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Obstructions At Airports  
User currently offlinegarnetpalmetto From United States of America, joined exactly 11 years ago today! , 5404 posts, RR: 53
Posted (2 years 2 months 1 week 1 day 9 hours ago) and read 3775 times:

So I've got a question that may seem fairly elementary - I'm looking at AirNav and I see that quite often airports have ratios attached to obstruction clearance. For instance:

"Obstructions: 16 ft. trees, 334 ft. from runway, 309 ft. right of centerline, 8:1 slope to clear"

I've figured out how to calculate that ratio but what, exactly, does it mean? Any explanation in layman's terms would be most appreciated!


South Carolina - too small to be its own country, too big to be a mental asylum.
7 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (2 years 2 months 1 week 1 day 2 hours ago) and read 3676 times:

Quoting garnetpalmetto (Thread starter):
I've figured out how to calculate that ratio but what, exactly, does it mean? Any explanation in layman's terms would be most appreciated!

8:1 slope means you need to gain 1' of elevation for every 8' you fly forward in order to clear the obstacle.

Tom.


User currently offlinegarnetpalmetto From United States of America, joined exactly 11 years ago today! , 5404 posts, RR: 53
Reply 2, posted (2 years 2 months 1 week 1 day ago) and read 3650 times:

Quoting tdscanuck (Reply 1):

8:1 slope means you need to gain 1' of elevation for every 8' you fly forward in order to clear the obstacle.

Is it not the other way around? I thought the ratio was expressed rise over run, but this makes it sound like it'd be run over rise.

Does it only work that way for takeoffs or for landings also? Is a 1:1 slope to clear "worse" than say a 25:1? Or a 0:1?



South Carolina - too small to be its own country, too big to be a mental asylum.
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (2 years 2 months 1 week 1 day ago) and read 3637 times:

Quoting garnetpalmetto (Reply 2):
Quoting tdscanuck (Reply 1):
8:1 slope means you need to gain 1' of elevation for every 8' you fly forward in order to clear the obstacle.

Is it not the other way around?

I'm not aware of any commercial fixed wing aircraft capable of 8' elevation increase for 1' of forward motion, except in highly unusual dynamic maneuvers. Many fighters can't even climb that fast.

Quoting garnetpalmetto (Reply 2):
Does it only work that way for takeoffs or for landings also? Is a 1:1 slope to clear "worse" than say a 25:1? Or a 0:1?

1:1 is worse than 25:1. 1:1 is a 45-degree angle of climb (fighter territory again). 25:1 is very shallow. 0:1 is straight up.

Tom.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10095 posts, RR: 26
Reply 4, posted (2 years 2 months 1 week 23 hours ago) and read 3626 times:
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Quoting garnetpalmetto (Thread starter):
"Obstructions: 16 ft. trees, 334 ft. from runway, 309 ft. right of centerline, 8:1 slope to clear"

Now for the real question:

334 / 16 = 20.875

8 / 1 = 8

Far as I know, 20:1 is the FAA-assured clearance plane for visual runways.....or something vaguely similar to that. So where does the 8:1 come from in this case?

Here's another example from my former home airport, KBED (this one is the opposite):

Obstructions: 120 ft. hill, lighted, 1050 ft. from runway, 734 ft. right of centerline, 34:1 slope to clear

1050 / 120 = 8.75
34 / 1 = 34

Thanks for bringing this up Garnet....I've wondered this for a long time.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 5, posted (2 years 2 months 1 week 19 hours ago) and read 3579 times:

Quoting garnetpalmetto (Reply 2):
Is it not the other way around? I thought the ratio was expressed rise over run, but this makes it sound like it'd be run over rise.

As tdscanuck says, no way an airliner can do 1:1 or "better", i.e. 45 degrees except in the very short term. Your pax would not be happy.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinegarnetpalmetto From United States of America, joined exactly 11 years ago today! , 5404 posts, RR: 53
Reply 6, posted (2 years 2 months 1 week 15 hours ago) and read 3547 times:

Thanks for the great replies so far - as for the second part of my question - do these clearance ratios only apply to takeoffs or would they also apply to landings too?


South Carolina - too small to be its own country, too big to be a mental asylum.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 7, posted (2 years 2 months 1 week 4 hours ago) and read 3421 times:

Quoting garnetpalmetto (Reply 6):
these clearance ratios only apply to takeoffs or would they also apply to landings too?

They do since you have to account for go-arounds. 

To answer your question, for the glide path you have to account for obstacles. However on landings you would have a fixed gradient (say 3%). If there's an obstacle in the way the threshold would be displaced forward along the runway until the glide path cleared the obstacle plus a margin.

That is, on take-off planes climb at different angles and rates. On landing they descend at the same angle. But still different rates. The higher your airspeed, the higher your descent rate due to the fixed glide path angle.



"There are no stupid questions, but there are a lot of inquisitive idiots."
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