[TowerTalk] Towers near airports

[jimlux]

On 7/30/18 11:58 AM, Chris Hoelzle wrote:
> I am putting up a couple towers and the standard 100:1 glide path (tower
> free zone) does not seem to take into account where your property is in
> relation to the airport.
> 
> My property is 90 degrees off the runway. Directly off the side of the
> runway.


But if someone comes in on a low circling approach, they're not 
necessarily in line with the extended centerline. There are all manner 
of approaches which come in at some angle and then change.  And, on take 
off, it's extremely common to have a turn immediately after take off for 
noise abatement or safety (e.g. Santa Monica departing to the west, you 
turn a bit to the left so you depart over a golf course, rather than houses)

Also, they may be covering for the future - there isn't an approach or 
departure *now*, but there might be in the future.  For instance, 
helicopter approaches often come in from the side (so they don't get in 
the way of fixed wing traffic on the main runway).

I think, also, the 100:1 is a "you don't have to tell anyone" threshold 
- if you're under the limit, they don't care.  Just because you're above 
the limit doesn't mean it's forbidden, it just means you have to 
coordinate with someone.

  "Prior to completing registration with the Commission, an antenna 
structure owner must have notified the FAA (via FAA Form 7460-1) and 
received a final determination of 'no hazard' from the FAA."


The other thing that might be relevant is the "interfere with a 
navigational aid" - your tower to the side of the airport's runway might 
affect some radio navigational aid. - unlikely, but that's why they want 
you to contact them.  Then they run their analysis and send you the 
determination, and you're good to go.



> 
> Using the calculations - I am 8650 feet from the side of the airport, it
> still seems that they want me not to exceed 86 feet in height.
> 
> Have I missed something?
> 


here's the gory details:
https://www.ecfr.gov/cgi-bin/text-idx?rgn=div5&node=14:2.0.1.2.9#se14.2.77_117


look at section 77.19 - the imaginary surface is flat out to 5000 or 
10000 ft at 150 ft above runway elevation (not AGL), then slopes up at 
20:1 for another 4000 ft horizontally, etc.  Mre than one airport has 
the runway on a hill relative to the surrounding terrain - SMO (Santa 
Monica) and AVX (Catalina) are two that I've flown out of.


This is sufficiently complex that basically, you tell them what you want 
to do, and they'll run the math against all the regulations, and tell 
you whether it passes or fails.  They're normally not trying to make 
life hard for you, but they have a genuine concern for planes not 
running into stuff.