[TowerTalk] Towers near airports

[jimlux]

On 7/31/18 3:40 PM, kstover at ac0h.net wrote:
> I read in the reg that if you have obstructions, trees, that are higher than
> your planned tower you are exempt as long as you keep the tower lower than
> the trees.
> 
> R. Kevin Stover    AC0H
> 
> 
> ###  what happens if you have a bunch of  100+  foot tall fir trees, or
> any other type of trees  on your property ??    Does any type of aircraft,
> or  helocopter  actually fly below 100  ft ?   We have seaplanes landing
> here
> locally downtown, yet the entire downtown, esp right on the water front is
> ringed with tall condos.  Local airport has  a lot of 100 ft +  fir trees
> in close proximity
> to the airport.   Like hundreds of em.... but they are off to the sides.
> Then we also have the helijet
> terminal, located  next to the cruise ship terminal...which is also right
> across the street  from more tall
> condos.
> 
> ## Once you get into the requirement for painting and lighting the tower,
> then its an ongoing
> maintenance issue..and also a legal liability issue.  I would not be
> surprised if they didnt
> require UPS power to keep the lights on for short....or  extended power
> outages.   You may
> want to think this through very very carefully  before you proceed with
> anything.
> 
>


The concern with towers vs trees (aside from trees always having less 
regulation) is that trees come in groups, so are less of a "see and 
avoid" collision hazard. A forest is hard to avoid seeing.

Towers, particularly guyed towers, are a real problem, because they 
aren't as visible as a big tree.  You probably don't have a 100 foot fir 
tree that is 2 feet wide from the base. It's probably more like 20 feet 
in diameter a good ways up from the ground.

On a hazy day, with the wrong sun angle, a tower can be almost 
invisible, until it's too late to take evasive action.

Your basic visual acuity is about 1 arc minute (0.3 milliradian), in the 
day time, with no haze or impairments

At 1 mile, a 18" wide tower is right about the limit of visual acuity. A 
20 ft wide tree is easily visible. Guy wires are invisible.

If you're puttering along at 100 mi/hr in your plane, you cover that 
mile in about 30-40 seconds. Even if you instantly rack the plane into a 
45 degree bank (unlikely if you're low and slow, that's good way to 
explore the stall/spin behavior of your plane), you're still going to 
cover a lot of ground.

This is how people run into 500 ft high towers, even if they're painted.
It's also why strobes (esp the double flash) work so well. the first 
flash attracts your attention, and you actually *see* the second flash, 
from a long way away.

The blinking red light is fine at night, but worthless in the day time.