"John R" wrote in message ...

Quonset State (RI) airport (OQU) has class D airspace and is just
south of Providence (PVD), which has class C airspace.

The northern arc of the Class D airspace isn't charted with a
dashed blue line, so the best I can do is assume that this
boundary is coexistent with the Class C surface area.


The "northern arc" isn't charted because that area which would otherwise be
Class D if the PVD Class C airspace was not there is always Class C when the
OQU Class D airspace is in effect.



Some of
the class D airspace is under Class C with a floor of 1700 feet,
some under Class C with a floor of 1300 feet, and some is outside
the Class C airspace. To make things confusing on the chart, the
dividing line between the 1700' floor and 1300' floor is beneath
the [25] Class D ceiling symbol.


Yeah, one would think they could have done a better job of symbol placement.



So according to the chart, the Class D airspace extends to 2500
feet. However, Quonset tower was sternly warning pilots the
other day that their Class D only went to 1300 feet and not to
exceed that without the blessing of Providence. So why does the
chart say 2500 feet for the Class D inside the Class C ring?


The standard ceiling for Class D airspace is 2500 AGL, but it's charted in
MSL values. Since field elevation at OQU is just 18 MSL they're the same.
Due to the hierarchy of airspace, the OQU Class D airspace reaches 2500 MSL
only outside the ten mile PVD Class C ring. Inside the ring the Class D
ceiling is the floor of the overlying Class C airspace.