Class E Airspace History?

[overdrive148]

Long story short, I've been checking airspace for solar sites by hand for the last couple hours for work and I started thinking - why does Class E start at 1200' AGL and 700' AGL?

I know why it is shaped the way it is, why it drops down lower in certain areas, etc. But I never stopped to think about why those specific numbers were used. Is it an interesting story like "how railroad gauges today are based on the width of Roman horse chariots" or is it some cut and dry reason of "we needed a number and this is it"?

 

[MauleSkinner]

Those were the numbers they had lying around in a warehouse somewhere, and had no other plans for them?

 

[flyingron]

Those numbers are 200' above 500 and 1000' respectively.

 

[John Collins]

To add to Ron's comment, the 200 foot adder is to account for uncharted obstacles and/or tree growth. Obstacles over 200 feet high need FAA approval, see FAR 77.9(a).

77.9 Construction or alteration requiring notice.
If requested by the FAA, or if you propose any of the following types of construction or alteration, you must file notice with the FAA of:
(a) Any construction or alteration that is more than 200 ft. AGL at its site.

Basic IFR cruise altitudes or the MIA according to 91.177 (a)(2)

(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.

The 700 foot AGL base for class E area is used where both IFR and VFR flight is expected below IFR cruise altitudes, around airports with IFR approaches. This increases the visibility and cloud separation requirements for VFR aircraft near the areas used for IFR approaches. In some cases, the E airspace extends to the surface around airports to provide more protection from VFR aircraft to aircraft making IFR approaches. The surface E airspace is often found around non towered airports that are served by certificated operators.

 

[Clip4]

First there was controlled airspace just in case the flying machine was invented. 700 ft and 1200 ft were the average vertical limits a muzzle loading musket and muzzle loading rifle could shoot. Some whiskey was involved and everything else is the regulation we have today.