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Airport Signs & Markings: Displaced Threshold

You may have heard the term before: "Displaced Threshold;" but you may not know what it actually means. Your airport may not have one, so you probably have never seen what one looks like, or even given it all that much thought.

But a displaced threshold is a common thing to see at an airport, and there are at least a couple of good reasons that your airport might have a displaced threshold or two (or more!).

First - what does a displaced threshold look like? How do you know when you are seeing a displaced threshold?

Let's start by talking about what a "threshold" is - on airport runways, the threshold is the beginning of the runway.  It's as simple as that.

Or is it?

The runway threshold is marked with white lines parallel to the length of the runway (see image below):

Runway MarkingsSee at the bottom of the image, the two sets of long skinny white lines, just below the runway number?

Those are your "Threshold Markings" - telling you where the runway begins.  Notice that, in this case, there is no white line perpendicular to the runway length just before the threshold markings, because the pavement just starts right there.

You can't see it in the picture, but the runway threshold will also have green lights to show a pilot approaching to land where the runway begins at night. If you approached the end of the runway from the other side, you would see red lights instead of green, signifying the runway ending (see image below).

Runway Threshold light

See?  Green on one side, red on the other!  Neat, huh?

Anyway, a "displaced" threshold is a way to start the actual runway somewhere beyond where the pavement starts.

Why would they do that?  Don't you generally want as much pavement as possible to land on or takeoff from?

There are at least a couple of reasons why a runway threshold would be displaced:

  1. increases obstacle clearance
  2. reduces the noise footprint below any approaching aircraft

How does a displaced threshold do this?  By elevating the glideslope over any given point along the landing approach.

Whaaaaaa...?

Think about it - actually don't.  Let me draw you a picture.

Displaced Threshold Approach

See how the displaced threshold approach is higher at any given point than the normal approach?  So the noise from an approaching aircraft will be less to anyone on the ground because the plane is farther away (higher) - making noise abatement a valid reason for a displaced threshold.  It also means that if there were some sort of building or tower in line with the runway, that a landing aircraft could avoid it by flying over it - also a valid reason for a displaced threshold.

So what does a displaced threshold look like?  Pretty much the same as a regular threshold, only there will be a white line perpendicular to the length of the runway, AND there will be an indication about what (if anything) the unused pavement CAN be used for:

Displaced Threshold MarkingsOn the left, you'll see the taxiway line extending right up to the displaced runway threshold.  In this case, you cannot use the pavement for anything but taxi (or overrun, if landing from the other direction).

 

In the center, you'll see chevrons (they will be yellow in the real world) - meaning this area cannot be used for taxi or for takeoff run (but again, could be used for overrun if landing from the other direction).  Usually you will see the taxiway enter the runway at or after the displaced threshold in this case.

 

And on the right, you see arrows (white in real life), meaning the area cannot be used for landing, but should be used for the takeoff run.  The common denominator between any of them is that you MUST land at or beyond the threshold, not prior to it in any case.  The meanings are mainly important for taking off from this direction - then it's important to know what the markings mean.


Do you know of any other reasons a displaced threshold might exist? Do you have displaced thresholds at your airport? Land your knowledge in the comments below!

Andrew Hartley is a Certificated Flight Instructor in Columbus, Ohio.

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Learn to Fly: What it Looks Like – Aileron

Smart Flight Training's goal is to take the mystery out of aviation for those who want to learn to fly by discussing flight training terms and showing pictures and video where we can.

We hope that reading about, seeing, and watching aviation in action will help you learn to fly more efficiently - saving you money at your flight school and during all of your flight training!

This Wednesday, we will take a look at one of the "control surfaces" of you flight training aircraft, the aileron.

Ailerons

An aileron (French for "little wing") is a movable flap on the wing of an airplane used to control the plane's side-to-side movements. Ailerons cause one wing tip to move up and the other wing tip to move down, helping a plane turn. To bank to the left, a pilot must raise the left aileron and lower the right aileron. Ailerons are located on the back of the wing (known as the "trailing edge") and near the wing tips.

Aileron Image

All ailerons work the same way, whether you are a student learning to fly a light sport aircraft or an airline pilot flying a Boeing 747.

As you continue to learn to fly, you'll find out more than you ever thought you would want to know about ailerons, including their effect on the yaw of an aircraft in addition to the roll, and what you can do if you lose the ability to control the ailerons in flight.

But all of that is for a future flight training post! Until then, enjoy this YouTube video of ailerons at work... Notice that the ailerons do not have to move very much to do their job!


Learn to fly faster and smarter with Smart Flight Training: join our mailing list and get involved in the community by leaving a comment below!

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