What is the floor of Class E airspace over Johnston Airport (M90)?

To understand why the floor of Class E airspace over Johnston Airport (M90) is designated at 1200 feet AGL, it’s crucial to recognize how Class E airspace is typically defined in relation to surrounding airspace structure and operational needs.

Class E airspace is often established to facilitate IFR (Instrument Flight Rules) operations and can begin at various altitudes depending on the proximity to airports and the specific regulations of each area. Generally, for most airports in areas where no other controlled airspace exists, Class E begins at 1200 feet above ground level. Specifically, if the airport is not located within 20 nautical miles of a Class B, Class C, or a Class D airspace that operates below that altitude, Class E will usually extend from 1200 feet AGL upwards.

In the case of Johnston Airport (M90), assuming it fits the criteria where it is not surrounded by other controlled airspace that would dictate a different floor, the designation of Class E airspace starting at 1200 feet AGL aligns with the standard practice. This altitude provides a buffer for aircraft transitioning through the airspace while also allowing for appropriate vertical separation between different flight operations, especially those under IFR.

The other altitude options (100