garnetpalmetto
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Obstructions At Airports

Mon Aug 13, 2012 7:32 pm

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!
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tdscanuck
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RE: Obstructions At Airports

Tue Aug 14, 2012 2:01 am

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.
 
garnetpalmetto
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RE: Obstructions At Airports

Tue Aug 14, 2012 4:00 am

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.
 
tdscanuck
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RE: Obstructions At Airports

Tue Aug 14, 2012 4:30 am

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.
 
vikkyvik
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RE: Obstructions At Airports

Tue Aug 14, 2012 5:30 am

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.
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Starlionblue
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RE: Obstructions At Airports

Tue Aug 14, 2012 9:17 am

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." - John Ringo
 
garnetpalmetto
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RE: Obstructions At Airports

Tue Aug 14, 2012 12:58 pm

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.
 
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Starlionblue
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RE: Obstructions At Airports

Wed Aug 15, 2012 12:38 am

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.
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