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At all airports where I've looked out the window during taxi, the taxiway intersections have been painted with nice circular arcs connecting (the centerline of) each entry to the intersection with each exit.

I've always assumed these lines were there to guide planes through the turns -- that is, if the nose wheel follows the curved line exactly then you're guaranteed that there'll be pavement under the path the main gear will follow, and there will be sufficient wingtip clearances for the aircraft types allowed to use the intersection, etc.

However, in this question it comes to light that pilots can't even see the curved centerlines while they're turning and rely on spatial intuition instead. Very well, that seems to work fine in practice.

Which leads to the question: Who are the curved centerlines for, then? What would go wrong if they weren't there and instead just the straight centerlines of the intersecting taxiways continued through?

Possibly the pilots of smaller aircraft than the intersection is built for (say, a Dash-8 in an intersection that can take A380s?) can see the curved lines ahead of them while they turn. But in that case they should have plenty of room to do the turn by intuition too, right?

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The line along taxiway centreline is meant to provide guidance before initiating the turn. You're right that in a large aircraft they would lose sight of the centreline when they're over it, but it gives the pilot an idea of where they need to keep the nose in order to keep the outer main wheels behind them on the taxiway surface. 

For example, in Canada, Transport Canada document TP312 (Aerodrome Standards and Recommended Practices) define how you need to place the centreline in order for the outer main wheels to remain at least a certain distance from the taxiway edge. (I believe the United States would have very similar rules; I just don't know the names of the appropriate documents)

Taxiway Curves

3.4.1.6 Recommendation - Changes in direction of taxiways should be as few and small as possible. The radii of the curves should be compatible with the manoeuvring capability and normal taxiing speeds of the aeroplanes for which the taxiway is intended. The design of the curve should be such that, when the cockpit of the aeroplane remains over the taxiway centre line markings, the clearance distance between the outer main wheels of the aeroplane and the edge of the taxiway should not be less than those specified in 3.4.1.3.

Think about when a large aircraft has to make a big turn (>90degrees). If there was no indication on the surface about how to take the turn in a large aircraft, you could inadvertently take it too early and run the main wheels off the pavement.

Here's an example of an aircraft following the centreline with its nose as best as possible in order to keep the main wheels in the right spot. taxiway curves

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  • $\begingroup$ Your references and explanation match the intuitive idea of the purpose I had before. But I have trouble seeing how the line can serve that purpose when the pilot cannot see it anyway. I suppose it's right that the pilot will know it is too early to turn if he can still see the curve he's supposed to follow, though ... $\endgroup$ – Henning Makholm Jul 26 '14 at 21:13
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    $\begingroup$ @HenningMakholm it isn't necessary to see the nose wheel rolling over the line in order for it to provide effective guidance - Taxiing a Cessna is like driving a small car: you can see all the markings pretty well & it's easy to use them for reference. Taxiing a 747 is like driving a cement truck: you can't see the markings close to you, but that's OK because you saw them a few seconds ago, and you can see the ones coming up, so you learn to anticipate and put the wheels where you want them. $\endgroup$ – voretaq7 Jul 26 '14 at 23:15

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