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Displaced Thresholds  
User currently offlineYX717fan From United States of America, joined Feb 2006, 50 posts, RR: 0
Posted (9 years 7 months 1 week 1 day 23 hours ago) and read 1610 times:

Hi. I have always noticed on runways that most have a displaced threshold at least at one end. What is their purpose? How come sometimes they are extremely long? Thanks a bunch!


Keep Midwest my Midwest
2 replies: All unread, jump to last
Reply 1, posted (9 years 7 months 1 week 1 day 22 hours ago) and read 1595 times:

Here is the wikipedia defenition....
"A displaced threshold is a runway threshold located at a point other than the physical beginning or end of the runway. The portion of the runway so displaced may be used for takeoff but not for landing. Landing aircraft may use the displaced area on the opposite end for roll out.

Most often the offset threshold is in place to give arriving aircraft clearance over an obstruction while still allowing departing aircraft the maximum amount of runway available. A displaced threshold may also be introduced if a beginning section of the runway in no longer able to sustain the continuous impact from landing aircraft. In such a case, aircraft are expected to land beyond the displaced threshold. Departing aircraft are permitted to use the displaced section of the runway for takeoffs or landing rollouts because those aircraft are not impacting the runway with the force of a landing aircraft.

Displaced thresholds have arrows as the center line of the runway. A thick white line with usually four arrows pointing in the direction of the runway denotes the end of the threshold and the beginning of the runway."

Hope this helps. btw, I knew what a displaced threshold was before I found this on wikipedia, I just was to lazy to type an explanation.



Usually, if the threshold is displaced, it is because of obstructions, such as trees, powerlines, or buildings off the end of the runway which prohibit you from making a normal descent and landing on the begining of the runway. Officially, your not supposed to land on them but you can if you want to because that part of the pavement still supports landing aircraft.The displaced part is only used for takeoff or taxi up for takeoff. Also the displaced part does not go into the overall runway lenght. Even if you have a runway that is 5000 feet but has a displaced threshold of 2000, the legal runway lengh for landing is only 3000. That threshold picture above signinficantly reduces the runway landing lenght for runway23.

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