have sloped runways and we have some ideas how to make it work. We just need to find time to do it. (Then again, it has been a long time since we shipped FS2004.)

Comments (19)
  1. Patrick says:

    Great post!

    As a commercial pilot and computer programmer (C++/C#), I probably can’t find a more relevant blog. Fascinating information.

    I gave this a lot of thought, and the only solution I could find for this situation is to have a much finer-grained DEM, which isn’t possible both because of our current measuring sources, and limitations on computer hardware to render it.

    Maybe Flight Simulator 2020 will have sloped runways.

  2. Dave says:

    The problem with trying to recreate a sloped runway in FS, is there will always be someone who complains that the slope is too severe or not sloped enough.

    Why try and add a feature that will cause more strain on a PC, than just leaving things the way they are. Sure it would be a nice to have feature, but as you have mentioned, it creates a new pandoras box.

  3. scott967 says:

    Thanks for the timely input. I think the No 1 issue I see is not so much "sloped runways" but rather "sloped flattens" to reduce the problem of plateaus/cliffs where the flatten transitions to DEM terrain. If all the vertices of the airport flatten were forced to be coplanar, rather than same elevation, then the designer could adjust the tilt to better match. Your point about dealing with 3d objects would still have to a consideration though.

    scott s.


  4. JJAllen says:

    As a programmer of 20+ years, I know that the trade-off quotient usually decides a feature-set. All I can say in this case is whoopee-fiz to sloping runways! Just forget about it. On a feature list of 1000 items I would place this one at 1001 only because in real life, the goal is to make runways as flat as possible. Wouldn’t everyone rather see a CG person waiting (and waving) at the gate?

    Hope that I didn’t step on anyones toes just had to through in my 2 cents.


  5. Hello Jason,

    Interesting article !

    Thanx a lot for all these explanations !

    I followed a similar pattern in my sim, I apply flatten areas around the airports at the airports altitude.

    I don’t support mesh/mesh collision anyways so that made it an easy choice =)


  6. John Dow says:

    What I would dearly love is a simple utility to add grass and gravel runway textures to existing mesh, so that local farm and utility strips can be added in a similar way to the way Runway12 lets us add buildings etc to scenery.

    A simple utility where you click each end of the desired runway directly onto the existing scenery, which then lays down a grass or gravel texture between the points, and then the user clicks on a further set of points to set the exclude area for autogen would be one of the greatest additions to fs that you could provide for us low and slow vfr flyers!!!

  7. Max says:

    Interesting !

    We already have many freewares as Paneva ( ) or in payware, LLH ( )

    Alps’ pilots wish you to find a solution for the next FS and i think developpers will appreciate your tips when the new FS will come…

  8. christian says:

    Thanks very much on your update on the sloping runway issue. I thought a lot about this myself (as you’ll probably know 😉 ).

    In my personal opinion, technology allowing sloping runways is the one single feature that is missing from a high-res terrain modellers point of view – ie it looks really ugly in the places where it is needed. once we can do sloping runways, we can create perfect terrains.

    I could just ramble on about technology stuff, but I’m sure the developers have that under control. All I want to suggest is that you guys have a look at my New Zealand scenery ( If you haven’t heard of it and like to check it out, let me know and I’ll send you guys a free copy (

    The scenery is using a 20m mesh and some airports are sitting on sloping terrain. This has the consequence that the flatten cuts into the terrain on one end and sits on a plateau on the other – and it looks really bad. On the other hand, the scenery adds about 3500 airstrips and these are simple airstrips without flattens and only using grass VTPs. It’s amazing how well this works, you can really land on all these bush strips with GA aircraft without any problems at all. I found a couple that don’t work all that well, but the vast majority works astonishingly well (the whole concept allows for some very interesting take-offs and landings, I recon some of the private operators in NZ are simply nuts). The only technical challenge I can see is to blend a high-res texture onto the runway – ie for concrete runways we don’t want the VTP blurring, but textures with a whole lot of detail (runway markings) and preferentially shadows which I know makes this not a no-brainer to tackle.

    As for the floating buildings – a designer can (and should) always wack a flatten underneath. In fact, I’m currently trialing this method.

    To sum up – I wouldn’t worry about the data issues, just leave unknown or suspect airports flat (on a side note I know of some spectecular examples where some private NZ airports have some very dodgy mapping parameters). We 3rd party designers can take care of the details, it would just be nice to have the capability of blending runways into high-res terrain. It may not make much sense for you guys to implement this feature if it isn’t used much out of the box, but with high-res spatial data progressively finding its way into MSFS I still believe it’s the most needed feature from a scenery point of view. At least if you want to fly in a wild untamed place like NZ.



  9. Travis says:

    YAP (Yet Another Programmer) chiming in … sometimes, if a problem’s solution is too difficult, you can often change the problem.

    You seem to be in binary mode – either we try to sort out the data we have and get runways ‘right’, or we flatten everything and the problem goes away.

    There is a third solution – what about a couple of sloping runways? You currently have a couple of high-definition airports; you could add a couple of high-definition runways.

  10. Sorry about the delay in posting this comment – I missed your article when it first came up, and have since been busy collating my latest thoughts on a topic which has been something of a ‘pet’ of mine since before VFR Terrain was released.

    I was originally intending to post my comments here, but while drafting them it became apparent that they were probably too lengthy to work as a blog comment, particularly as there is no formatting available as far as I can see. I’ve therefore posted them as a new topic on the Visual Flight Forums. If the link at the foot of this comment doesn’t work, please click on my name at the head of this comment.

    For those of you not wanting to read the full post, here’s a summary of what it contains:

    Proposing ‘variably sloping’ rather than just ‘sloping’ runways with reasons; Examines real-world runway characteristics; Proposes shifting the emphasis from design time to load time to ease data access and compatibility problems; Discusses compatibility issues with existing 3D scenery and proposes possible solutions; Discusses why it is important that non-flat runways are modelled; Discusses additional considerations such as blending with surrounding terrain, differences along the length of individual runways, and impact (sic) on landing gear behaviour; Discusses effect on 3D object scenery, and proposes new design time capability to enhance compatibility of 3D ‘scenes’ with non-flat terrain in general (not just at airports);

    Discusses the problems associated with the ‘foundations’ of 3D objects in non-flat terrain.

    You can find my full comments in the following Visual Flight Forum post:

  11. pixelpoke says:


    "You seem to be in binary mode – either we try to sort out the data we have and get runways ‘right’, or we flatten everything and the problem goes away. "

    Well, it may *seem* binary, but Mike’s only touched on one or two points of the issue.

    There are more than a few of us here on the team who have given this some thought, and we all *want* to do it.

    You will find that a lot of people have lots of suggestions about how to solve this issue or that. It is also important to stress that it’s not a lack of understanding that prevents us from working on the issue now, but instead it’s an all too perfect understanding.

    It’s a wholistic view.

    The amount of time involved to implement a couple of examples would be a tragic misuse of our resources.

    Sure, it would allow a few 3rd party people to produce work, but we’re already double and triple booked on the feature sets we’re working on right now. Feature sets, I might add, that set the stage for doing this *right* next time.

    Doing it "right" means a worldwide default experience for *all* of our users, with enough expandability and customization for the 3rd party to take true advantage of.



  12. Hi.

    When we are talking about sloped runways;

    I really want to have the pleasure of landing on sloped runways in FSX.

    I saw what you wrote about "worlds larges skateboard-park", and here’s a comment to that: most of the airports who have runways who crosses each other, are so big that we can’t see if hey are sloped or not.

    The runways we really need a slope on, is runways like, for example; Honningsvaag, Valan Airfield. It’s placed in the north of Norway.

    That airport just have one runway: 08 to 26.

    26 is 11 meters higher than 26. Freddy Wilhelmsen have made a sloped runway to that airport.

    First you need to download Norway Airports which can be done here:

    Then you can download the update for Valan here:

    Install these two things, start FS and search for ENHV.

    To get a realistic look for the landscape, download Norway Scenic from here:

    If you don’t want to do all these things, look on the bottom of the AIP-map for Valan to see the high-difference:

    Now you have a perfect airport with a perfect sloped runway and with a perfect and realistic terrain around (I know because I live there). The only airliner in Norway who’s flying to Valan is Wideroe.

    They are using an upgraded version of the DeHavilland Dash 8-103, but if you can’t find it, try with the Dash 8-Q202 on (I’m using that one) or use the default Beechcraft King Air 350.

    If you want all from landing chart, aerodrome ground movement chart etc. to parking/docking chart, ACFT PRKNG and much more, for Norway, visit

    Left-click on a red mark on the map to see different airports.

    Another example of a runway woh REALLY should be sloped is the runway on Courchevel Airfield in France.

    From one end to another is the elevation 55 meters (the runway is 535 meters long)  and the maximum slope is on 18,5 %!

    This runway should really be sloped. I’ve seen pictures of a scenery with a sloped runway for FS2004, but I don’t know where the download is…

    If you haven’t seen Courchevel, visit this site and open you’r eyes:

    My point is: not all runways have to be sloped, only them who are so short that we actually can see the slope.

    I understand that it’s too hard to put a slope on runways who crosses each others. Everyone should try to land and take off from the airfields in the north, and the airfields/airports should have their slope so it will be even funnier.

  13. You want sloped runways in FS2004?  Here you go:

    I’ve seen videos of people trying to land the PMDG 747-400 at Lukla with this scenery.  Great fun! 🙂

  14. X-Plane has done this for about a year now, why can’t you guys do this?

  15. Alan Whitlock says:

    To me the big issue is to blend the non flat airport into the surrounding mesh and get rid of the "cliffs".

    I agree with the view that says we are not expecting Microsoft to provide all the airports with flatness data out of the box, just come up with a method that allows third parties to provide the detail, especially to merge the meshes in a tidy fashion.


  16. Barry says:

    I would be happy if they corrected the airports (mostly grass fields) that rose hundreds of feet in the air, there are more in FSX than FS9. For example, due north of KITH.

  17. Shamir says:

    Maybe, in FS11 we can get this kind of runway on the DETAILED airports

  18. Dreddhk - Neil says:

    I’ve yet to land on a flat runway excpet in flight sim – I was really hoping that FSX would have real runways, and boy was I dissapointed. I think slopping runways are as important as the weather! We may not spend much time on the gorund, but it is here that the two most imporatant parts of light occur – getting airbourne itself, and perhaps slightly more importantly, landing!

  19. Andy Ford says:

    Now that we have SP1 for FSX does this allow sloped runways???

    It strikes me as fine that FSX (or any version) doesn’t ship with sloped runways, but what we really need is the ability to do it in the SDK. That way freeware developers and other 3rd parties can make sloped runways using their knowledge of local airfields.

    The fact that the SDK now allows us to make sloped flattens also goes a long way to addressing the side effects of sloped runways.

Comments are closed.

Skip to main content