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I was looking at these two questions about the single direction takeoff-only runway 18 at Frankfurt Airport (EDDF) when I noticed that it has a full-length taxiway (taxiway Y5) to its east.

As far as I could tell based on online sources, this runway was always designed to be used in its current southbound takeoff-only configuration. Thus the only situation I could imagine this taxiway being useful is in the case of a rejected takeoff. I couldn't find good information on the internet about the frequency of rejected takeoffs in commercial aircraft, but intuitively it seems they can't be common enough for a whole taxiway to be dedicated them. If turning around after a rejected takeoff is a concern, why couldn't the runway include a runway turnaround area at the south end?

aerial view of south end of runway 18 at EDDF, also showing taxiway Y5 and taxiway Y9

One rather implausible reason I could think of is that the designers wanted an area for the brakes to cool down on an aircraft after a high speed rejected takeoff without having the aircraft occupying the runway during that time, but high speed rejected takeoffs are so rare that I find this very unlikely.

Even if that is the case, the combination of the southernmost part of taxiway Y5 and taxiway Y9 (the short northwest-southeast taxiway near the south end of runway 18 connecting it to taxiway Y5) makes for an adequately large "brake cooling area". After the brakes are cool, the aircraft can backtrack north along the runway, occupying it for a few minutes at most, and making the long northern part of taxiway Y5 seem unnecessary.

If that is not the case, why does taxiway Y9 exist at all? I find the existence of taxiway Y9 even more confusing than the existence of Y5, as the only case I could think of that leads to taxiway Y9 being used is when an aircraft wants to backtrack northward along runway 18, either by reaching the south end of the runway, exiting via taxiway Y5, and reentering the runway via taxiway Y9, or almost reaching the south end of runway 18, exiting via taxiway Y9, and reentering the south end of runway 18 facing northward via taxiway Y5.

The only closely analogous setup to runway 18 at EDDF that I could find is runway 11 at ZUTF in Chengdu, China. (I'd be interested to see more examples, but such things are hard to look for on the internet.) It has a parallel taxiway (taxiway K) to its north. Some sources give the runway numbering as 11/29 but inspection of aerial imagery confirms that the 29 side does not have a painted number, and there are no landing-related markings at all in either direction, so overall it closely resembles the situation at EDDF.

However, taxiway K is in this case connected to runway 11 at around the midpoint and at the far end (I couldn't find a publicly available airport diagram that shows the numbers of these connecting exit taxiways). Looking on aerial imagery, one can see that these exits are marked "NO ENTRY" in the direction of entering the runway and taxiway K itself is marked "NO ENTRY" past the third entry into the runway counting from the west. This seems to align with the theory that this taxiway is only used in a rejected takeoff.

aerial view of runway 11 and taxiway K at ZUTF

So basically my question is, what are these taxiways actually for? If they are for rejected takeoffs, how often do the get used for that purpose and how is it worth constructing a whole taxiway just to make handling this rather uncommon event slightly more efficient? And specifically for the EDDF example, what's the reasoning behind the weird configuration of taxiways at the southern end?

Edit: I went through historical aerial imagery a bit and saw that in some interval from around 2019-2020 to around 2021-2022, the main northern part of taxiway Y5 was closed. In that case, taxiway Y9 would indeed serve as part of an "enlarged" turnaround area, together with the southernmost part of taxiway Y5, to allow aircraft to backtrack along the runway. But the aerial imagery shows that taxiway Y9 (historically called taxiway Y7) had existed since at least 2000, so this closure of taxiway Y5 can't be the reason it was put in. But is it possible that potential closures of taxiway Y5 was anticipated as a possibility long ago and they put in Y7/Y9 for redundancy?

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Having that taxiway is cheaper/safer. I'll explain.

Runways are not flat. They slope up and down. Frankfurt's runway 18 slopes down (judging by Google Earth's accuracy it's a drop of 4–5 metres).

Now, when standing 3 m tall the horizon is approximately 6 km away. If the land slopes down, the horizon distance is shortened. It's an old guidance that when viewed from 3 m high without a full-length parallel taxiway, the whole runway would be visible; with such a taxiway, only half the runway:

3.1.17 Sight distance

Recommendation.— Where slope changes cannot be avoided, they should be such that there will be an unobstructed line of sight from:

— any point 3 m above a runway to all other points 3 m above the runway within a distance of at least half the length of the runway where the code letter is C, D, E or F [applies to Frankfurt]; [...]

Note.— Consideration will have to be given to providing an unobstructed line of sight over the entire length of a single runway where a full-length parallel taxiway is not available. Where an aerodrome has intersecting runways, additional criteria on the line of sight of the intersection area would need to be considered for operational safety. (Source: ICAO SARPs Annex 14 Vol 1 8th ed.)

That guidance can be found in the USA, Europe, and the general ICAO standards and recommendations. (Though in the USA for Part 139 airports it's more stringent, requiring an eye height of 5 ft / 1.5 m.)

What it does is give extra confidence to the pilot that if there was a backtracking airplane or vehicle, it would most likely be on the full-length parallel taxiway and no one has made a mistake.

But radars...

You might think nowadays such airports have surface radars and other protection layers: mishaps still happen. You can read a Eurocontrol analysis of runway incursion incidents over a period of 10 years here (PDF; skybrary.aero). Point is: protection layers are not impenetrable (the more, the better).

Even with no slopes, it's still better to have it, e.g. for low-visibility operations (again, it could be an airport car doing an inspection or the snow clearing team returning back).

Why is it cheaper? Building a 4 km runway with no slopes or sloping down then up against the natural contour is a massive earthworks undertaking, and a parallel taxiway has other benefits even if the runway is takeoff-only, e.g. a place for fire and rescue to stage for an emergency (emergency planes can use whatever runway they like).

In short: the runway slopes down and it's recommended to have that taxiway.

runway 18 from cockpit
Source: YouTube - Hayo Lücke - Take Off Frankfurt Airport FRA - Flight Deck Cockpit Condor Boeing 767-300 @ 14m03s

Above frame shows the sloping down of Frankfurt's runway 18 from a cockpit that is at least 4 m high. While a three-metre offset is not overlaid, it's clear that most of the concrete-paved part of the runway is not in view (more than the second half is paved in concrete). In Google Earth, one needs to twist the height graph to account for Earth's curvature as well. (And again, it's still very beneficial as explained above assuming no line-of-sight issues.)


What about that part on intersecting runways in the quotation? Slopes that give a clear view of the intersecting area are also recommended:

visible zone diagram
Source: ICAO Aerodrome Design Manual Part 1

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    $\begingroup$ Welcome (back?) to aviation.stackexchange.com! That is an excellent answer, I hope you stick around and write more of these in the future! $\endgroup$
    – DeltaLima
    Commented Oct 24, 2023 at 11:28
  • $\begingroup$ I became curious so I took a measurement on Google Earth (see image). It seems that even without the taxiway, runway 18 satisfies the 3 m rule (higher green line), but not the FAA 1.5 m rule (lower green line) (link 3.8.1.1). So technically they didn't have to build the taxiway? Of course I know Google Earth may be inaccurate. $\endgroup$
    – 331821
    Commented Oct 24, 2023 at 12:34
  • $\begingroup$ Ground surveillance radar/systems like ASDE-X were still well in the future when the runway was designed & built. They can provide some assurance now, but not in the 60's or 80's. $\endgroup$
    – Ralph J
    Commented Oct 24, 2023 at 13:56
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    $\begingroup$ @331821: It's fine to take it seriously :) One thing I forgot to mention is you can't measure angles on such charts since the distances are super-squeezed in the x-axis. So here's something to try, and even use to post a complementary answer to your own question: fix the y-axis scale, draw the pilot's FOV cone, and make it orthogonal to the surface the plane is on, say the early down-sloping part of the runway. $\endgroup$
    – user71566
    Commented Oct 24, 2023 at 16:13
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    $\begingroup$ Hi @RTO :) I haven't a clue what eye height West Germany used back then; regardless, what I'm saying is even if the line of sight is marginal, it's still very beneficial to have that taxiway. $\endgroup$
    – user71566
    Commented Oct 24, 2023 at 19:59
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In my opinion, there is a parallel taxiway for runway 18 because when it was originally conceived in the 1960's (according to this website) and opened for traffic in 1984, it was constructed, as it would typically be, with a parallel taxiway (designated "Y5") running the entire length of the runway.

Even if it was always designed to be used as a departure only, single-direction runway (runway 18) having a parallel taxiway allows an aircraft rejecting its takeoff to turn off at the end and taxi back using the parallel taxiway. Also, since there is not an expanded-width surface area at the end of runway 18 designed to facilitate aircraft making the 180 degree turn for a taxi-back on the runway, I would guess that the runway was never designed or envisioned to accommodate a taxi-back. (of course many aircraft could successfully make a 180 degree turn within the existing width of the runway without an expanded-width surface area designed specifically for that purpose)

Further, since Frankfurt is one of the busiest airports in Europe, designing a runway that requires a long "taxi-back" on the actual runway in the event of a rejected takeoff would delay any subsequent aircraft awaiting departure (using runway 18).

As to why taxiway "Y9" exists is unclear. Not only does it seem to be paved with asphalt as opposed to runway 18, which is apparently paved with concrete (at least the last two-thirds), the hold lines (see the image below) are depicted as applying to runway 18. There is not much of runway 18 remaining at the point taxiway "Y9" intersects it. So "Y9" could possibly be a taxiway addition to the runway after it (runway 18) was completed and now used for some local purpose.

enter image description here

enter image description here

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    $\begingroup$ I think Y9 exists because it was a temporary solution before the full-length taxiway was build. Originally, there was a 90° taxi-off at the end of RWY18, connecting to a short parallel taxiway that was led back to the runway via what is now Y9. The full-length parallel Y5 was only constructed later (although I am convinced it was planned from the beginning). See this image, taken a few days before opening RWY 18. It shows what is now Y9, but not the full length Y5. $\endgroup$
    – DeltaLima
    Commented Oct 25, 2023 at 14:02
  • $\begingroup$ @DeltaLima Great find! Thanks $\endgroup$
    – user22445
    Commented Oct 25, 2023 at 14:26

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