Runway extension, effect on crossing height

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
A runway is being extended by 1000' (the approach end).
Previously, an airplane on a 3° glideslope would pass 500' over the school on the approach, based upon touching down on the numbers.
With the new length, and still touching down on the numbers, what is the new crossing height over the school.
Not a theoretical question, this airport and situation exists. The numbers may not be exact however.
I get somewhere in the range of 50-60' range as a guess but it is 35 years since I did trig.
 
You'll be just about 50' lower (so you'll cross the school 450' overhead).

Edit - I got beat to the punch.

Edit edit - but still ahead of some! ;)
 
Figuring that you cross the school at 500 ft., the school is 9,540 ft. from the numbers of the original runway.

Extending the runway 1,000 ft. on the approach end subtracts 1,000 ft. from that calculation, so the school would now be 8,540 ft. from the runway.

tan (3) = x / 8540
x = 447.56 ft.

You would cross the school at 447 ft., about 52 feet lower than the original runway.
 
Let'sgoflying! said:
A runway is being extended by 1000' (the approach end).
Previously, an airplane on a 3° glideslope would pass 500' over the school on the approach, based upon touching down on the numbers.
With the new length, and still touching down on the numbers, what is the new crossing height over the school.
Not a theoretical question, this airport and situation exists. The numbers may not be exact however.
I get somewhere in the range of 50-60' range as a guess but it is 35 years since I did trig.
Click to expand...

What runway at what airport?
 
Thanks
I used the one in sixty rule to approximate, while answering calls from center, not bad.
5T6
 
Last edited:
the location of the TD point is changing, 1000' closer to the school. Someone draw a picture.
 
Let'sgoflying! said:
the location of the TD point is changing, 1000' closer to the school. Someone draw a picture.
Click to expand...

Correct, just subtract 1,000 ft. from the original distance between the runway and the school. That original distance by trig is 9540 ft so the new distance from the school to the numbers (touchdown point) would be 8540 ft. Using the tangent of 3 and solving for the unknown value, the height above the school would be about 448 ft.
 
Typically, one of two things happens -- the threshold stays where it is as a displaced threshold, or it moves with the new runway end. In the latter case, they'll move everything back that 1000 feet the runway is being extended and the TCH over the new threshold will be determined based on the new touchdown zone, but probably won't change much from the TCH over the original threshold. In the former case, nothing changes at all, including the TCH, since the landing threshold doesn't change.
 
A 3 degree slope is about 330ft / nm.
1000ft is about one-sixth of a nm.
1/6th of 330 is about 55ft.
You'll be 55ft lower over the school.

And I'd venture a guess that some VFR pilots fly a lower approach today and are already that low, if they are flying a long 1.5 mile long final.
 
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