Arrows on runway

[CRJ_simpilot]

What is this called?

 

 

 

 

[mallcott]

Displaced Threshold.

 

Technnically, an offset (although rarely offset from runway centreline) taxiway. The arrows indicate it is not stressed for repeated Take-Offs and Landings.

[f16jockey_2]

Displaced threshold.

 

Wim

[CRJ_simpilot]

Now my second question:

 

For what reason?

 

And another small question:

 

Do you start off there like how the Sim has placed my F-22 or further up? I can only imagine it doesn't matter.

 

Why a displaced threshold anyway? Why not just make the whole thing one long usable runway and be done with it?

[tiger1962]

The threshold isn't built to take the weight or impact of a landing aircraft, it's for taxiing on or taking off from only.

[Chock]

As noted, it's a displaced threshold, i.e. it is actually part of the runway's entire length, and it can be used for part of the take off roll, or a roll out, but it's not stressed for a touchdown. But...

 

On the face of it, it would seem that it would be sensible to simply make this extra bit of concrete capable of being able to take the weight of a touchdown, however, there are a few reasons why this isn't always practical or even preferable, beyond of course any sort of civil engineering reasons. As you probably know, there are often two hold points on the taxiway when you get near the runway entrance, one of these will be further away from the runway than the other; the further away one will be designated as an ILS hold point, i.e. the that's the one where a large airliner can be stopped and its mass will not interfere with the instrument landing system antennas.

 

Airliners are a pretty big lump of metal, and so it is certainly is possible for them to cause interference to the ILS signal if they are too near it. Having a displaced threshold, well away from the antennas for the ILS, allows arliners to get right near the runway and thus able to line up and go fairly quickly without them risking causing interference by their presence close to the runway.

 

It's also worth noting that at night, any airliner holding near a displaced threshold will likely be well clear of the PAPI lights and is therefore less likely to cause visual interference to an incoming aircraft using the PAPI lights for guidance, since an airliner is likely to have red and white lights on it, just as the PAPI does. Similarly, when an airport has a road passing across the end of the runway, the lights of traffic might potentially interfere with a pilot looking at the PAPI lights, and just possibly, the metal of a lot of traffic and any radio equipment it is using etc, could cause interference to the ILS antennas, so this is another reason why there is sometimes a displaced threshold.

 

Then there's the cost too. Since it is displaced, and not intended to be landed upon, it is cheaper to build that part of the runway, as it doesn't have to be as hard wearing or have anywhere near the load bearing capacity of the main runway where the impact of a touchdown occurs.

[mallcott]

Now my second question:

 

For what reason?

 

And another small question:

 

Do you start off there like how the Sim has placed my F-22 or further up? I can only imagine it doesn't matter.

 

Why a displaced threshold anyway? Why not just make the whole thing one long usable runway and be done with it?

 

Often because it was added later, after the original runway was certified at its original length

Or because the ILS was set up further down the runway

Or because the approach path would come too close to existing obstacles, like house or trees

Or because the typical aircraft using it need a wide angle turn

Or because the airfield/airport has access to that end of the airport and elsewhere is blocked

 

There are literally thousands of reasons...

[av8tor98]

Just as important is the obstacle clearance...especially in populated areas.

[lnuss]

Just as important is the obstacle clearance...especially in populated areas.

That's true, Joe -- many places have displaced thresholds with no obstacle problem for the reasons others have cited above, but some (usually smaller fields) are, indeed, primarily to allow extra runway for takeoff while recognizing obstacle clearance needed for landings on that runway.

[CRJ_simpilot]

All very good points. Thanks.